draft-ietf-openpgp-rfc4880bis-03.txt

Network Working Group                                            W. Koch
Internet-Draft
Updates: 4880 (if approved)                            December 29, 2017
Intended status: Standards Track
Expires: July 2, 2018


                         OpenPGP Message Format
                    draft-ietf-openpgp-rfc4880bis-03

Abstract

   { Work in progress to update the OpenPGP specification from RFC4880 }

   This document is maintained in order to publish all necessary
   information needed to develop interoperable applications based on the
   OpenPGP format.  It is not a step-by-step cookbook for writing an
   application.  It describes only the format and methods needed to
   read, check, generate, and write conforming packets crossing any
   network.  It does not deal with storage and implementation questions.
   It does, however, discuss implementation issues necessary to avoid
   security flaws.

   OpenPGP software uses a combination of strong public-key and
   symmetric cryptography to provide security services for electronic
   communications and data storage.  These services include
   confidentiality, key management, authentication, and digital
   signatures.  This document specifies the message formats used in
   OpenPGP.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 2, 2018.





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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  {1} Introduction  . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  {1.1} Terms . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  {2} General functions . . . . . . . . . . . . . . . . . . . .   6
     2.1.  {2.1} Confidentiality via Encryption  . . . . . . . . . .   6
     2.2.  {2.2} Authentication via Digital Signature  . . . . . . .   7
     2.3.  {2.3} Compression . . . . . . . . . . . . . . . . . . . .   8
     2.4.  {2.4} Conversion to Radix-64  . . . . . . . . . . . . . .   8
     2.5.  {2.5} Signature-Only Applications . . . . . . . . . . . .   8
   3.  {3} Data Element Formats  . . . . . . . . . . . . . . . . . .   9
     3.1.  {3.1} Scalar Numbers  . . . . . . . . . . . . . . . . . .   9
     3.2.  {3.2} Multiprecision Integers . . . . . . . . . . . . . .   9
     3.3.  {3.3} Key IDs . . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  {3.4} Text  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  {3.5} Time Fields . . . . . . . . . . . . . . . . . . . .  10
     3.6.  {3.6} Keyrings  . . . . . . . . . . . . . . . . . . . . .  10
     3.7.  {3.7} String-to-Key (S2K) Specifiers  . . . . . . . . . .  10
       3.7.1.  {3.7.1} String-to-Key (S2K) Specifier Types . . . . .  10
       3.7.2.  {3.7.2} String-to-Key Usage . . . . . . . . . . . . .  12
   4.  {4} Packet Syntax . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  {4.1} Overview  . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  {4.2} Packet Headers  . . . . . . . . . . . . . . . . . .  14
       4.2.1.  {4.2.1} Old Format Packet Lengths . . . . . . . . . .  14
       4.2.2.  {4.2.2} New Format Packet Lengths . . . . . . . . . .  15
       4.2.3.  {4.2.3} Packet Length Examples  . . . . . . . . . . .  16
     4.3.  {4.3} Packet Tags . . . . . . . . . . . . . . . . . . . .  17
   5.  {5} Packet Types  . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  {5.1} Public-Key Encrypted Session Key Packets (Tag 1)  .  18
     5.2.  {5.2} Signature Packet (Tag 2)  . . . . . . . . . . . . .  19
       5.2.1.  {5.2.1} Signature Types . . . . . . . . . . . . . . .  19
       5.2.2.  {5.2.2} Version 3 Signature Packet Format . . . . . .  22
       5.2.3.  {5.2.3} Version 4 Signature Packet Format . . . . . .  24



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       5.2.4.  {5.2.4} Computing Signatures  . . . . . . . . . . . .  40
     5.3.  {5.3} Symmetric-Key Encrypted Session Key Packets (Tag 3)  42
     5.4.  {5.4} One-Pass Signature Packets (Tag 4)  . . . . . . . .  43
     5.5.  {5.5} Key Material Packet . . . . . . . . . . . . . . . .  44
       5.5.1.  {5.5.1} Key Packet Variants . . . . . . . . . . . . .  44
       5.5.2.  {5.5.2} Public-Key Packet Formats . . . . . . . . . .  45
       5.5.3.  {5.5.3} Secret-Key Packet Formats . . . . . . . . . .  46
     5.6.  Algorithm-specific Parts of Keys  . . . . . . . . . . . .  48
       5.6.1.  Algorithm-Specific Part for RSA Keys  . . . . . . . .  48
       5.6.2.  Algorithm-Specific Part for DSA Keys  . . . . . . . .  49
       5.6.3.  Algorithm-Specific Part for Elgamal Keys  . . . . . .  49
       5.6.4.  Algorithm-Specific Part for ECDSA Keys  . . . . . . .  49
       5.6.5.  Algorithm-Specific Part for EdDSA Keys  . . . . . . .  50
       5.6.6.  Algorithm-Specific Part for ECDH Keys . . . . . . . .  50
     5.7.  {5.6} Compressed Data Packet (Tag 8)  . . . . . . . . . .  51
     5.8.  {5.7} Symmetrically Encrypted Data Packet (Tag 9) . . . .  52
     5.9.  {5.8} Marker Packet (Obsolete Literal Packet) (Tag 10)  .  52
     5.10. {5.9} Literal Data Packet (Tag 11)  . . . . . . . . . . .  53
     5.11. {5.10} Trust Packet (Tag 12)  . . . . . . . . . . . . . .  54
     5.12. {5.11} User ID Packet (Tag 13)  . . . . . . . . . . . . .  54
     5.13. {5.12} User Attribute Packet (Tag 17) . . . . . . . . . .  54
       5.13.1.  {5.12.1} The Image Attribute Subpacket . . . . . . .  55
       5.13.2.  User ID Attribute Subpacket  . . . . . . . . . . . .  56
     5.14. {5.13} Sym. Encrypted Integrity Protected Data Packet
           (Tag 18)  . . . . . . . . . . . . . . . . . . . . . . . .  56
     5.15. {5.14} Modification Detection Code Packet (Tag 19)  . . .  59
     5.16. AEAD Encrypted Data Packet (Tag 20) . . . . . . . . . . .  60
       5.16.1.  EAX Mode . . . . . . . . . . . . . . . . . . . . . .  61
       5.16.2.  OCB Mode . . . . . . . . . . . . . . . . . . . . . .  61
   6.  {6} Radix-64 Conversions  . . . . . . . . . . . . . . . . . .  62
     6.1.  {6.1} An Implementation of the CRC-24 in "C"  . . . . . .  63
     6.2.  {6.2} Forming ASCII Armor . . . . . . . . . . . . . . . .  63
     6.3.  {6.3} Encoding Binary in Radix-64 . . . . . . . . . . . .  66
     6.4.  {6.4} Decoding Radix-64 . . . . . . . . . . . . . . . . .  67
     6.5.  {6.5} Examples of Radix-64  . . . . . . . . . . . . . . .  68
     6.6.  {6.6} Example of an ASCII Armored Message . . . . . . . .  68
   7.  {7} Cleartext Signature Framework . . . . . . . . . . . . . .  69
     7.1.  {7.1} Dash-Escaped Text . . . . . . . . . . . . . . . . .  69
   8.  {8} Regular Expressions . . . . . . . . . . . . . . . . . . .  70
   9.  {9} Constants . . . . . . . . . . . . . . . . . . . . . . . .  71
     9.1.  {9.1} Public-Key Algorithms . . . . . . . . . . . . . . .  71
     9.2.  ECC Curve OID . . . . . . . . . . . . . . . . . . . . . .  72
     9.3.  {9.2} Symmetric-Key Algorithms  . . . . . . . . . . . . .  72
     9.4.  {9.3} Compression Algorithms  . . . . . . . . . . . . . .  73
     9.5.  {9.4} Hash Algorithms . . . . . . . . . . . . . . . . . .  73
     9.6.  AEAD Algorithms . . . . . . . . . . . . . . . . . . . . .  74
   10. {10} IANA Considerations  . . . . . . . . . . . . . . . . . .  74
     10.1.  {10.1} New String-to-Key Specifier Types . . . . . . . .  75



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     10.2.  {10.2} New Packets . . . . . . . . . . . . . . . . . . .  75
       10.2.1.  {10.2.1} User Attribute Types  . . . . . . . . . . .  75
       10.2.2.  {10.2.1.1} Image Format Subpacket Types  . . . . . .  75
       10.2.3.  {10.2.2} New Signature Subpackets  . . . . . . . . .  76
       10.2.4.  {10.2.3} New Packet Versions . . . . . . . . . . . .  78
     10.3.  {10.3} New Algorithms  . . . . . . . . . . . . . . . . .  78
       10.3.1.  {10.3.1} Public-Key Algorithms . . . . . . . . . . .  79
       10.3.2.  {10.3.2} Symmetric-Key Algorithms  . . . . . . . . .  79
       10.3.3.  {10.3.3} Hash Algorithms . . . . . . . . . . . . . .  79
       10.3.4.  {10.3.4} Compression Algorithms  . . . . . . . . . .  80
   11. {11} Packet Composition . . . . . . . . . . . . . . . . . . .  80
     11.1.  {11.1} Transferable Public Keys  . . . . . . . . . . . .  80
     11.2.  {11.2} Transferable Secret Keys  . . . . . . . . . . . .  82
     11.3.  {11.3} OpenPGP Messages  . . . . . . . . . . . . . . . .  82
     11.4.  {11.4} Detached Signatures . . . . . . . . . . . . . . .  83
   12. {12} Enhanced Key Formats . . . . . . . . . . . . . . . . . .  83
     12.1.  {12.1} Key Structures  . . . . . . . . . . . . . . . . .  83
     12.2.  {12.2} Key IDs and Fingerprints  . . . . . . . . . . . .  85
   13. Elliptic Curve Cryptography . . . . . . . . . . . . . . . . .  86
     13.1.  Supported ECC Curves . . . . . . . . . . . . . . . . . .  87
     13.2.  ECDSA and ECDH Conversion Primitives . . . . . . . . . .  87
     13.3.  EdDSA Point Format . . . . . . . . . . . . . . . . . . .  88
     13.4.  Key Derivation Function  . . . . . . . . . . . . . . . .  88
     13.5.  EC DH Algorithm (ECDH) . . . . . . . . . . . . . . . . .  89
   14. {13} Notes on Algorithms  . . . . . . . . . . . . . . . . . .  91
     14.1.  {13.1} PKCS#1 Encoding in OpenPGP  . . . . . . . . . . .  91
       14.1.1.  {13.1.1} EME-PKCS1-v1_5-ENCODE . . . . . . . . . . .  91
       14.1.2.  {13.1.2} EME-PKCS1-v1_5-DECODE . . . . . . . . . . .  92
       14.1.3.  {13.1.3} EMSA-PKCS1-v1_5 . . . . . . . . . . . . . .  93
     14.2.  {13.2} Symmetric Algorithm Preferences . . . . . . . . .  94
     14.3.  {13.3} Other Algorithm Preferences . . . . . . . . . . .  95
       14.3.1.  {13.3.1} Compression Preferences . . . . . . . . . .  95
       14.3.2.  {13.3.2} Hash Algorithm Preferences  . . . . . . . .  96
     14.4.  {13.4} Plaintext . . . . . . . . . . . . . . . . . . . .  96
     14.5.  {13.5} RSA . . . . . . . . . . . . . . . . . . . . . . .  96
     14.6.  {13.6} DSA . . . . . . . . . . . . . . . . . . . . . . .  96
     14.7.  {13.7} Elgamal . . . . . . . . . . . . . . . . . . . . .  97
     14.8.  EdDSA  . . . . . . . . . . . . . . . . . . . . . . . . .  97
     14.9.  {13.8} Reserved Algorithm Numbers  . . . . . . . . . . .  97
     14.10. {13.9} OpenPGP CFB Mode  . . . . . . . . . . . . . . . .  98
     14.11. {13.10} Private or Experimental Parameters . . . . . . .  99
     14.12. {13.12} Meta-Considerations for Expansion  . . . . . . .  99
   15. {14} Security Considerations  . . . . . . . . . . . . . . . . 100
   16. Compatibility Profiles  . . . . . . . . . . . . . . . . . . . 106
     16.1.  OpenPGP ECC Profile  . . . . . . . . . . . . . . . . . . 106
     16.2.  Suite-B Profile  . . . . . . . . . . . . . . . . . . . . 107
     16.3.  Security Strength at 192 Bits  . . . . . . . . . . . . . 107
     16.4.  Security Strength at 128 Bits  . . . . . . . . . . . . . 107



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   17. {15} Implementation Nits  . . . . . . . . . . . . . . . . . . 107
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . . 109
     18.1.  Normative References . . . . . . . . . . . . . . . . . . 109
     18.2.  Informative References . . . . . . . . . . . . . . . . . 112
     18.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 113
   Appendix A.  Test vectors . . . . . . . . . . . . . . . . . . . . 113
     A.1.  Sample EdDSA key  . . . . . . . . . . . . . . . . . . . . 113
     A.2.  Sample EdDSA signature  . . . . . . . . . . . . . . . . . 114
   Appendix B.  ECC Point compression flag bytes . . . . . . . . . . 114
   Appendix C.  Changes since RFC-4880 . . . . . . . . . . . . . . . 115
   Appendix D.  The principal authors of RFC-4880 are as follows:  . 115
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 116

1.  {1} Introduction

   { This is work in progress to update OpenPGP.  Editorial notes are
   enclosed in curly braces.  The section numbers from RFC4880 are also
   indicated in curly braces. }

   This document provides information on the message-exchange packet
   formats used by OpenPGP to provide encryption, decryption, signing,
   and key management functions.  It is a revision of RFC 2440, "OpenPGP
   Message Format", which itself replaces RFC 1991, "PGP Message
   Exchange Formats" [RFC1991] [RFC2440].

   This document obsoletes: RFC 5581 (Camellia cipher) and RFC 6637 (ECC
   for OpenPGP)

1.1.  {1.1} Terms

   o  OpenPGP - This is a term for security software that uses PGP 5.x
      as a basis, formalized in RFC 2440 and this document.

   o  PGP - Pretty Good Privacy.  PGP is a family of software systems
      developed by Philip R.  Zimmermann from which OpenPGP is based.

   o  PGP 2.6.x - This version of PGP has many variants, hence the term
      PGP 2.6.x.  It used only RSA, MD5, and IDEA for its cryptographic
      transforms.  An informational RFC, RFC 1991, was written
      describing this version of PGP.

   o  PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
      community and also in the predecessor of this document, RFC 1991.
      It has new formats and corrects a number of problems in the PGP
      2.6.x design.  It is referred to here as PGP 5.x because that
      software was the first release of the "PGP 3" code base.





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   o  GnuPG - GNU Privacy Guard, also called GPG.  GnuPG is an OpenPGP
      implementation that avoids all encumbered algorithms.
      Consequently, early versions of GnuPG did not include RSA public
      keys.  GnuPG may or may not have (depending on version) support
      for IDEA or other encumbered algorithms.

   "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP
   Corporation and are used with permission.  The term "OpenPGP" refers
   to the protocol described in this and related documents.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
   FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
   APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
   this document when used to describe namespace allocation are to be
   interpreted as described in [RFC2434].

2.  {2} General functions

   OpenPGP provides data integrity services for messages and data files
   by using these core technologies:

   o  digital signatures

   o  encryption

   o  compression

   o  Radix-64 conversion

   In addition, OpenPGP provides key management and certificate
   services, but many of these are beyond the scope of this document.

2.1.  {2.1} Confidentiality via Encryption

   OpenPGP combines symmetric-key encryption and public-key encryption
   to provide confidentiality.  When made confidential, first the object
   is encrypted using a symmetric encryption algorithm.  Each symmetric
   key is used only once, for a single object.  A new "session key" is
   generated as a random number for each object (sometimes referred to
   as a session).  Since it is used only once, the session key is bound
   to the message and transmitted with it.  To protect the key, it is
   encrypted with the receiver's public key.  The sequence is as
   follows:




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   1.  The sender creates a message.

   2.  The sending OpenPGP generates a random number to be used as a
       session key for this message only.

   3.  The session key is encrypted using each recipient's public key.
       These "encrypted session keys" start the message.

   4.  The sending OpenPGP encrypts the message using the session key,
       which forms the remainder of the message.  Note that the message
       is also usually compressed.

   5.  The receiving OpenPGP decrypts the session key using the
       recipient's private key.

   6.  The receiving OpenPGP decrypts the message using the session key.
       If the message was compressed, it will be decompressed.

   With symmetric-key encryption, an object may be encrypted with a
   symmetric key derived from a passphrase (or other shared secret), or
   a two-stage mechanism similar to the public-key method described
   above in which a session key is itself encrypted with a symmetric
   algorithm keyed from a shared secret.

   Both digital signature and confidentiality services may be applied to
   the same message.  First, a signature is generated for the message
   and attached to the message.  Then the message plus signature is
   encrypted using a symmetric session key.  Finally, the session key is
   encrypted using public-key encryption and prefixed to the encrypted
   block.

2.2.  {2.2} Authentication via Digital Signature

   The digital signature uses a hash code or message digest algorithm,
   and a public-key signature algorithm.  The sequence is as follows:

   1.  The sender creates a message.

   2.  The sending software generates a hash code of the message.

   3.  The sending software generates a signature from the hash code
       using the sender's private key.

   4.  The binary signature is attached to the message.

   5.  The receiving software keeps a copy of the message signature.





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   6.  The receiving software generates a new hash code for the received
       message and verifies it using the message's signature.  If the
       verification is successful, the message is accepted as authentic.

2.3.  {2.3} Compression

   OpenPGP implementations SHOULD compress the message after applying
   the signature but before encryption.

   If an implementation does not implement compression, its authors
   should be aware that most OpenPGP messages in the world are
   compressed.  Thus, it may even be wise for a space-constrained
   implementation to implement decompression, but not compression.

   Furthermore, compression has the added side effect that some types of
   attacks can be thwarted by the fact that slightly altered, compressed
   data rarely uncompresses without severe errors.  This is hardly
   rigorous, but it is operationally useful.  These attacks can be
   rigorously prevented by implementing and using Modification Detection
   Codes as described in sections following.

2.4.  {2.4} Conversion to Radix-64

   OpenPGP's underlying native representation for encrypted messages,
   signature certificates, and keys is a stream of arbitrary octets.
   Some systems only permit the use of blocks consisting of seven-bit,
   printable text.  For transporting OpenPGP's native raw binary octets
   through channels that are not safe to raw binary data, a printable
   encoding of these binary octets is needed.  OpenPGP provides the
   service of converting the raw 8-bit binary octet stream to a stream
   of printable ASCII characters, called Radix-64 encoding or ASCII
   Armor.

   Implementations SHOULD provide Radix-64 conversions.

2.5.  {2.5} Signature-Only Applications

   OpenPGP is designed for applications that use both encryption and
   signatures, but there are a number of problems that are solved by a
   signature-only implementation.  Although this specification requires
   both encryption and signatures, it is reasonable for there to be
   subset implementations that are non-conformant only in that they omit
   encryption.








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3.  {3} Data Element Formats

   This section describes the data elements used by OpenPGP.

3.1.  {3.1} Scalar Numbers

   Scalar numbers are unsigned and are always stored in big-endian
   format.  Using n[k] to refer to the kth octet being interpreted, the
   value of a two-octet scalar is ((n[0] << 8) + n[1]).  The value of a
   four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
   n[3]).

3.2.  {3.2} Multiprecision Integers

   Multiprecision integers (also called MPIs) are unsigned integers used
   to hold large integers such as the ones used in cryptographic
   calculations.

   An MPI consists of two pieces: a two-octet scalar that is the length
   of the MPI in bits followed by a string of octets that contain the
   actual integer.

   These octets form a big-endian number; a big-endian number can be
   made into an MPI by prefixing it with the appropriate length.

   Examples:

   (all numbers are in hexadecimal)

   The string of octets [00 01 01] forms an MPI with the value 1.  The
   string [00 09 01 FF] forms an MPI with the value of 511.

   Additional rules:

   The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.

   The length field of an MPI describes the length starting from its
   most significant non-zero bit.  Thus, the MPI [00 02 01] is not
   formed correctly.  It should be [00 01 01].

   Unused bits of an MPI MUST be zero.

   Also note that when an MPI is encrypted, the length refers to the
   plaintext MPI.  It may be ill-formed in its ciphertext.







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3.3.  {3.3} Key IDs

   A Key ID is an eight-octet scalar that identifies a key.
   Implementations SHOULD NOT assume that Key IDs are unique.  The
   section "Enhanced Key Formats" below describes how Key IDs are
   formed.

3.4.  {3.4} Text

   Unless otherwise specified, the character set for text is the UTF-8
   [RFC3629] encoding of Unicode [ISO10646].

3.5.  {3.5} Time Fields

   A time field is an unsigned four-octet number containing the number
   of seconds elapsed since midnight, 1 January 1970 UTC.

3.6.  {3.6} Keyrings

   A keyring is a collection of one or more keys in a file or database.
   Traditionally, a keyring is simply a sequential list of keys, but may
   be any suitable database.  It is beyond the scope of this standard to
   discuss the details of keyrings or other databases.

3.7.  {3.7} String-to-Key (S2K) Specifiers

   String-to-key (S2K) specifiers are used to convert passphrase strings
   into symmetric-key encryption/decryption keys.  They are used in two
   places, currently: to encrypt the secret part of private keys in the
   private keyring, and to convert passphrases to encryption keys for
   symmetrically encrypted messages.

3.7.1.  {3.7.1} String-to-Key (S2K) Specifier Types

   There are three types of S2K specifiers currently supported, and some
   reserved values:

                +-------------+---------------------------+
                |          ID | S2K Type                  |
                +-------------+---------------------------+
                |           0 | Simple S2K                |
                |           1 | Salted S2K                |
                |           2 | Reserved value            |
                |           3 | Iterated and Salted S2K   |
                |  100 to 110 | Private/Experimental S2K  |
                +-------------+---------------------------+

   These are described in the following Sections.



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3.7.1.1.  {3.7.1.1} Simple S2K

   This directly hashes the string to produce the key data.  See below
   for how this hashing is done.

      Octet 0:        0x00
      Octet 1:        hash algorithm

   Simple S2K hashes the passphrase to produce the session key.  The
   manner in which this is done depends on the size of the session key
   (which will depend on the cipher used) and the size of the hash
   algorithm's output.  If the hash size is greater than the session key
   size, the high-order (leftmost) octets of the hash are used as the
   key.

   If the hash size is less than the key size, multiple instances of the
   hash context are created -- enough to produce the required key data.
   These instances are preloaded with 0, 1, 2, ... octets of zeros (that
   is to say, the first instance has no preloading, the second gets
   preloaded with 1 octet of zero, the third is preloaded with two
   octets of zeros, and so forth).

   As the data is hashed, it is given independently to each hash
   context.  Since the contexts have been initialized differently, they
   will each produce different hash output.  Once the passphrase is
   hashed, the output data from the multiple hashes is concatenated,
   first hash leftmost, to produce the key data, with any excess octets
   on the right discarded.

3.7.1.2.  {3.7.1.2} Salted S2K

   This includes a "salt" value in the S2K specifier -- some arbitrary
   data -- that gets hashed along with the passphrase string, to help
   prevent dictionary attacks.

      Octet 0:        0x01
      Octet 1:        hash algorithm
      Octets 2-9:     8-octet salt value

   Salted S2K is exactly like Simple S2K, except that the input to the
   hash function(s) consists of the 8 octets of salt from the S2K
   specifier, followed by the passphrase.

3.7.1.3.  {3.7.1.3} Iterated and Salted S2K

   This includes both a salt and an octet count.  The salt is combined
   with the passphrase and the resulting value is hashed repeatedly.




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   This further increases the amount of work an attacker must do to try
   dictionary attacks.

      Octet  0:        0x03
      Octet  1:        hash algorithm
      Octets 2-9:      8-octet salt value
      Octet  10:       count, a one-octet, coded value

   The count is coded into a one-octet number using the following
   formula:

      #define EXPBIAS 6
          count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);

   The above formula is in C, where "Int32" is a type for a 32-bit
   integer, and the variable "c" is the coded count, Octet 10.

   Iterated-Salted S2K hashes the passphrase and salt data multiple
   times.  The total number of octets to be hashed is specified in the
   encoded count in the S2K specifier.  Note that the resulting count
   value is an octet count of how many octets will be hashed, not an
   iteration count.

   Initially, one or more hash contexts are set up as with the other S2K
   algorithms, depending on how many octets of key data are needed.
   Then the salt, followed by the passphrase data, is repeatedly hashed
   until the number of octets specified by the octet count has been
   hashed.  The one exception is that if the octet count is less than
   the size of the salt plus passphrase, the full salt plus passphrase
   will be hashed even though that is greater than the octet count.
   After the hashing is done, the data is unloaded from the hash
   context(s) as with the other S2K algorithms.

3.7.2.  {3.7.2} String-to-Key Usage

   Implementations SHOULD use salted or iterated-and-salted S2K
   specifiers, as simple S2K specifiers are more vulnerable to
   dictionary attacks.

3.7.2.1.  {3.7.2.1} Secret-Key Encryption

   An S2K specifier can be stored in the secret keyring to specify how
   to convert the passphrase to a key that unlocks the secret data.
   Older versions of PGP just stored a cipher algorithm octet preceding
   the secret data or a zero to indicate that the secret data was
   unencrypted.  The MD5 hash function was always used to convert the
   passphrase to a key for the specified cipher algorithm.




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   For compatibility, when an S2K specifier is used, the special value
   253, 254, or 255 is stored in the position where the hash algorithm
   octet would have been in the old data structure.  This is then
   followed immediately by a one-octet algorithm identifier, and then by
   the S2K specifier as encoded above.

   Therefore, preceding the secret data there will be one of these
   possibilities:

      0:                 secret data is unencrypted (no passphrase)
      255, 254, or 253:  followed by algorithm octet and S2K specifier
      Cipher alg:        use Simple S2K algorithm using MD5 hash

   This last possibility, the cipher algorithm number with an implicit
   use of MD5 and IDEA, is provided for backward compatibility; it MAY
   be understood, but SHOULD NOT be generated, and is deprecated.

   These are followed by an Initial Vector of the same length as the
   block size of the cipher for the decryption of the secret values, if
   they are encrypted, and then the secret-key values themselves.

3.7.2.2.  {3.7.2.2} Symmetric-Key Message Encryption

   OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet
   at the front of a message.  This is used to allow S2K specifiers to
   be used for the passphrase conversion or to create messages with a
   mix of symmetric-key ESKs and public-key ESKs.  This allows a message
   to be decrypted either with a passphrase or a public-key pair.

   PGP 2.X always used IDEA with Simple string-to-key conversion when
   encrypting a message with a symmetric algorithm.  This is deprecated,
   but MAY be used for backward-compatibility.

4.  {4} Packet Syntax

   This section describes the packets used by OpenPGP.

4.1.  {4.1} Overview

   An OpenPGP message is constructed from a number of records that are
   traditionally called packets.  A packet is a chunk of data that has a
   tag specifying its meaning.  An OpenPGP message, keyring,
   certificate, and so forth consists of a number of packets.  Some of
   those packets may contain other OpenPGP packets (for example, a
   compressed data packet, when uncompressed, contains OpenPGP packets).

   Each packet consists of a packet header, followed by the packet body.
   The packet header is of variable length.



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4.2.  {4.2} Packet Headers

   The first octet of the packet header is called the "Packet Tag".  It
   determines the format of the header and denotes the packet contents.
   The remainder of the packet header is the length of the packet.

   Note that the most significant bit is the leftmost bit, called bit 7.
   A mask for this bit is 0x80 in hexadecimal.

             +---------------+
        PTag |7 6 5 4 3 2 1 0|
             +---------------+
        Bit 7 -- Always one
        Bit 6 -- New packet format if set

   PGP 2.6.x only uses old format packets.  Thus, software that
   interoperates with those versions of PGP must only use old format
   packets.  If interoperability is not an issue, the new packet format
   is RECOMMENDED.  Note that old format packets have four bits of
   packet tags, and new format packets have six; some features cannot be
   used and still be backward-compatible.

   Also note that packets with a tag greater than or equal to 16 MUST
   use new format packets.  The old format packets can only express tags
   less than or equal to 15.

   Old format packets contain:

        Bits 5-2 -- packet tag
        Bits 1-0 -- length-type

   New format packets contain:

        Bits 5-0 -- packet tag

4.2.1.  {4.2.1} Old Format Packet Lengths

   The meaning of the length-type in old format packets is:

   0  The packet has a one-octet length.  The header is 2 octets long.

   1  The packet has a two-octet length.  The header is 3 octets long.

   2  The packet has a four-octet length.  The header is 5 octets long.

   3  The packet is of indeterminate length.  The header is 1 octet
      long, and the implementation must determine how long the packet
      is.  If the packet is in a file, this means that the packet



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      extends until the end of the file.  In general, an implementation
      SHOULD NOT use indeterminate-length packets except where the end
      of the data will be clear from the context, and even then it is
      better to use a definite length, or a new format header.  The new
      format headers described below have a mechanism for precisely
      encoding data of indeterminate length.

4.2.2.  {4.2.2} New Format Packet Lengths

   New format packets have four possible ways of encoding length:

   1.  A one-octet Body Length header encodes packet lengths of up to
       191 octets.

   2.  A two-octet Body Length header encodes packet lengths of 192 to
       8383 octets.

   3.  A five-octet Body Length header encodes packet lengths of up to
       4,294,967,295 (0xFFFFFFFF) octets in length.  (This actually
       encodes a four-octet scalar number.)

   4.  When the length of the packet body is not known in advance by the
       issuer, Partial Body Length headers encode a packet of
       indeterminate length, effectively making it a stream.

4.2.2.1.  {4.2.2.1} One-Octet Lengths

   A one-octet Body Length header encodes a length of 0 to 191 octets.
   This type of length header is recognized because the one octet value
   is less than 192.  The body length is equal to:

      bodyLen = 1st_octet;

4.2.2.2.  {4.2.2.2} Two-Octet Lengths

   A two-octet Body Length header encodes a length of 192 to 8383
   octets.  It is recognized because its first octet is in the range 192
   to 223.  The body length is equal to:

      bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

4.2.2.3.  {4.2.2.3} Five-Octet Lengths

   A five-octet Body Length header consists of a single octet holding
   the value 255, followed by a four-octet scalar.  The body length is
   equal to:





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      bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
                (4th_octet << 8)  | 5th_octet

   This basic set of one, two, and five-octet lengths is also used
   internally to some packets.

4.2.2.4.  {4.2.2.4} Partial Body Lengths

   A Partial Body Length header is one octet long and encodes the length
   of only part of the data packet.  This length is a power of 2, from 1
   to 1,073,741,824 (2 to the 30th power).  It is recognized by its one
   octet value that is greater than or equal to 224, and less than 255.
   The Partial Body Length is equal to:

      partialBodyLen = 1 << (1st_octet & 0x1F);

   Each Partial Body Length header is followed by a portion of the
   packet body data.  The Partial Body Length header specifies this
   portion's length.  Another length header (one octet, two-octet, five-
   octet, or partial) follows that portion.  The last length header in
   the packet MUST NOT be a Partial Body Length header.  Partial Body
   Length headers may only be used for the non-final parts of the
   packet.

   Note also that the last Body Length header can be a zero-length
   header.

   An implementation MAY use Partial Body Lengths for data packets, be
   they literal, compressed, or encrypted.  The first partial length
   MUST be at least 512 octets long.  Partial Body Lengths MUST NOT be
   used for any other packet types.

4.2.3.  {4.2.3} Packet Length Examples

   These examples show ways that new format packets might encode the
   packet lengths.

   A packet with length 100 may have its length encoded in one octet:
   0x64.  This is followed by 100 octets of data.

   A packet with length 1723 may have its length encoded in two octets:
   0xC5, 0xFB.  This header is followed by the 1723 octets of data.

   A packet with length 100000 may have its length encoded in five
   octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.

   It might also be encoded in the following octet stream: 0xEF, first
   32768 octets of data; 0xE1, next two octets of data; 0xE0, next one



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   octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693
   octets of data.  This is just one possible encoding, and many
   variations are possible on the size of the Partial Body Length
   headers, as long as a regular Body Length header encodes the last
   portion of the data.

   Please note that in all of these explanations, the total length of
   the packet is the length of the header(s) plus the length of the
   body.

4.3.  {4.3} Packet Tags

   The packet tag denotes what type of packet the body holds.  Note that
   old format headers can only have tags less than 16, whereas new
   format headers can have tags as great as 63.  The defined tags (in
   decimal) are as follows:

    +-----------+-----------------------------------------------------+
    |       Tag | Packet Type                                         |
    +-----------+-----------------------------------------------------+
    |         0 | Reserved - a packet tag MUST NOT have this value    |
    |         1 | Public-Key Encrypted Session Key Packet             |
    |         2 | Signature Packet                                    |
    |         3 | Symmetric-Key Encrypted Session Key Packet          |
    |         4 | One-Pass Signature Packet                           |
    |         5 | Secret-Key Packet                                   |
    |         6 | Public-Key Packet                                   |
    |         7 | Secret-Subkey Packet                                |
    |         8 | Compressed Data Packet                              |
    |         9 | Symmetrically Encrypted Data Packet                 |
    |        10 | Marker Packet                                       |
    |        11 | Literal Data Packet                                 |
    |        12 | Trust Packet                                        |
    |        13 | User ID Packet                                      |
    |        14 | Public-Subkey Packet                                |
    |        17 | User Attribute Packet                               |
    |        18 | Sym. Encrypted and Integrity Protected Data Packet  |
    |        19 | Modification Detection Code Packet                  |
    |        20 | AEAD Encrypted Data Packet                          |
    |  60 to 63 | Private or Experimental Values                      |
    +-----------+-----------------------------------------------------+

5.  {5} Packet Types








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5.1.  {5.1} Public-Key Encrypted Session Key Packets (Tag 1)

   A Public-Key Encrypted Session Key packet holds the session key used
   to encrypt a message.  Zero or more Public-Key Encrypted Session Key
   packets and/or Symmetric-Key Encrypted Session Key packets may
   precede a Symmetrically Encrypted Data Packet, which holds an
   encrypted message.  The message is encrypted with the session key,
   and the session key is itself encrypted and stored in the Encrypted
   Session Key packet(s).  The Symmetrically Encrypted Data Packet is
   preceded by one Public-Key Encrypted Session Key packet for each
   OpenPGP key to which the message is encrypted.  The recipient of the
   message finds a session key that is encrypted to their public key,
   decrypts the session key, and then uses the session key to decrypt
   the message.

   The body of this packet consists of:

   o  A one-octet number giving the version number of the packet type.
      The currently defined value for packet version is 3.

   o  An eight-octet number that gives the Key ID of the public key to
      which the session key is encrypted.  If the session key is
      encrypted to a subkey, then the Key ID of this subkey is used here
      instead of the Key ID of the primary key.

   o  A one-octet number giving the public-key algorithm used.

   o  A string of octets that is the encrypted session key.  This string
      takes up the remainder of the packet, and its contents are
      dependent on the public-key algorithm used.

      Algorithm Specific Fields for RSA encryption:

      *  Multiprecision integer (MPI) of RSA encrypted value m**e mod n.

      Algorithm Specific Fields for Elgamal encryption:

      *  MPI of Elgamal (Diffie-Hellman) value g**k mod p.

      *  MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.

      Algorithm-Specific Fields for ECDH encryption:

      *  MPI of an EC point representing an ephemeral public key.

      *  a one-octet size, followed by a symmetric key encoded using the
         method described in Section 13.5.




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   The value "m" in the above formulas is derived from the session key
   as follows.  First, the session key is prefixed with a one-octet
   algorithm identifier that specifies the symmetric encryption
   algorithm used to encrypt the following Symmetrically Encrypted Data
   Packet.  Then a two-octet checksum is appended, which is equal to the
   sum of the preceding session key octets, not including the algorithm
   identifier, modulo 65536.  This value is then encoded as described in
   PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
   form the "m" value used in the formulas above.  See Section 13.1 of
   this document for notes on OpenPGP's use of PKCS#1.

   Note that when an implementation forms several PKESKs with one
   session key, forming a message that can be decrypted by several keys,
   the implementation MUST make a new PKCS#1 encoding for each key.

   An implementation MAY accept or use a Key ID of zero as a "wild card"
   or "speculative" Key ID.  In this case, the receiving implementation
   would try all available private keys, checking for a valid decrypted
   session key.  This format helps reduce traffic analysis of messages.

5.2.  {5.2} Signature Packet (Tag 2)

   A Signature packet describes a binding between some public key and
   some data.  The most common signatures are a signature of a file or a
   block of text, and a signature that is a certification of a User ID.

   Two versions of Signature packets are defined.  Version 3 provides
   basic signature information, while version 4 provides an expandable
   format with subpackets that can specify more information about the
   signature.  PGP 2.6.x only accepts version 3 signatures.

   Implementations SHOULD accept V3 signatures.  Implementations SHOULD
   generate V4 signatures.

   Note that if an implementation is creating an encrypted and signed
   message that is encrypted to a V3 key, it is reasonable to create a
   V3 signature.

5.2.1.  {5.2.1} Signature Types

   There are a number of possible meanings for a signature, which are
   indicated in a signature type octet in any given signature.  Please
   note that the vagueness of these meanings is not a flaw, but a
   feature of the system.  Because OpenPGP places final authority for
   validity upon the receiver of a signature, it may be that one
   signer's casual act might be more rigorous than some other
   authority's positive act.  See Section 5.2.4, "Computing Signatures",




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   for detailed information on how to compute and verify signatures of
   each type.

   These meanings are as follows:

   0x00  Signature of a binary document.  This means the signer owns it,
      created it, or certifies that it has not been modified.

   0x01  Signature of a canonical text document.  This means the signer
      owns it, created it, or certifies that it has not been modified.
      The signature is calculated over the text data with its line
      endings converted to <CR><LF>.

   0x02  Standalone signature.  This signature is a signature of only
      its own subpacket contents.  It is calculated identically to a
      signature over a zero-length binary document.  Note that it
      doesn't make sense to have a V3 standalone signature.

   0x10  Generic certification of a User ID and Public-Key packet.  The
      issuer of this certification does not make any particular
      assertion as to how well the certifier has checked that the owner
      of the key is in fact the person described by the User ID.

   0x11  Persona certification of a User ID and Public-Key packet.  The
      issuer of this certification has not done any verification of the
      claim that the owner of this key is the User ID specified.

   0x12  Casual certification of a User ID and Public-Key packet.  The
      issuer of this certification has done some casual verification of
      the claim of identity.

   0x13  Positive certification of a User ID and Public-Key packet.  The
      issuer of this certification has done substantial verification of
      the claim of identity.

      Most OpenPGP implementations make their "key signatures" as 0x10
      certifications.  Some implementations can issue 0x11-0x13
      certifications, but few differentiate between the types.

   0x18  Subkey Binding Signature This signature is a statement by the
      top-level signing key that indicates that it owns the subkey.
      This signature is calculated directly on the primary key and
      subkey, and not on any User ID or other packets.  A signature that
      binds a signing subkey MUST have an Embedded Signature subpacket
      in this binding signature that contains a 0x19 signature made by
      the signing subkey on the primary key and subkey.





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   0x19  Primary Key Binding Signature This signature is a statement by
      a signing subkey, indicating that it is owned by the primary key
      and subkey.  This signature is calculated the same way as a 0x18
      signature: directly on the primary key and subkey, and not on any
      User ID or other packets.

   0x1F  Signature directly on a key This signature is calculated
      directly on a key.  It binds the information in the Signature
      subpackets to the key, and is appropriate to be used for
      subpackets that provide information about the key, such as the
      Revocation Key subpacket.  It is also appropriate for statements
      that non-self certifiers want to make about the key itself, rather
      than the binding between a key and a name.

   0x20  Key revocation signature The signature is calculated directly
      on the key being revoked.  A revoked key is not to be used.  Only
      revocation signatures by the key being revoked, or by an
      authorized revocation key, should be considered valid revocation
      signatures.

   0x28  Subkey revocation signature The signature is calculated
      directly on the subkey being revoked.  A revoked subkey is not to
      be used.  Only revocation signatures by the top-level signature
      key that is bound to this subkey, or by an authorized revocation
      key, should be considered valid revocation signatures.

   0x30  Certification revocation signature This signature revokes an
      earlier User ID certification signature (signature class 0x10
      through 0x13) or direct-key signature (0x1F).  It should be issued
      by the same key that issued the revoked signature or an authorized
      revocation key.  The signature is computed over the same data as
      the certificate that it revokes, and should have a later creation
      date than that certificate.

   0x40  Timestamp signature.  This signature is only meaningful for the
      timestamp contained in it.

   0x50  Third-Party Confirmation signature.  This signature is a
      signature over some other OpenPGP Signature packet(s).  It is
      analogous to a notary seal on the signed data.  A third-party
      signature SHOULD include Signature Target subpacket(s) to give
      easy identification.  Note that we really do mean SHOULD.  There
      are plausible uses for this (such as a blind party that only sees
      the signature, not the key or source document) that cannot include
      a target subpacket.






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5.2.2.  {5.2.2} Version 3 Signature Packet Format

   The body of a version 3 Signature Packet contains:

   o  One-octet version number (3).

   o  One-octet length of following hashed material.  MUST be 5.

   o  One-octet signature type.

   o  Four-octet creation time.

   o  Eight-octet Key ID of signer.

   o  One-octet public-key algorithm.

   o  One-octet hash algorithm.

   o  Two-octet field holding left 16 bits of signed hash value.

   o  One or more multiprecision integers comprising the signature.
      This portion is algorithm specific, as described below.

      The concatenation of the data to be signed, the signature type,
      and creation time from the Signature packet (5 additional octets)
      is hashed.  The resulting hash value is used in the signature
      algorithm.  The high 16 bits (first two octets) of the hash are
      included in the Signature packet to provide a quick test to reject
      some invalid signatures.

      Algorithm-Specific Fields for RSA signatures:

      *  Multiprecision integer (MPI) of RSA signature value m**d mod n.

      Algorithm-Specific Fields for DSA and ECDSA signatures:

      *  MPI of DSA or ECDSA value r.

      *  MPI of DSA or ECDSA value s.

   The signature calculation is based on a hash of the signed data, as
   described above.  The details of the calculation are different for
   DSA signatures than for RSA signatures.

   With RSA signatures, the hash value is encoded using PKCS#1 encoding
   type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447.  This
   requires inserting the hash value as an octet string into an ASN.1
   structure.  The object identifier for the type of hash being used is



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   included in the structure.  The hexadecimal representations for the
   currently defined hash algorithms are as follows:

    - MD5:        0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05

    - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01

    - SHA-1:      0x2B, 0x0E, 0x03, 0x02, 0x1A

    - SHA2-224:   0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04

    - SHA2-256:   0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01

    - SHA2-384:   0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02

    - SHA2-512:   0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03

   The ASN.1 Object Identifiers (OIDs) are as follows:

    - MD5:        1.2.840.113549.2.5

    - RIPEMD-160: 1.3.36.3.2.1

    - SHA-1:      1.3.14.3.2.26

    - SHA2-224:   2.16.840.1.101.3.4.2.4

    - SHA2-256:   2.16.840.1.101.3.4.2.1

    - SHA2-384:   2.16.840.1.101.3.4.2.2

    - SHA2-512:   2.16.840.1.101.3.4.2.3

   The full hash prefixes for these are as follows:

















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    - MD5:        0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
                  0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
                  0x04, 0x10

    - RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
                  0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14

    - SHA-1:      0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
                  0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14

    - SHA2-224:   0x30, 0x2D, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                  0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,
                  0x00, 0x04, 0x1C

    - SHA2-256:   0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                  0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
                  0x00, 0x04, 0x20

    - SHA2-384:   0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                  0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
                  0x00, 0x04, 0x30

    - SHA2-512:   0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                  0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
                  0x00, 0x04, 0x40

   DSA signatures MUST use hashes that are equal in size to the number
   of bits of q, the group generated by the DSA key's generator value.

   If the output size of the chosen hash is larger than the number of
   bits of q, the hash result is truncated to fit by taking the number
   of leftmost bits equal to the number of bits of q.  This (possibly
   truncated) hash function result is treated as a number and used
   directly in the DSA signature algorithm.

5.2.3.  {5.2.3} Version 4 Signature Packet Format

   The body of a version 4 Signature packet contains:

   o  One-octet version number (4).

   o  One-octet signature type.

   o  One-octet public-key algorithm.

   o  One-octet hash algorithm.





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   o  Two-octet scalar octet count for following hashed subpacket data.
      Note that this is the length in octets of all of the hashed
      subpackets; a pointer incremented by this number will skip over
      the hashed subpackets.

   o  Hashed subpacket data set (zero or more subpackets).

   o  Two-octet scalar octet count for the following unhashed subpacket
      data.  Note that this is the length in octets of all of the
      unhashed subpackets; a pointer incremented by this number will
      skip over the unhashed subpackets.

   o  Unhashed subpacket data set (zero or more subpackets).

   o  Two-octet field holding the left 16 bits of the signed hash value.

   o  One or more multiprecision integers comprising the signature.
      This portion is algorithm specific:

      Algorithm-Specific Fields for RSA signatures:

      *  Multiprecision integer (MPI) of RSA signature value m**d mod n.

      Algorithm-Specific Fields for DSA or ECDSA signatures:

      *  MPI of DSA or ECDSA value r.

      *  MPI of DSA or ECDSA value s.

      Algorithm-Specific Fields for EdDSA signatures:

      *  MPI of EdDSA compressed value r.

      *  MPI of EdDSA compressed value s.

   The compressed version of R and S for use with EdDSA is described in
   [I-D.irtf-cfrg-eddsa].  The version 3 signature format MUST NOT be
   used with EdDSA.

   The concatenation of the data being signed and the signature data
   from the version number through the hashed subpacket data (inclusive)
   is hashed.  The resulting hash value is what is signed.  The left 16
   bits of the hash are included in the Signature packet to provide a
   quick test to reject some invalid signatures.

   There are two fields consisting of Signature subpackets.  The first
   field is hashed with the rest of the signature data, while the second
   is unhashed.  The second set of subpackets is not cryptographically



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   protected by the signature and should include only advisory
   information.

   The algorithms for converting the hash function result to a signature
   are described in a section below.

5.2.3.1.  {5.2.3.1} Signature Subpacket Specification

   A subpacket data set consists of zero or more Signature subpackets.
   In Signature packets, the subpacket data set is preceded by a two-
   octet scalar count of the length in octets of all the subpackets.  A
   pointer incremented by this number will skip over the subpacket data
   set.

   Each subpacket consists of a subpacket header and a body.  The header
   consists of:

   o  the subpacket length (1, 2, or 5 octets),

   o  the subpacket type (1 octet),

   and is followed by the subpacket-specific data.

   The length includes the type octet but not this length.  Its format
   is similar to the "new" format packet header lengths, but cannot have
   Partial Body Lengths.  That is:

      if the 1st octet <  192, then
          lengthOfLength = 1
          subpacketLen = 1st_octet

      if the 1st octet >= 192 and < 255, then
          lengthOfLength = 2
          subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

      if the 1st octet = 255, then
          lengthOfLength = 5
          subpacket length = [four-octet scalar starting at 2nd_octet]

   The value of the subpacket type octet may be:











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         +-------------+-----------------------------------------+
         |        Type | Description                             |
         +-------------+-----------------------------------------+
         |           0 | Reserved                                |
         |           1 | Reserved                                |
         |           2 | Signature Creation Time                 |
         |           3 | Signature Expiration Time               |
         |           4 | Exportable Certification                |
         |           5 | Trust Signature                         |
         |           6 | Regular Expression                      |
         |           7 | Revocable                               |
         |           8 | Reserved                                |
         |           9 | Key Expiration Time                     |
         |          10 | Placeholder for backward compatibility  |
         |          11 | Preferred Symmetric Algorithms          |
         |          12 | Revocation Key                          |
         |    13 to 15 | Reserved                                |
         |          16 | Issuer                                  |
         |    17 to 19 | Reserved                                |
         |          20 | Notation Data                           |
         |          21 | Preferred Hash Algorithms               |
         |          22 | Preferred Compression Algorithms        |
         |          23 | Key Server Preferences                  |
         |          24 | Preferred Key Server                    |
         |          25 | Primary User ID                         |
         |          26 | Policy URI                              |
         |          27 | Key Flags                               |
         |          28 | Signer's User ID                        |
         |          29 | Reason for Revocation                   |
         |          30 | Features                                |
         |          31 | Signature Target                        |
         |          32 | Embedded Signature                      |
         |          33 | Issuer Fingerprint                      |
         |          34 | Preferred AEAD Algorithms               |
         |  100 to 110 | Private or experimental                 |
         +-------------+-----------------------------------------+

   An implementation SHOULD ignore any subpacket of a type that it does
   not recognize.

   Bit 7 of the subpacket type is the "critical" bit.  If set, it
   denotes that the subpacket is one that is critical for the evaluator
   of the signature to recognize.  If a subpacket is encountered that is
   marked critical but is unknown to the evaluating software, the
   evaluator SHOULD consider the signature to be in error.

   An evaluator may "recognize" a subpacket, but not implement it.  The
   purpose of the critical bit is to allow the signer to tell an



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   evaluator that it would prefer a new, unknown feature to generate an
   error than be ignored.

   Implementations SHOULD implement the four preferred algorithm
   subpackets (11, 21, 22, and 34), as well as the "Reason for
   Revocation" subpacket.  Note, however, that if an implementation
   chooses not to implement some of the preferences, it is required to
   behave in a polite manner to respect the wishes of those users who do
   implement these preferences.

5.2.3.2.  {5.2.3.2} Signature Subpacket Types

   A number of subpackets are currently defined.  Some subpackets apply
   to the signature itself and some are attributes of the key.
   Subpackets that are found on a self-signature are placed on a
   certification made by the key itself.  Note that a key may have more
   than one User ID, and thus may have more than one self-signature, and
   differing subpackets.

   A subpacket may be found either in the hashed or unhashed subpacket
   sections of a signature.  If a subpacket is not hashed, then the
   information in it cannot be considered definitive because it is not
   part of the signature proper.

5.2.3.3.  {5.2.3.3} Notes on Self-Signatures

   A self-signature is a binding signature made by the key to which the
   signature refers.  There are three types of self-signatures, the
   certification signatures (types 0x10-0x13), the direct-key signature
   (type 0x1F), and the subkey binding signature (type 0x18).  For
   certification self-signatures, each User ID may have a self-
   signature, and thus different subpackets in those self-signatures.
   For subkey binding signatures, each subkey in fact has a self-
   signature.  Subpackets that appear in a certification self-signature
   apply to the user name, and subpackets that appear in the subkey
   self-signature apply to the subkey.  Lastly, subpackets on the
   direct-key signature apply to the entire key.

   Implementing software should interpret a self-signature's preference
   subpackets as narrowly as possible.  For example, suppose a key has
   two user names, Alice and Bob. Suppose that Alice prefers the
   symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES.  If the
   software locates this key via Alice's name, then the preferred
   algorithm is CAST5; if software locates the key via Bob's name, then
   the preferred algorithm is IDEA.  If the key is located by Key ID,
   the algorithm of the primary User ID of the key provides the
   preferred symmetric algorithm.




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   Revoking a self-signature or allowing it to expire has a semantic
   meaning that varies with the signature type.  Revoking the self-
   signature on a User ID effectively retires that user name.  The self-
   signature is a statement, "My name X is tied to my signing key K" and
   is corroborated by other users' certifications.  If another user
   revokes their certification, they are effectively saying that they no
   longer believe that name and that key are tied together.  Similarly,
   if the users themselves revoke their self-signature, then the users
   no longer go by that name, no longer have that email address, etc.
   Revoking a binding signature effectively retires that subkey.
   Revoking a direct-key signature cancels that signature.  Please see
   the "Reason for Revocation" subpacket (Section 5.2.3.23) for more
   relevant detail.

   Since a self-signature contains important information about the key's
   use, an implementation SHOULD allow the user to rewrite the self-
   signature, and important information in it, such as preferences and
   key expiration.

   It is good practice to verify that a self-signature imported into an
   implementation doesn't advertise features that the implementation
   doesn't support, rewriting the signature as appropriate.

   An implementation that encounters multiple self-signatures on the
   same object may resolve the ambiguity in any way it sees fit, but it
   is RECOMMENDED that priority be given to the most recent self-
   signature.

5.2.3.4.  {5.2.3.4} Signature Creation Time

   (4-octet time field)

   The time the signature was made.

   MUST be present in the hashed area.

5.2.3.5.  {5.2.3.5} Issuer

   (8-octet Key ID)

   The OpenPGP Key ID of the key issuing the signature.  If the version
   of that key is greater than 4, this subpacket MUST NOT be included in
   the signature.








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5.2.3.6.  {5.2.3.6} Key Expiration Time

   (4-octet time field)

   The validity period of the key.  This is the number of seconds after
   the key creation time that the key expires.  If this is not present
   or has a value of zero, the key never expires.  This is found only on
   a self-signature.

5.2.3.7.  {5.2.3.7} Preferred Symmetric Algorithms

   (array of one-octet values)

   Symmetric algorithm numbers that indicate which algorithms the key
   holder prefers to use.  The subpacket body is an ordered list of
   octets with the most preferred listed first.  It is assumed that only
   algorithms listed are supported by the recipient's software.
   Algorithm numbers are in Section 9.  This is only found on a self-
   signature.

5.2.3.8.  Preferred AEAD Algorithms

   (array of one-octet values)

   AEAD algorithm numbers that indicate which AEAD algorithms the key
   holder prefers to use.  The subpacket body is an ordered list of
   octets with the most preferred listed first.  It is assumed that only
   algorithms listed are supported by the recipient's software.
   Algorithm numbers are in Section [1].  This is only found on a self-
   signature.  Note that support for the AEAD Encrypted Data packet in
   the general is indicated by a Feature Flag.

5.2.3.9.  {5.2.3.8} Preferred Hash Algorithms

   (array of one-octet values)

   Message digest algorithm numbers that indicate which algorithms the
   key holder prefers to receive.  Like the preferred symmetric
   algorithms, the list is ordered.  Algorithm numbers are in Section 9.
   This is only found on a self-signature.

5.2.3.10.  {5.2.3.9} Preferred Compression Algorithms

   (array of one-octet values)

   Compression algorithm numbers that indicate which algorithms the key
   holder prefers to use.  Like the preferred symmetric algorithms, the
   list is ordered.  Algorithm numbers are in Section 9.  If this



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   subpacket is not included, ZIP is preferred.  A zero denotes that
   uncompressed data is preferred; the key holder's software might have
   no compression software in that implementation.  This is only found
   on a self-signature.

5.2.3.11.  {5.2.3.10} Signature Expiration Time

   (4-octet time field)

   The validity period of the signature.  This is the number of seconds
   after the signature creation time that the signature expires.  If
   this is not present or has a value of zero, it never expires.

5.2.3.12.  {5.2.3.11} Exportable Certification

   (1 octet of exportability, 0 for not, 1 for exportable)

   This subpacket denotes whether a certification signature is
   "exportable", to be used by other users than the signature's issuer.
   The packet body contains a Boolean flag indicating whether the
   signature is exportable.  If this packet is not present, the
   certification is exportable; it is equivalent to a flag containing a
   1.

   Non-exportable, or "local", certifications are signatures made by a
   user to mark a key as valid within that user's implementation only.

   Thus, when an implementation prepares a user's copy of a key for
   transport to another user (this is the process of "exporting" the
   key), any local certification signatures are deleted from the key.

   The receiver of a transported key "imports" it, and likewise trims
   any local certifications.  In normal operation, there won't be any,
   assuming the import is performed on an exported key.  However, there
   are instances where this can reasonably happen.  For example, if an
   implementation allows keys to be imported from a key database in
   addition to an exported key, then this situation can arise.

   Some implementations do not represent the interest of a single user
   (for example, a key server).  Such implementations always trim local
   certifications from any key they handle.

5.2.3.13.  {5.2.3.12} Revocable

   (1 octet of revocability, 0 for not, 1 for revocable)

   Signature's revocability status.  The packet body contains a Boolean
   flag indicating whether the signature is revocable.  Signatures that



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   are not revocable have any later revocation signatures ignored.  They
   represent a commitment by the signer that he cannot revoke his
   signature for the life of his key.  If this packet is not present,
   the signature is revocable.

5.2.3.14.  {5.2.3.13} Trust Signature

   (1 octet "level" (depth), 1 octet of trust amount)

   Signer asserts that the key is not only valid but also trustworthy at
   the specified level.  Level 0 has the same meaning as an ordinary
   validity signature.  Level 1 means that the signed key is asserted to
   be a valid trusted introducer, with the 2nd octet of the body
   specifying the degree of trust.  Level 2 means that the signed key is
   asserted to be trusted to issue level 1 trust signatures, i.e., that
   it is a "meta introducer".  Generally, a level n trust signature
   asserts that a key is trusted to issue level n-1 trust signatures.
   The trust amount is in a range from 0-255, interpreted such that
   values less than 120 indicate partial trust and values of 120 or
   greater indicate complete trust.  Implementations SHOULD emit values
   of 60 for partial trust and 120 for complete trust.

5.2.3.15.  {5.2.3.14} Regular Expression

   (null-terminated regular expression)

   Used in conjunction with trust Signature packets (of level > 0) to
   limit the scope of trust that is extended.  Only signatures by the
   target key on User IDs that match the regular expression in the body
   of this packet have trust extended by the trust Signature subpacket.
   The regular expression uses the same syntax as the Henry Spencer's
   "almost public domain" regular expression [REGEX] package.  A
   description of the syntax is found in Section 8 below.

5.2.3.16.  {5.2.3.15} Revocation Key

   (1 octet of class, 1 octet of public-key algorithm ID, 20 or 32
   octets of fingerprint)

   V4 keys use the full 20 octet fingerprint; V5 keys use the full 32
   octet fingerprint

   Authorizes the specified key to issue revocation signatures for this
   key.  Class octet must have bit 0x80 set.  If the bit 0x40 is set,
   then this means that the revocation information is sensitive.  Other
   bits are for future expansion to other kinds of authorizations.  This
   is found on a self-signature.




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   If the "sensitive" flag is set, the keyholder feels this subpacket
   contains private trust information that describes a real-world
   sensitive relationship.  If this flag is set, implementations SHOULD
   NOT export this signature to other users except in cases where the
   data needs to be available: when the signature is being sent to the
   designated revoker, or when it is accompanied by a revocation
   signature from that revoker.  Note that it may be appropriate to
   isolate this subpacket within a separate signature so that it is not
   combined with other subpackets that need to be exported.

5.2.3.17.  {5.2.3.16} Notation Data

      (4 octets of flags, 2 octets of name length (M),
                          2 octets of value length (N),
                          M octets of name data,
                          N octets of value data)

   This subpacket describes a "notation" on the signature that the
   issuer wishes to make.  The notation has a name and a value, each of
   which are strings of octets.  There may be more than one notation in
   a signature.  Notations can be used for any extension the issuer of
   the signature cares to make.  The "flags" field holds four octets of
   flags.

   All undefined flags MUST be zero.  Defined flags are as follows:

      First octet: 0x80 = human-readable.  This note value is text.
      Other octets: none.

   Notation names are arbitrary strings encoded in UTF-8.  They reside
   in two namespaces: The IETF namespace and the user namespace.

   The IETF namespace is registered with IANA.  These names MUST NOT
   contain the "@" character (0x40).  This is a tag for the user
   namespace.

   Names in the user namespace consist of a UTF-8 string tag followed by
   "@" followed by a DNS domain name.  Note that the tag MUST NOT
   contain an "@" character.  For example, the "sample" tag used by
   Example Corporation could be "sample@example.com".

   Names in a user space are owned and controlled by the owners of that
   domain.  Obviously, it's bad form to create a new name in a DNS space
   that you don't own.

   Since the user namespace is in the form of an email address,
   implementers MAY wish to arrange for that address to reach a person
   who can be consulted about the use of the named tag.  Note that due



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   to UTF-8 encoding, not all valid user space name tags are valid email
   addresses.

   If there is a critical notation, the criticality applies to that
   specific notation and not to notations in general.

   The following subsections define a set of standard notations.

5.2.3.17.1.  The 'manu' Notation

   The "manu" notation is a string that declares the device
   manufacturer's name.  The certifier key is asserting this string
   (which may or may not be related to the User ID of the certifier's
   key).

5.2.3.17.2.  The 'make' Notation

   This notation defines the product make.  It is a free form string.

5.2.3.17.3.  The 'model' Notation

   This notation defines the product model name/number.  It is a free
   form string.

5.2.3.17.4.  The 'prodid' Notation

   This notation contains the product identifier.  It is a free form
   string.

5.2.3.17.5.  The 'pvers' Notation

   This notation defines the product version number (which could be a
   release number, year, or some other identifier to differentiate
   different versions of the same make/model).  It is a free form
   string.

5.2.3.17.6.  The 'lot' Notation

   This notation defines the product lot number (which is an indicator
   of the batch of product).  It is a free form string.

5.2.3.17.7.  The 'qty' Notation

   This notation defines the quantity of items in this package.  It is a
   decimal integer representation with no punctuation, e.g. "10",
   "1000", "10000", etc.





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5.2.3.17.8.  The 'loc' and 'dest' Notations

   The "loc" and 'dest' notations declare a GeoLocation as defined by
   RFC 5870 [RFC5870] but without the leading "geo:" header.  For
   example, if you had a GeoLocation URI of "geo:13.4125,103.8667" you
   would encode that in these notations as "13.4125,103.8667".

   The 'loc' notation is meant to encode the geo location where the
   signature was made.  The 'dest' notation is meant to encode the geo
   location where the device is "destined" (i.e., a "destination" for
   the device).

5.2.3.17.9.  The 'hash' Notation

   A 'hash' notation is a means to include external data in the contents
   of a signature without including the data itself.  This is done by
   hashing the external data separately and then including the data's
   name and hash in the signature via this notation.  This is useful,
   for example, to have an external "manifest," "image," or other data
   that might not be vital to the signature itself but still needs to be
   protected and authenticated without requiring a second signature.

   The 'hash' notation has the following structure: * A single byte
   specifying the length of the name of the hashed data * A UTF-8 string
   of the name of the hashed data * A single byte specifying the hash
   algorithm (see section 9.4) * The binary hash output of the hashed
   data using the specified algorithm.  (The length of this data is
   implicit based on the algorithm specified).

   Due to its nature a 'hash' notation is not human readable and MUST
   NOT be marked as such when used.

5.2.3.18.  {5.2.3.17} Key Server Preferences

   (N octets of flags)

   This is a list of one-bit flags that indicate preferences that the
   key holder has about how the key is handled on a key server.  All
   undefined flags MUST be zero.

   First octet: 0x80 = No-modify the key holder requests that this key
   only be modified or updated by the key holder or an administrator of
   the key server.

   This is found only on a self-signature.






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5.2.3.19.  {5.2.3.18} Preferred Key Server

   (String)

   This is a URI of a key server that the key holder prefers be used for
   updates.  Note that keys with multiple User IDs can have a preferred
   key server for each User ID.  Note also that since this is a URI, the
   key server can actually be a copy of the key retrieved by ftp, http,
   finger, etc.

5.2.3.20.  {5.2.3.19} Primary User ID

   (1 octet, Boolean)

   This is a flag in a User ID's self-signature that states whether this
   User ID is the main User ID for this key.  It is reasonable for an
   implementation to resolve ambiguities in preferences, etc. by
   referring to the primary User ID.  If this flag is absent, its value
   is zero.  If more than one User ID in a key is marked as primary, the
   implementation may resolve the ambiguity in any way it sees fit, but
   it is RECOMMENDED that priority be given to the User ID with the most
   recent self-signature.

   When appearing on a self-signature on a User ID packet, this
   subpacket applies only to User ID packets.  When appearing on a self-
   signature on a User Attribute packet, this subpacket applies only to
   User Attribute packets.  That is to say, there are two different and
   independent "primaries" -- one for User IDs, and one for User
   Attributes.

5.2.3.21.  {5.2.3.20} Policy URI

   (String)

   This subpacket contains a URI of a document that describes the policy
   under which the signature was issued.

5.2.3.22.  {5.2.3.21} Key Flags

   (N octets of flags)

   This subpacket contains a list of binary flags that hold information
   about a key.  It is a string of octets, and an implementation MUST
   NOT assume a fixed size.  This is so it can grow over time.  If a
   list is shorter than an implementation expects, the unstated flags
   are considered to be zero.  The defined flags are as follows:

   0x01  This key may be used to certify other keys.



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   0x02  This key may be used to sign data.

   0x04  This key may be used to encrypt communications.

   0x08  This key may be used to encrypt storage.

   0x10  The private component of this key may have been split by a
      secret-sharing mechanism.

   0x20  This key may be used for authentication.

   0x80  The private component of this key may be in the possession of
      more than one person.

   Usage notes:

   The flags in this packet may appear in self-signatures or in
   certification signatures.  They mean different things depending on
   who is making the statement --- for example, a certification
   signature that has the "sign data" flag is stating that the
   certification is for that use.  On the other hand, the
   "communications encryption" flag in a self-signature is stating a
   preference that a given key be used for communications.  Note
   however, that it is a thorny issue to determine what is
   "communications" and what is "storage".  This decision is left wholly
   up to the implementation; the authors of this document do not claim
   any special wisdom on the issue and realize that accepted opinion may
   change.

   The "split key" (0x10) and "group key" (0x80) flags are placed on a
   self-signature only; they are meaningless on a certification
   signature.  They SHOULD be placed only on a direct-key signature
   (type 0x1F) or a subkey signature (type 0x18), one that refers to the
   key the flag applies to.

5.2.3.23.  {5.2.3.22} Signer's User ID

   (String)

   This subpacket allows a keyholder to state which User ID is
   responsible for the signing.  Many keyholders use a single key for
   different purposes, such as business communications as well as
   personal communications.  This subpacket allows such a keyholder to
   state which of their roles is making a signature.

   This subpacket is not appropriate to use to refer to a User Attribute
   packet.




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5.2.3.24.  {5.2.3.23} Reason for Revocation

   (1 octet of revocation code, N octets of reason string)

   This subpacket is used only in key revocation and certification
   revocation signatures.  It describes the reason why the key or
   certificate was revoked.

   The first octet contains a machine-readable code that denotes the
   reason for the revocation:

   +----------+--------------------------------------------------------+
   |     Code | Reason                                                 |
   +----------+--------------------------------------------------------+
   |        0 | No reason specified (key revocations or cert           |
   |          | revocations)                                           |
   |        1 | Key is superseded (key revocations)                    |
   |        2 | Key material has been compromised (key revocations)    |
   |        3 | Key is retired and no longer used (key revocations)    |
   |       32 | User ID information is no longer valid (cert           |
   |          | revocations)                                           |
   |  100-110 | Private Use                                            |
   +----------+--------------------------------------------------------+

   Following the revocation code is a string of octets that gives
   information about the Reason for Revocation in human-readable form
   (UTF-8).  The string may be null, that is, of zero length.  The
   length of the subpacket is the length of the reason string plus one.
   An implementation SHOULD implement this subpacket, include it in all
   revocation signatures, and interpret revocations appropriately.
   There are important semantic differences between the reasons, and
   there are thus important reasons for revoking signatures.

   If a key has been revoked because of a compromise, all signatures
   created by that key are suspect.  However, if it was merely
   superseded or retired, old signatures are still valid.  If the
   revoked signature is the self-signature for certifying a User ID, a
   revocation denotes that that user name is no longer in use.  Such a
   revocation SHOULD include a 0x20 code.

   Note that any signature may be revoked, including a certification on
   some other person's key.  There are many good reasons for revoking a
   certification signature, such as the case where the keyholder leaves
   the employ of a business with an email address.  A revoked
   certification is no longer a part of validity calculations.






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5.2.3.25.  {5.2.3.24} Features

   (N octets of flags)

   The Features subpacket denotes which advanced OpenPGP features a
   user's implementation supports.  This is so that as features are
   added to OpenPGP that cannot be backwards-compatible, a user can
   state that they can use that feature.  The flags are single bits that
   indicate that a given feature is supported.

   This subpacket is similar to a preferences subpacket, and only
   appears in a self-signature.

   An implementation SHOULD NOT use a feature listed when sending to a
   user who does not state that they can use it.

   Defined features are as follows:

   First octet:

   0x01 - Modification Detection (packets 18 and 19)

   0x02 - AEAD Encrypted Data Packet (packet 20) and version 5
          Symmetric-Key Encrypted Session Key Packets (packet 3)

   0x04 - Version 5 Public-Key Packet format and corresponding new
          fingerprint format

   If an implementation implements any of the defined features, it
   SHOULD implement the Features subpacket, too.

   An implementation may freely infer features from other suitable
   implementation-dependent mechanisms.

5.2.3.26.  {5.2.3.25} Signature Target

   (1 octet public-key algorithm, 1 octet hash algorithm, N octets hash)

   This subpacket identifies a specific target signature to which a
   signature refers.  For revocation signatures, this subpacket provides
   explicit designation of which signature is being revoked.  For a
   third-party or timestamp signature, this designates what signature is
   signed.  All arguments are an identifier of that target signature.

   The N octets of hash data MUST be the size of the hash of the
   signature.  For example, a target signature with a SHA-1 hash MUST
   have 20 octets of hash data.




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5.2.3.27.  {5.2.3.26} Embedded Signature

   (1 signature packet body)

   This subpacket contains a complete Signature packet body as specified
   in Section 5.2 above.  It is useful when one signature needs to refer
   to, or be incorporated in, another signature.

5.2.3.28.  Issuer Fingerprint

   (1 octet key version number, N octets of fingerprint)

   The OpenPGP Key fingerprint of the key issuing the signature.  This
   subpacket SHOULD be included in all signatures.  If the version of
   the issuing key is 4 and an Issuer subpacket is also included in the
   signature, the key ID of the Issuer subpacket MUST match the low 64
   bits of the fingerprint.

   Note that the length N of the fingerprint for a version 4 key is 20
   octets; for a version 5 key N is 32.

5.2.4.  {5.2.4} Computing Signatures

   All signatures are formed by producing a hash over the signature
   data, and then using the resulting hash in the signature algorithm.

   For binary document signatures (type 0x00), the document data is
   hashed directly.  For text document signatures (type 0x01), the
   document is canonicalized by converting line endings to <CR><LF>, and
   the resulting data is hashed.

   When a signature is made over a key, the hash data starts with the
   octet 0x99, followed by a two-octet length of the key, and then body
   of the key packet.  (Note that this is an old-style packet header for
   a key packet with two-octet length.)  A subkey binding signature
   (type 0x18) or primary key binding signature (type 0x19) then hashes
   the subkey using the same format as the main key (also using 0x99 as
   the first octet).  Primary key revocation signatures (type 0x20) hash
   only the key being revoked.  Subkey revocation signature (type 0x28)
   hash first the primary key and then the subkey being revoked.

   A certification signature (type 0x10 through 0x13) hashes the User ID
   being bound to the key into the hash context after the above data.  A
   V3 certification hashes the contents of the User ID or attribute
   packet packet, without any header.  A V4 or V5 certification hashes
   the constant 0xB4 for User ID certifications or the constant 0xD1 for
   User Attribute certifications, followed by a four-octet number giving




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   the length of the User ID or User Attribute data, and then the User
   ID or User Attribute data.

   When a signature is made over a Signature packet (type 0x50), the
   hash data starts with the octet 0x88, followed by the four-octet
   length of the signature, and then the body of the Signature packet.
   (Note that this is an old-style packet header for a Signature packet
   with the length-of-length set to zero.)  The unhashed subpacket data
   of the Signature packet being hashed is not included in the hash, and
   the unhashed subpacket data length value is set to zero.

   Once the data body is hashed, then a trailer is hashed.  A V3
   signature hashes five octets of the packet body, starting from the
   signature type field.  This data is the signature type, followed by
   the four-octet signature time.  A V4 or V5 signature hashes the
   packet body starting from its first field, the version number,
   through the end of the hashed subpacket data.  Thus, the fields
   hashed are the signature version, the signature type, the public-key
   algorithm, the hash algorithm, the hashed subpacket length, and the
   hashed subpacket body.

   V4 signatures also hash in a final trailer of six octets: the version
   of the Signature packet, i.e., 0x04; 0xFF; and a four-octet, big-
   endian number that is the length of the hashed data from the
   Signature packet (note that this number does not include these final
   six octets). {FIXME: truncated or wrap that number on overflow}

   V5 signatures instead hash in a ten-octet trailer: the version of the
   Signature packet, i.e., 0x05; 0xFF; and an eight-octet, big-endian
   number that is the length of the hashed data from the Signature
   packet (note that this number does not include these final ten
   octets).

   After all this has been hashed in a single hash context, the
   resulting hash field is used in the signature algorithm and placed at
   the end of the Signature packet.

5.2.4.1.  {5.2.4.1} Subpacket Hints

   It is certainly possible for a signature to contain conflicting
   information in subpackets.  For example, a signature may contain
   multiple copies of a preference or multiple expiration times.  In
   most cases, an implementation SHOULD use the last subpacket in the
   signature, but MAY use any conflict resolution scheme that makes more
   sense.  Please note that we are intentionally leaving conflict
   resolution to the implementer; most conflicts are simply syntax
   errors, and the wishy-washy language here allows a receiver to be




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   generous in what they accept, while putting pressure on a creator to
   be stingy in what they generate.

   Some apparent conflicts may actually make sense -- for example,
   suppose a keyholder has a V3 key and a V4 key that share the same RSA
   key material.  Either of these keys can verify a signature created by
   the other, and it may be reasonable for a signature to contain an
   issuer subpacket for each key, as a way of explicitly tying those
   keys to the signature.

5.3.  {5.3} Symmetric-Key Encrypted Session Key Packets (Tag 3)

   The Symmetric-Key Encrypted Session Key packet holds the symmetric-
   key encryption of a session key used to encrypt a message.  Zero or
   more Public-Key Encrypted Session Key packets and/or Symmetric-Key
   Encrypted Session Key packets may precede a Symmetrically Encrypted
   Data packet that holds an encrypted message.  The message is
   encrypted with a session key, and the session key is itself encrypted
   and stored in the Encrypted Session Key packet or the Symmetric-Key
   Encrypted Session Key packet.

   If the Symmetrically Encrypted Data packet is preceded by one or more
   Symmetric-Key Encrypted Session Key packets, each specifies a
   passphrase that may be used to decrypt the message.  This allows a
   message to be encrypted to a number of public keys, and also to one
   or more passphrases.  This packet type is new and is not generated by
   PGP 2.x or PGP 5.0.

   A version 4 Symmetric-Key Encrypted Session Key packet consists of:

   o  A one-octet version number with value 4.

   o  A one-octet number describing the symmetric algorithm used.

   o  A string-to-key (S2K) specifier, length as defined above.

   o  Optionally, the encrypted session key itself, which is decrypted
      with the string-to-key object.

   If the encrypted session key is not present (which can be detected on
   the basis of packet length and S2K specifier size), then the S2K
   algorithm applied to the passphrase produces the session key for
   decrypting the file, using the symmetric cipher algorithm from the
   Symmetric-Key Encrypted Session Key packet.

   If the encrypted session key is present, the result of applying the
   S2K algorithm to the passphrase is used to decrypt just that
   encrypted session key field, using CFB mode with an IV of all zeros.



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   The decryption result consists of a one-octet algorithm identifier
   that specifies the symmetric-key encryption algorithm used to encrypt
   the following Symmetrically Encrypted Data packet, followed by the
   session key octets themselves.

   Note: because an all-zero IV is used for this decryption, the S2K
   specifier MUST use a salt value, either a Salted S2K or an Iterated-
   Salted S2K.  The salt value will ensure that the decryption key is
   not repeated even if the passphrase is reused.

   A version 5 Symmetric-Key Encrypted Session Key packet consists of:

   o  A one-octet version number with value 5.

   o  A one-octet cipher algorithm.

   o  A one-octet AEAD algorithm.

   o  A string-to-key (S2K) specifier, length as defined above.

   o  A starting initialization vector of size specified by the AEAD
      algorithm.

   o  The encrypted session key itself, which is decrypted with the
      string-to-key object using the given cipher and AEAD mode.

   o  A final, summary authentication tag for the AEAD mode.

   The encrypted session key is encrypted exactly as an AEAD Encrypted
   Data packet with a chunk size octet of 10 would be.  This implicit
   chunk size octet is included in the normal calculations of additional
   data.

5.4.  {5.4} One-Pass Signature Packets (Tag 4)

   The One-Pass Signature packet precedes the signed data and contains
   enough information to allow the receiver to begin calculating any
   hashes needed to verify the signature.  It allows the Signature
   packet to be placed at the end of the message, so that the signer can
   compute the entire signed message in one pass.

   A One-Pass Signature does not interoperate with PGP 2.6.x or earlier.

   The body of this packet consists of:

   o  A one-octet version number.  The current version is 3.





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   o  A one-octet signature type.  Signature types are described in
      Section 5.2.1.

   o  A one-octet number describing the hash algorithm used.

   o  A one-octet number describing the public-key algorithm used.

   o  An eight-octet number holding the Key ID of the signing key.

   o  A one-octet number holding a flag showing whether the signature is
      nested.  A zero value indicates that the next packet is another
      One-Pass Signature packet that describes another signature to be
      applied to the same message data.

   Note that if a message contains more than one one-pass signature,
   then the Signature packets bracket the message; that is, the first
   Signature packet after the message corresponds to the last one-pass
   packet and the final Signature packet corresponds to the first one-
   pass packet.

5.5.  {5.5} Key Material Packet

   A key material packet contains all the information about a public or
   private key.  There are four variants of this packet type, and two
   major versions.  Consequently, this section is complex.

5.5.1.  {5.5.1} Key Packet Variants

5.5.1.1.  {5.5.1.1} Public-Key Packet (Tag 6)

   A Public-Key packet starts a series of packets that forms an OpenPGP
   key (sometimes called an OpenPGP certificate).

5.5.1.2.  {5.5.1.2} Public-Subkey Packet (Tag 14)

   A Public-Subkey packet (tag 14) has exactly the same format as a
   Public-Key packet, but denotes a subkey.  One or more subkeys may be
   associated with a top-level key.  By convention, the top-level key
   provides signature services, and the subkeys provide encryption
   services.

   Note: in PGP 2.6.x, tag 14 was intended to indicate a comment packet.
   This tag was selected for reuse because no previous version of PGP
   ever emitted comment packets but they did properly ignore them.
   Public-Subkey packets are ignored by PGP 2.6.x and do not cause it to
   fail, providing a limited degree of backward compatibility.





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5.5.1.3.  {5.5.1.3} Secret-Key Packet (Tag 5)

   A Secret-Key packet contains all the information that is found in a
   Public-Key packet, including the public-key material, but also
   includes the secret-key material after all the public-key fields.

5.5.1.4.  {5.5.1.4} Secret-Subkey Packet (Tag 7)

   A Secret-Subkey packet (tag 7) is the subkey analog of the Secret Key
   packet and has exactly the same format.

5.5.2.  {5.5.2} Public-Key Packet Formats

   There are three versions of key-material packets.  Version 3 packets
   were first generated by PGP 2.6.  Version 4 keys first appeared in
   PGP 5.0 and are the preferred key version for OpenPGP.

   OpenPGP implementations MUST create keys with version 4 format.  V3
   keys are deprecated; an implementation MUST NOT generate a V3 key,
   but MAY accept it.

   A version 3 public key or public-subkey packet contains:

   o  A one-octet version number (3).

   o  A four-octet number denoting the time that the key was created.

   o  A two-octet number denoting the time in days that this key is
      valid.  If this number is zero, then it does not expire.

   o  A one-octet number denoting the public-key algorithm of this key.

   o  A series of multiprecision integers comprising the key material:

      *  a multiprecision integer (MPI) of RSA public modulus n;

      *  an MPI of RSA public encryption exponent e.

   V3 keys are deprecated.  They contain three weaknesses.  First, it is
   relatively easy to construct a V3 key that has the same Key ID as any
   other key because the Key ID is simply the low 64 bits of the public
   modulus.  Secondly, because the fingerprint of a V3 key hashes the
   key material, but not its length, there is an increased opportunity
   for fingerprint collisions.  Third, there are weaknesses in the MD5
   hash algorithm that make developers prefer other algorithms.  See
   below for a fuller discussion of Key IDs and fingerprints.





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   V2 keys are identical to the deprecated V3 keys except for the
   version number.  An implementation MUST NOT generate them and MAY
   accept or reject them as it sees fit.

   The version 4 format is similar to the version 3 format except for
   the absence of a validity period.  This has been moved to the
   Signature packet.  In addition, fingerprints of version 4 keys are
   calculated differently from version 3 keys, as described in the
   section "Enhanced Key Formats".

   A version 4 packet contains:

   o  A one-octet version number (4).

   o  A four-octet number denoting the time that the key was created.

   o  A one-octet number denoting the public-key algorithm of this key.

   o  A series of values comprising the key material.  This is
      algorithm-specific and described in section XXXX.

   The version 5 format is similar to the version 4 format except for
   the addition of a count for the key material.  This count helps
   parsing secret key packets (which are an extension of the public key
   packet format) in the case of an unknown algoritm.  In addition,
   fingerprints of version 5 keys are calculated differently from
   version 4 keys, as described in the section "Enhanced Key Formats".

   A version 5 packet contains:

   o  A one-octet version number (5).

   o  A four-octet number denoting the time that the key was created.

   o  A one-octet number denoting the public-key algorithm of this key.

   o  A four-octet scalar octet count for the following key material.

   o  A series of values comprising the key material.  This is
      algorithm-specific and described in section XXXX.

5.5.3.  {5.5.3} Secret-Key Packet Formats

   The Secret-Key and Secret-Subkey packets contain all the data of the
   Public-Key and Public-Subkey packets, with additional algorithm-
   specific secret-key data appended, usually in encrypted form.

   The packet contains:



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   o  A Public-Key or Public-Subkey packet, as described above.

   o  One octet indicating string-to-key usage conventions.  Zero
      indicates that the secret-key data is not encrypted. 255 or 254
      indicates that a string-to-key specifier is being given.  Any
      other value is a symmetric-key encryption algorithm identifier.  A
      version 5 packet MUST NOT use the value 255.

   o  Only for a version 5 packet, a one-octet scalar octet count of the
      next 3 optional fields.

   o  [Optional] If string-to-key usage octet was 255, 254, or 253, a
      one- octet symmetric encryption algorithm.

   o  [Optional] If string-to-key usage octet was 253, a one-octet AEAD
      algorithm.

   o  [Optional] If string-to-key usage octet was 255, 254, or 253, a
      string-to-key specifier.  The length of the string-to-key
      specifier is implied by its type, as described above.

   o  [Optional] If secret data is encrypted (string-to-key usage octet
      not zero), an Initial Vector (IV) of the same length as the
      cipher's block size.

   o  Only for a version 5 packet, a four-octet scalar octet count for
      the following key material.

   o  Plain or encrypted series of values comprising the secret key
      material.  This is algorithm-specific and described in section
      XXXX.

   o  If the string-to-key usage octet is zero or 255, then a two-octet
      checksum of the plaintext of the algorithm-specific portion (sum
      of all octets, mod 65536).  If the string-to-key usage octet was
      254, then a 20-octet SHA-1 hash of the plaintext of the algorithm-
      specific portion.  This checksum or hash is encrypted together
      with the algorithm-specific fields (if string-to-key usage octet
      is not zero).  If the string-to-key usage octet was 253, then an
      AEAD authentication tag is included here.  Note that for all other
      values, a two-octet checksum is required.

   Note that the version 5 packet format adds two count values to help
   parsing packets with unknown S2K or public key algorithms.

   Secret MPI values can be encrypted using a passphrase.  If a string-
   to-key specifier is given, that describes the algorithm for
   converting the passphrase to a key, else a simple MD5 hash of the



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   passphrase is used.  Implementations MUST use a string-to-key
   specifier; the simple hash is for backward compatibility and is
   deprecated, though implementations MAY continue to use existing
   private keys in the old format.  The cipher for encrypting the MPIs
   is specified in the Secret-Key packet.

   Encryption/decryption of the secret data is done in CFB mode using
   the key created from the passphrase and the Initial Vector from the
   packet.  A different mode is used with V3 keys (which are only RSA)
   than with other key formats.  With V3 keys, the MPI bit count prefix
   (i.e., the first two octets) is not encrypted.  Only the MPI non-
   prefix data is encrypted.  Furthermore, the CFB state is
   resynchronized at the beginning of each new MPI value, so that the
   CFB block boundary is aligned with the start of the MPI data.

   With V4 and V5 keys, a simpler method is used.  All secret MPI values
   are encrypted, including the MPI bitcount prefix.

   If the string-to-key usage octet is 253, the encrypted MPI values are
   encrypted as one combined plaintext exactly as an AEAD Encrypted Data
   packet with a chunk size octet of 10 would be.  This implicit chunk
   size octet is included in the normal calculations of additional data.

   The two-octet checksum that follows the algorithm-specific portion is
   the algebraic sum, mod 65536, of the plaintext of all the algorithm-
   specific octets (including MPI prefix and data).  With V3 keys, the
   checksum is stored in the clear.  With V4 keys, the checksum is
   encrypted like the algorithm-specific data.  This value is used to
   check that the passphrase was correct.  However, this checksum is
   deprecated; an implementation SHOULD NOT use it, but should rather
   use the SHA-1 hash denoted with a usage octet of 254.  The reason for
   this is that there are some attacks that involve undetectably
   modifying the secret key.

5.6.  Algorithm-specific Parts of Keys

   The public and secret key format specifies algorithm-specific parts
   of a key.  The following sections describe them in detail.

5.6.1.  Algorithm-Specific Part for RSA Keys

   The public key is this series of multiprecision integers:

   o  MPI of RSA public modulus n;

   o  MPI of RSA public encryption exponent e.

   The secret key is this series of multiprecision integers:



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   o  MPI of RSA secret exponent d;

   o  MPI of RSA secret prime value p;

   o  MPI of RSA secret prime value q (p < q);

   o  MPI of u, the multiplicative inverse of p, mod q.

5.6.2.  Algorithm-Specific Part for DSA Keys

   The public key is this series of multiprecision integers:

   o  MPI of DSA prime p;

   o  MPI of DSA group order q (q is a prime divisor of p-1);

   o  MPI of DSA group generator g;

   o  MPI of DSA public-key value y (= g**x mod p where x is secret).

   The secret key is this single multiprecision integer:

   o  MPI of DSA secret exponent x.

5.6.3.  Algorithm-Specific Part for Elgamal Keys

   The public key is this series of multiprecision integers:

   o  MPI of Elgamal prime p;

   o  MPI of Elgamal group generator g;

   o  MPI of Elgamal public key value y (= g**x mod p where x is
      secret).

   The secret key is this single multiprecision integer:

   o  MPI of Elgamal secret exponent x.

5.6.4.  Algorithm-Specific Part for ECDSA Keys

   The public key is this series of values:

   o  a variable-length field containing a curve OID, formatted as
      follows:

      *  a one-octet size of the following field; values 0 and 0xFF are
         reserved for future extensions,



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      *  the octets representing a curve OID, defined in section
         11{FIXME};

   o  a MPI of an EC point representing a public key.

   The secret key is this single multiprecision integer:

   o  MPI of an integer representing the secret key, which is a scalar
      of the public EC point.

5.6.5.  Algorithm-Specific Part for EdDSA Keys

   The public key is this series of values:

   o  a variable-length field containing a curve OID, formatted as
      follows:

      *  a one-octet size of the following field; values 0 and 0xFF are
         reserved for future extensions,

      *  the octets representing a curve OID, defined in section
         NN{FIXME};

   o  a MPI of an EC point representing a public key Q as described
      under EdDSA Point Format below.

   The secret key is this single multiprecision integer:

   o  MPI of an integer representing the secret key, which is a scalar
      of the public EC point.

5.6.6.  Algorithm-Specific Part for ECDH Keys

   The public key is this series of values:

   o  a variable-length field containing a curve OID, formatted as
      follows:

      *  a one-octet size of the following field; values 0 and 0xFF are
         reserved for future extensions,

      *  the octets representing a curve OID, defined in
         Section 11{FIXME};

   o  a MPI of an EC point representing a public key;

   o  a variable-length field containing KDF parameters, formatted as
      follows:



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      *  a one-octet size of the following fields; values 0 and 0xff are
         reserved for future extensions;

      *  a one-octet value 1, reserved for future extensions;

      *  a one-octet hash function ID used with a KDF;

      *  a one-octet algorithm ID for the symmetric algorithm used to
         wrap the symmetric key used for the message encryption; see
         Section 8 for details.

   Observe that an ECDH public key is composed of the same sequence of
   fields that define an ECDSA key, plus the KDF parameters field.

   The secret key is this single multiprecision integer:

   o  MPI of an integer representing the secret key, which is a scalar
      of the public EC point.

5.7.  {5.6} Compressed Data Packet (Tag 8)

   The Compressed Data packet contains compressed data.  Typically, this
   packet is found as the contents of an encrypted packet, or following
   a Signature or One-Pass Signature packet, and contains a literal data
   packet.

   The body of this packet consists of:

   o  One octet that gives the algorithm used to compress the packet.

   o  Compressed data, which makes up the remainder of the packet.

   A Compressed Data Packet's body contains an block that compresses
   some set of packets.  See section "Packet Composition" for details on
   how messages are formed.

   ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951]
   DEFLATE blocks.  Note that PGP V2.6 uses 13 bits of compression.  If
   an implementation uses more bits of compression, PGP V2.6 cannot
   decompress it.

   ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB-
   style blocks.

   BZip2-compressed packets are compressed using the BZip2 [BZ2]
   algorithm.





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5.8.  {5.7} Symmetrically Encrypted Data Packet (Tag 9)

   The Symmetrically Encrypted Data packet contains data encrypted with
   a symmetric-key algorithm.  When it has been decrypted, it contains
   other packets (usually a literal data packet or compressed data
   packet, but in theory other Symmetrically Encrypted Data packets or
   sequences of packets that form whole OpenPGP messages).

   The body of this packet consists of:

   o  Encrypted data, the output of the selected symmetric-key cipher
      operating in OpenPGP's variant of Cipher Feedback (CFB) mode.

   The symmetric cipher used may be specified in a Public-Key or
   Symmetric-Key Encrypted Session Key packet that precedes the
   Symmetrically Encrypted Data packet.  In that case, the cipher
   algorithm octet is prefixed to the session key before it is
   encrypted.  If no packets of these types precede the encrypted data,
   the IDEA algorithm is used with the session key calculated as the MD5
   hash of the passphrase, though this use is deprecated.

   The data is encrypted in CFB mode, with a CFB shift size equal to the
   cipher's block size.  The Initial Vector (IV) is specified as all
   zeros.  Instead of using an IV, OpenPGP prefixes a string of length
   equal to the block size of the cipher plus two to the data before it
   is encrypted.  The first block-size octets (for example, 8 octets for
   a 64-bit block length) are random, and the following two octets are
   copies of the last two octets of the IV.  For example, in an 8-octet
   block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
   octet 8.  In a cipher of length 16, octet 17 is a repeat of octet 15
   and octet 18 is a repeat of octet 16.  As a pedantic clarification,
   in both these examples, we consider the first octet to be numbered 1.

   After encrypting the first block-size-plus-two octets, the CFB state
   is resynchronized.  The last block-size octets of ciphertext are
   passed through the cipher and the block boundary is reset.

   The repetition of 16 bits in the random data prefixed to the message
   allows the receiver to immediately check whether the session key is
   incorrect.  See the "Security Considerations" section for hints on
   the proper use of this "quick check".

5.9.  {5.8} Marker Packet (Obsolete Literal Packet) (Tag 10)

   An experimental version of PGP used this packet as the Literal
   packet, but no released version of PGP generated Literal packets with
   this tag.  With PGP 5.x, this packet has been reassigned and is
   reserved for use as the Marker packet.



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   The body of this packet consists of:

   o  The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).

   Such a packet MUST be ignored when received.  It may be placed at the
   beginning of a message that uses features not available in PGP 2.6.x
   in order to cause that version to report that newer software is
   necessary to process the message.

5.10.  {5.9} Literal Data Packet (Tag 11)

   A Literal Data packet contains the body of a message; data that is
   not to be further interpreted.

   The body of this packet consists of:

   o  A one-octet field that describes how the data is formatted.

      If it is a 'b' (0x62), then the Literal packet contains binary
      data.  If it is a 't' (0x74), then it contains text data, and thus
      may need line ends converted to local form, or other text-mode
      changes.  The tag 'u' (0x75) means the same as 't', but also
      indicates that implementation believes that the literal data
      contains UTF-8 text.  If it is a 'm' (0x6d), then it contains a
      MIME message body part [RFC2045].

      Early versions of PGP also defined a value of 'l' as a 'local'
      mode for machine-local conversions.  RFC 1991 [RFC1991]
      incorrectly stated this local mode flag as '1' (ASCII numeral
      one).  Both of these local modes are deprecated.

   o  File name as a string (one-octet length, followed by a file name).
      This may be a zero-length string.  Commonly, if the source of the
      encrypted data is a file, this will be the name of the encrypted
      file.  An implementation MAY consider the file name in the Literal
      packet to be a more authoritative name than the actual file name.

      If the special name "_CONSOLE" is used, the message is considered
      to be "for your eyes only".  This advises that the message data is
      unusually sensitive, and the receiving program should process it
      more carefully, perhaps avoiding storing the received data to
      disk, for example.

   o  A four-octet number that indicates a date associated with the
      literal data.  Commonly, the date might be the modification date
      of a file, or the time the packet was created, or a zero that
      indicates no specific time.




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   o  The remainder of the packet is literal data.

      Text data is stored with <CR><LF> text endings (i.e., network-
      normal line endings).  These should be converted to native line
      endings by the receiving software.

5.11.  {5.10} Trust Packet (Tag 12)

   The Trust packet is used only within keyrings and is not normally
   exported.  Trust packets contain data that record the user's
   specifications of which key holders are trustworthy introducers,
   along with other information that implementing software uses for
   trust information.  The format of Trust packets is defined by a given
   implementation.

   Trust packets SHOULD NOT be emitted to output streams that are
   transferred to other users, and they SHOULD be ignored on any input
   other than local keyring files.

5.12.  {5.11} User ID Packet (Tag 13)

   A User ID packet consists of UTF-8 text that is intended to represent
   the name and email address of the key holder.  By convention, it
   includes an RFC 2822 [RFC2822] mail name-addr, but there are no
   restrictions on its content.  The packet length in the header
   specifies the length of the User ID.

5.13.  {5.12} User Attribute Packet (Tag 17)

   The User Attribute packet is a variation of the User ID packet.  It
   is capable of storing more types of data than the User ID packet,
   which is limited to text.  Like the User ID packet, a User Attribute
   packet may be certified by the key owner ("self-signed") or any other
   key owner who cares to certify it.  Except as noted, a User Attribute
   packet may be used anywhere that a User ID packet may be used.

   While User Attribute packets are not a required part of the OpenPGP
   standard, implementations SHOULD provide at least enough
   compatibility to properly handle a certification signature on the
   User Attribute packet.  A simple way to do this is by treating the
   User Attribute packet as a User ID packet with opaque contents, but
   an implementation may use any method desired.

   The User Attribute packet is made up of one or more attribute
   subpackets.  Each subpacket consists of a subpacket header and a
   body.  The header consists of:

   o  the subpacket length (1, 2, or 5 octets)



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   o  the subpacket type (1 octet)

   and is followed by the subpacket specific data.

   The following table lists the currently known subpackets:

                +----------+------------------------------+
                |     Type | Attribute Subpacket          |
                +----------+------------------------------+
                |        1 | Image Attribute Subpacket    |
                |   [TBD1] | User ID Attribute Subpacket  |
                |  100-110 | Private/Experimental Use     |
                +----------+------------------------------+

   An implementation SHOULD ignore any subpacket of a type that it does
   not recognize.

5.13.1.  {5.12.1} The Image Attribute Subpacket

   The Image Attribute subpacket is used to encode an image, presumably
   (but not required to be) that of the key owner.

   The Image Attribute subpacket begins with an image header.  The first
   two octets of the image header contain the length of the image
   header.  Note that unlike other multi-octet numerical values in this
   document, due to a historical accident this value is encoded as a
   little-endian number.  The image header length is followed by a
   single octet for the image header version.  The only currently
   defined version of the image header is 1, which is a 16-octet image
   header.  The first three octets of a version 1 image header are thus
   0x10, 0x00, 0x01.

   The fourth octet of a version 1 image header designates the encoding
   format of the image.  The only currently defined encoding format is
   the value 1 to indicate JPEG.  Image format types 100 through 110 are
   reserved for private or experimental use.  The rest of the version 1
   image header is made up of 12 reserved octets, all of which MUST be
   set to 0.

   The rest of the image subpacket contains the image itself.  As the
   only currently defined image type is JPEG, the image is encoded in
   the JPEG File Interchange Format (JFIF), a standard file format for
   JPEG images [JFIF].

   An implementation MAY try to determine the type of an image by
   examination of the image data if it is unable to handle a particular
   version of the image header or if a specified encoding format value
   is not recognized.



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5.13.2.  User ID Attribute Subpacket

   A User ID Attribute subpacket has type #[IANA -- assignment TBD1].

   A User ID Attribute subpacket, just like a User ID packet, consists
   of UTF-8 text that is intended to represent the name and email
   address of the key holder.  By convention, it includes an RFC 2822
   [RFC2822] mail name-addr, but there are no restrictions on its
   content.  For devices using OpenPGP for device certificates, it may
   just be the device identifier.  The packet length in the header
   specifies the length of the User ID.

   Because User Attribute subpackets can be used anywhere a User ID
   packet can be used, implementations MAY choose to trust a signed User
   Attribute subpacket that includes a User ID Attribute subpacket.

5.14.  {5.13} Sym. Encrypted Integrity Protected Data Packet (Tag 18)

   The Symmetrically Encrypted Integrity Protected Data packet is a
   variant of the Symmetrically Encrypted Data packet.  It is a new
   feature created for OpenPGP that addresses the problem of detecting a
   modification to encrypted data.  It is used in combination with a
   Modification Detection Code packet.

   There is a corresponding feature in the features Signature subpacket
   that denotes that an implementation can properly use this packet
   type.  An implementation MUST support decrypting these packets and
   SHOULD prefer generating them to the older Symmetrically Encrypted
   Data packet when possible.  Since this data packet protects against
   modification attacks, this standard encourages its proliferation.
   While blanket adoption of this data packet would create
   interoperability problems, rapid adoption is nevertheless important.
   An implementation SHOULD specifically denote support for this packet,
   but it MAY infer it from other mechanisms.

   For example, an implementation might infer from the use of a cipher
   such as Advanced Encryption Standard (AES) or Twofish that a user
   supports this feature.  It might place in the unhashed portion of
   another user's key signature a Features subpacket.  It might also
   present a user with an opportunity to regenerate their own self-
   signature with a Features subpacket.

   This packet contains data encrypted with a symmetric-key algorithm
   and protected against modification by the SHA-1 hash algorithm.  When
   it has been decrypted, it will typically contain other packets (often
   a Literal Data packet or Compressed Data packet).  The last decrypted
   packet in this packet's payload MUST be a Modification Detection Code
   packet.



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   The body of this packet consists of:

   o  A one-octet version number.  The only defined value is 1.  There
      won't be any future versions of this packet because the MDC system
      has been superseded by the AEAD Encrypted Data packet.

   o  Encrypted data, the output of the selected symmetric-key cipher
      operating in Cipher Feedback mode with shift amount equal to the
      block size of the cipher (CFB-n where n is the block size).

   The symmetric cipher used MUST be specified in a Public-Key or
   Symmetric-Key Encrypted Session Key packet that precedes the
   Symmetrically Encrypted Data packet.  In either case, the cipher
   algorithm octet is prefixed to the session key before it is
   encrypted.

   The data is encrypted in CFB mode, with a CFB shift size equal to the
   cipher's block size.  The Initial Vector (IV) is specified as all
   zeros.  Instead of using an IV, OpenPGP prefixes an octet string to
   the data before it is encrypted.  The length of the octet string
   equals the block size of the cipher in octets, plus two.  The first
   octets in the group, of length equal to the block size of the cipher,
   are random; the last two octets are each copies of their 2nd
   preceding octet.  For example, with a cipher whose block size is 128
   bits or 16 octets, the prefix data will contain 16 random octets,
   then two more octets, which are copies of the 15th and 16th octets,
   respectively.  Unlike the Symmetrically Encrypted Data Packet, no
   special CFB resynchronization is done after encrypting this prefix
   data.  See "OpenPGP CFB Mode" below for more details.

   The repetition of 16 bits in the random data prefixed to the message
   allows the receiver to immediately check whether the session key is
   incorrect.

   The plaintext of the data to be encrypted is passed through the SHA-1
   hash function, and the result of the hash is appended to the
   plaintext in a Modification Detection Code packet.  The input to the
   hash function includes the prefix data described above; it includes
   all of the plaintext, and then also includes two octets of values
   0xD3, 0x14.  These represent the encoding of a Modification Detection
   Code packet tag and length field of 20 octets.

   The resulting hash value is stored in a Modification Detection Code
   (MDC) packet, which MUST use the two octet encoding just given to
   represent its tag and length field.  The body of the MDC packet is
   the 20-octet output of the SHA-1 hash.





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   The Modification Detection Code packet is appended to the plaintext
   and encrypted along with the plaintext using the same CFB context.

   During decryption, the plaintext data should be hashed with SHA-1,
   including the prefix data as well as the packet tag and length field
   of the Modification Detection Code packet.  The body of the MDC
   packet, upon decryption, is compared with the result of the SHA-1
   hash.

   Any failure of the MDC indicates that the message has been modified
   and MUST be treated as a security problem.  Failures include a
   difference in the hash values, but also the absence of an MDC packet,
   or an MDC packet in any position other than the end of the plaintext.
   Any failure SHOULD be reported to the user.

     NON-NORMATIVE EXPLANATION

     The MDC system, as packets 18 and 19 are called, were created to
     provide an integrity mechanism that is less strong than a
     signature, yet stronger than bare CFB encryption.

     It is a limitation of CFB encryption that damage to the
     ciphertext will corrupt the affected cipher blocks and the block
     following.  Additionally, if data is removed from the end of a
     CFB-encrypted block, that removal is undetectable.  (Note also
     that CBC mode has a similar limitation, but data removed from
     the front of the block is undetectable.)

     The obvious way to protect or authenticate an encrypted block is
     to digitally sign it.  However, many people do not wish to
     habitually sign data, for a large number of reasons beyond the
     scope of this document.  Suffice it to say that many people
     consider properties such as deniability to be as valuable as
     integrity.

     OpenPGP addresses this desire to have more security than raw
     encryption and yet preserve deniability with the MDC system.  An
     MDC is intentionally not a MAC.  Its name was not selected by
     accident.  It is analogous to a checksum.

     Despite the fact that it is a relatively modest system, it has
     proved itself in the real world.  It is an effective defense to
     several attacks that have surfaced since it has been created.
     It has met its modest goals admirably.

     Consequently, because it is a modest security system, it has
     modest requirements on the hash function(s) it employs.  It does
     not rely on a hash function being collision-free, it relies on a



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     hash function being one-way.  If a forger, Frank, wishes to send
     Alice a (digitally) unsigned message that says, "I've always
     secretly loved you, signed Bob", it is far easier for him to
     construct a new message than it is to modify anything
     intercepted from Bob.  (Note also that if Bob wishes to
     communicate secretly with Alice, but without authentication or
     identification and with a threat model that includes forgers, he
     has a problem that transcends mere cryptography.)

     Note also that unlike nearly every other OpenPGP subsystem,
     there are no parameters in the MDC system.  It hard-defines
     SHA-1 as its hash function.  This is not an accident.  It is an
     intentional choice to avoid downgrade and cross-grade attacks
     while making a simple, fast system.  (A downgrade attack would
     be an attack that replaced SHA2-256 with SHA-1, for example.  A
     cross-grade attack would replace SHA-1 with another 160-bit
     hash, such as RIPE-MD/160, for example.)

     However, given the present state of hash function cryptanalysis
     and cryptography, it may be desirable to upgrade the MDC system
     to a new hash function.  See Section 13.11 in the "IANA
     Considerations" for guidance.

5.15.  {5.14} Modification Detection Code Packet (Tag 19)

   The Modification Detection Code packet contains a SHA-1 hash of
   plaintext data, which is used to detect message modification.  It is
   only used with a Symmetrically Encrypted Integrity Protected Data
   packet.  The Modification Detection Code packet MUST be the last
   packet in the plaintext data that is encrypted in the Symmetrically
   Encrypted Integrity Protected Data packet, and MUST appear in no
   other place.

   A Modification Detection Code packet MUST have a length of 20 octets.

   The body of this packet consists of:

   o  A 20-octet SHA-1 hash of the preceding plaintext data of the
      Symmetrically Encrypted Integrity Protected Data packet, including
      prefix data, the tag octet, and length octet of the Modification
      Detection Code packet.

   Note that the Modification Detection Code packet MUST always use a
   new format encoding of the packet tag, and a one-octet encoding of
   the packet length.  The reason for this is that the hashing rules for
   modification detection include a one-octet tag and one-octet length
   in the data hash.  While this is a bit restrictive, it reduces
   complexity.



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5.16.  AEAD Encrypted Data Packet (Tag 20)

   This packet contains data encrypted with an authenticated encryption
   and additional data (AEAD) construction.  When it has been decrypted,
   it will typically contain other packets (often a Literal Data packet
   or Compressed Data packet).

   The body of this packet consists of:

   o  A one-octet version number.  The only currently defined value is
      1.

   o  A one-octet cipher algorithm.

   o  A one-octet AEAD algorithm.

   o  A one-octet chunk size.

   o  A starting initialization vector of size specified by the AEAD
      algorithm.

   o  Encrypted data, the output of the selected symmetric-key cipher
      operating in the given AEAD mode.

   o  A final, summary authentication tag for the AEAD mode.

   An AEAD encrypted data packet consists of one or more chunks of data.
   The plaintext of each chunk is of a size specified using the chunk
   size octet using the method specified below.

   The encrypted data consists of the encryption of each chunk of
   plaintext, followed immediately by the relevant authentication tag.
   If the last chunk of plaintext is smaller than the chunk size, the
   ciphertext for that data may be shorter; it is nevertheless followed
   by a full authentication tag.

   For each chunk, the AEAD construction is given the packet header,
   version number, cipher algorithm octet, AEAD algorithm octet, chunk
   size octet, and an eight-octet, big-endian chunk index as additional
   data.  The index of the first chunk is zero.

   After the final chunk, the AEAD algorithm is used to produce a final
   authentication tag encrypting the empty string.  This AEAD instance
   is given the additional data specified above, plus an eight-octet,
   big-endian value specifying the total number of plaintext octets
   encrypted.  This allows detection of a truncated ciphertext.





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   The chunk size octet specifies the size of chunks using the following
   formula (in C), where c is the chunk size octet:

       chunk_size = ((uint64_t)1 << (c + 6))

   An implementation MUST support chunk size octets with values from 0
   to 56.  Chunk size octets with other values are reserved for future
   extensions.

   A new random initialization vector MUST be used for each message.

5.16.1.  EAX Mode

   The EAX algorithm can only use block ciphers with 16-octet blocks.
   The starting initialization vector and authentication tag are both 16
   octets long.

   The starting initialization vector for this mode MUST be unique and
   unpredictable.

   The nonce for EAX mode is computed by treating the starting
   initialization vector as a 16-octet, big-endian value and exclusive-
   oring the low eight octets of it with the chunk index.

   The security of EAX requires that the nonce is never reused, hence
   the requirement that the starting initialization vector be unique.

5.16.2.  OCB Mode

   The OCB Authenticated-Encryption Algorithm used in this document is
   defined in [RFC7253].

   OCB was initially defined in [OCB1] (now called "OCB1") for
   authenticated encryption, then as an authenticated encryption with
   associated data algorithm with tweakable blockciphers in [OCB2]
   ("OCB2"), and finally with performance enhancements as [OCB3]
   ("OCB3").

   The [RFC7253] algorithm differs from "OCB3" such that tag length is
   encoded into the internally formatted nonce.

   OCB usage requires specification of the following parameters:

   o  a blockcipher that operate on 128-bit (16-octet) blocks

   o  an authentication tag length of 128 bits





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   While OCB [RFC7253] allows the authentication tag length to be of any
   number up to 128 bits long, this document requires a fixed
   authentication tag length of 128 bits (16 octets) for simplicity.

   The nonce for a chunk of chunk index "i" in OCB processing is defined
   as:

   OCB-Nonce_{i} = IV[1..120] xor i

   Where,

   o  IV is the initialization vector of the message;

   o  IV[i..j] is the substring of IV consisting of bits i through j,
      inclusive, in big-endian format.

   The value of OCB-Nonce_{i} is always 120 bits (15 octets) long as the
   longest allowed nonce length of OCB mode according to [RFC7253].

   Security of OCB mode depends on the non-repeated nature of nonces
   used for the same key on distinct plaintext [RFC7253].  Therefore the
   initialization vector per message MUST be distinct, and OCB mode
   SHOULD only be used in environments when there is certainty to
   fulfilling this requirement.

6.  {6} Radix-64 Conversions

   As stated in the introduction, OpenPGP's underlying native
   representation for objects is a stream of arbitrary octets, and some
   systems desire these objects to be immune to damage caused by
   character set translation, data conversions, etc.

   In principle, any printable encoding scheme that met the requirements
   of the unsafe channel would suffice, since it would not change the
   underlying binary bit streams of the native OpenPGP data structures.
   The OpenPGP standard specifies one such printable encoding scheme to
   ensure interoperability.

   OpenPGP's Radix-64 encoding is composed of two parts: a base64
   encoding of the binary data and a checksum.  The base64 encoding is
   identical to the MIME base64 content-transfer-encoding [RFC2045].

   The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to
   four characters of radix-64 encoding by the same MIME base64
   transformation, preceded by an equal sign (=).  The CRC is computed
   by using the generator 0x864CFB and an initialization of 0xB704CE.
   The accumulation is done on the data before it is converted to radix-




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   64, rather than on the converted data.  A sample implementation of
   this algorithm is in the next section.

   The checksum with its leading equal sign MAY appear on the first line
   after the base64 encoded data.

   Rationale for CRC-24: The size of 24 bits fits evenly into printable
   base64.  The nonzero initialization can detect more errors than a
   zero initialization.

6.1.  {6.1} An Implementation of the CRC-24 in "C"

     <CODE BEGINS>
     #define CRC24_INIT 0xB704CEL
     #define CRC24_POLY 0x1864CFBL

     typedef long crc24;
     crc24 crc_octets(unsigned char *octets, size_t len)
     {
         crc24 crc = CRC24_INIT;
         int i;
         while (len--) {
             crc ^= (*octets++) << 16;
             for (i = 0; i < 8; i++) {
                 crc <<= 1;
                 if (crc & 0x1000000)
                     crc ^= CRC24_POLY;
             }
         }
         return crc & 0xFFFFFFL;
     }
     <CODE ENDS>

6.2.  {6.2} Forming ASCII Armor

   When OpenPGP encodes data into ASCII Armor, it puts specific headers
   around the Radix-64 encoded data, so OpenPGP can reconstruct the data
   later.  An OpenPGP implementation MAY use ASCII armor to protect raw
   binary data.  OpenPGP informs the user what kind of data is encoded
   in the ASCII armor through the use of the headers.

   Concatenating the following data creates ASCII Armor:

   o  An Armor Header Line, appropriate for the type of data

   o  Armor Headers

   o  A blank line



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   o  The ASCII-Armored data

   o  An Armor Checksum

   o  The Armor Tail, which depends on the Armor Header Line

   An Armor Header Line consists of the appropriate header line text
   surrounded by five (5) dashes ('-', 0x2D) on either side of the
   header line text.  The header line text is chosen based upon the type
   of data that is being encoded in Armor, and how it is being encoded.
   Header line texts include the following strings:

   BEGIN PGP MESSAGE  Used for signed, encrypted, or compressed files.

   BEGIN PGP PUBLIC KEY BLOCK  Used for armoring public keys.

   BEGIN PGP PRIVATE KEY BLOCK  Used for armoring private keys.

   BEGIN PGP MESSAGE, PART X/Y  Used for multi-part messages, where the
      armor is split amongst Y parts, and this is the Xth part out of Y.

   BEGIN PGP MESSAGE, PART X  Used for multi-part messages, where this
      is the Xth part of an unspecified number of parts.  Requires the
      MESSAGE-ID Armor Header to be used.

   BEGIN PGP SIGNATURE  Used for detached signatures, OpenPGP/MIME
      signatures, and cleartext signatures.  Note that PGP 2.x uses
      BEGIN PGP MESSAGE for detached signatures.

   Note that all these Armor Header Lines are to consist of a complete
   line.  That is to say, there is always a line ending preceding the
   starting five dashes, and following the ending five dashes.  The
   header lines, therefore, MUST start at the beginning of a line, and
   MUST NOT have text other than whitespace -- space (0x20), tab (0x09)
   or carriage return (0x0d) -- following them on the same line.  These
   line endings are considered a part of the Armor Header Line for the
   purposes of determining the content they delimit.  This is
   particularly important when computing a cleartext signature (see
   below).

   The Armor Headers are pairs of strings that can give the user or the
   receiving OpenPGP implementation some information about how to decode
   or use the message.  The Armor Headers are a part of the armor, not a
   part of the message, and hence are not protected by any signatures
   applied to the message.

   The format of an Armor Header is that of a key-value pair.  A colon
   (':' 0x38) and a single space (0x20) separate the key and value.



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   OpenPGP should consider improperly formatted Armor Headers to be
   corruption of the ASCII Armor.  Unknown keys should be reported to
   the user, but OpenPGP should continue to process the message.

   Note that some transport methods are sensitive to line length.  While
   there is a limit of 76 characters for the Radix-64 data
   (Section 6.3), there is no limit to the length of Armor Headers.
   Care should be taken that the Armor Headers are short enough to
   survive transport.  One way to do this is to repeat an Armor Header
   Key multiple times with different values for each so that no one line
   is overly long.

   Currently defined Armor Header Keys are as follows:

   o  "Version", which states the OpenPGP implementation and version
      used to encode the message.

   o  "Comment", a user-defined comment.  OpenPGP defines all text to be
      in UTF-8.  A comment may be any UTF-8 string.  However, the whole
      point of armoring is to provide seven-bit-clean data.
      Consequently, if a comment has characters that are outside the US-
      ASCII range of UTF, they may very well not survive transport.

   o  "Hash", a comma-separated list of hash algorithms used in this
      message.  This is used only in cleartext signed messages.

   o  "MessageID", a 32-character string of printable characters.  The
      string must be the same for all parts of a multi-part message that
      uses the "PART X" Armor Header.  MessageID strings should be
      unique enough that the recipient of the mail can associate all the
      parts of a message with each other.  A good checksum or
      cryptographic hash function is sufficient.

      The MessageID SHOULD NOT appear unless it is in a multi-part
      message.  If it appears at all, it MUST be computed from the
      finished (encrypted, signed, etc.) message in a deterministic
      fashion, rather than contain a purely random value.  This is to
      allow the legitimate recipient to determine that the MessageID
      cannot serve as a covert means of leaking cryptographic key
      information.

   o  "Charset", a description of the character set that the plaintext
      is in.  Please note that OpenPGP defines text to be in UTF-8.  An
      implementation will get best results by translating into and out
      of UTF-8.  However, there are many instances where this is easier
      said than done.  Also, there are communities of users who have no
      need for UTF-8 because they are all happy with a character set
      like ISO Latin-5 or a Japanese character set.  In such instances,



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      an implementation MAY override the UTF-8 default by using this
      header key.  An implementation MAY implement this key and any
      translations it cares to; an implementation MAY ignore it and
      assume all text is UTF-8.

   The blank line can either be zero-length or contain only whitespace,
   that is spaces (0x20), tabs (0x09) or carriage returns (0x0d).

   The Armor Tail Line is composed in the same manner as the Armor
   Header Line, except the string "BEGIN" is replaced by the string
   "END".

6.3.  {6.3} Encoding Binary in Radix-64

   The encoding process represents 24-bit groups of input bits as output
   strings of 4 encoded characters.  Proceeding from left to right, a
   24-bit input group is formed by concatenating three 8-bit input
   groups.  These 24 bits are then treated as four concatenated 6-bit
   groups, each of which is translated into a single digit in the
   Radix-64 alphabet.  When encoding a bit stream with the Radix-64
   encoding, the bit stream must be presumed to be ordered with the most
   significant bit first.  That is, the first bit in the stream will be
   the high-order bit in the first 8-bit octet, and the eighth bit will
   be the low-order bit in the first 8-bit octet, and so on.

        +--first octet--+-second octet--+--third octet--+
        |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
        +-----------+---+-------+-------+---+-----------+
        |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
        +--1.index--+--2.index--+--3.index--+--4.index--+

   Each 6-bit group is used as an index into an array of 64 printable
   characters from the table below.  The character referenced by the
   index is placed in the output string.

















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    Value Encoding  Value Encoding  Value Encoding  Value Encoding
        0 A            17 R            34 i            51 z
        1 B            18 S            35 j            52 0
        2 C            19 T            36 k            53 1
        3 D            20 U            37 l            54 2
        4 E            21 V            38 m            55 3
        5 F            22 W            39 n            56 4
        6 G            23 X            40 o            57 5
        7 H            24 Y            41 p            58 6
        8 I            25 Z            42 q            59 7
        9 J            26 a            43 r            60 8
       10 K            27 b            44 s            61 9
       11 L            28 c            45 t            62 +
       12 M            29 d            46 u            63 /
       13 N            30 e            47 v
       14 O            31 f            48 w         (pad) =
       15 P            32 g            49 x
       16 Q            33 h            50 y

   The encoded output stream must be represented in lines of no more
   than 76 characters each.

   Special processing is performed if fewer than 24 bits are available
   at the end of the data being encoded.  There are three possibilities:

   1.  The last data group has 24 bits (3 octets).  No special
       processing is needed.

   2.  The last data group has 16 bits (2 octets).  The first two
       6-bit groups are processed as above.  The third (incomplete)
       data group has two zero-value bits added to it, and is
       processed as above. A pad character (=) is added to the
       output.

   3.  The last data group has 8 bits (1 octet).  The first 6-bit
       group is processed as above.  The second (incomplete) data
       group has four zero-value bits added to it, and is processed
       as above.  Two pad characters (=) are added to the output.

6.4.  {6.4} Decoding Radix-64

   In Radix-64 data, characters other than those in the table, line
   breaks, and other white space probably indicate a transmission error,
   about which a warning message or even a message rejection might be
   appropriate under some circumstances.  Decoding software must ignore
   all white space.





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   Because it is used only for padding at the end of the data, the
   occurrence of any "=" characters may be taken as evidence that the
   end of the data has been reached (without truncation in transit).  No
   such assurance is possible, however, when the number of octets
   transmitted was a multiple of three and no "=" characters are
   present.

6.5.  {6.5} Examples of Radix-64

   Input data:  0x14FB9C03D97E
   Hex:     1   4    F   B    9   C     | 0   3    D   9    7   E
   8-bit:   00010100 11111011 10011100  | 00000011 11011001 01111110
   6-bit:   000101 001111 101110 011100 | 000000 111101 100101 111110
   Decimal: 5      15     46     28       0      61     37     62
   Output:  F      P      u      c        A      9      l      +
   Input data:  0x14FB9C03D9
   Hex:     1   4    F   B    9   C     | 0   3    D   9
   8-bit:   00010100 11111011 10011100  | 00000011 11011001
                                                   pad with 00
   6-bit:   000101 001111 101110 011100 | 000000 111101 100100
   Decimal: 5      15     46     28       0      61     36
                                                      pad with =
   Output:  F      P      u      c        A      9      k      =
   Input data:  0x14FB9C03
   Hex:     1   4    F   B    9   C     | 0   3
   8-bit:   00010100 11111011 10011100  | 00000011
                                          pad with 0000
   6-bit:   000101 001111 101110 011100 | 000000 110000
   Decimal: 5      15     46     28       0      48
                                               pad with =      =
   Output:  F      P      u      c        A      w      =      =

6.6.  {6.6} Example of an ASCII Armored Message

   -----BEGIN PGP MESSAGE-----
   Version: OpenPrivacy 0.99

   yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
   vBSFjNSiVHsuAA==
   =njUN
   -----END PGP MESSAGE-----

   Note that this example has extra indenting; an actual armored message
   would have no leading whitespace.







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7.  {7} Cleartext Signature Framework

   It is desirable to be able to sign a textual octet stream without
   ASCII armoring the stream itself, so the signed text is still
   readable without special software.  In order to bind a signature to
   such a cleartext, this framework is used, which follows the same
   basic format and restrictions as the ASCII armoring described above
   in "Forming ASCII Armor" (Section 6.2).  (Note that this framework is
   not intended to be reversible.  RFC 3156 [RFC3156] defines another
   way to sign cleartext messages for environments that support MIME.)

   The cleartext signed message consists of:

   o  The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
      single line,

   o  One or more "Hash" Armor Headers,

   o  Exactly one blank line not included into the message digest,

   o  The dash-escaped cleartext that is included into the message
      digest,

   o  The ASCII armored signature(s) including the '-----BEGIN PGP
      SIGNATURE-----' Armor Header and Armor Tail Lines.


   If the "Hash" Armor Header is given, the specified message digest
   algorithm(s) are used for the signature.  If there are no such
   headers, MD5 is used.  If MD5 is the only hash used, then an
   implementation MAY omit this header for improved V2.x compatibility.
   If more than one message digest is used in the signature, the "Hash"
   armor header contains a comma-delimited list of used message digests.

   Current message digest names are described below with the algorithm
   IDs.

   An implementation SHOULD add a line break after the cleartext, but
   MAY omit it if the cleartext ends with a line break.  This is for
   visual clarity.

7.1.  {7.1} Dash-Escaped Text

   The cleartext content of the message must also be dash-escaped.

   Dash-escaped cleartext is the ordinary cleartext where every line
   starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
   (0x2D) and space ' ' (0x20).  This prevents the parser from



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   recognizing armor headers of the cleartext itself.  An implementation
   MAY dash-escape any line, SHOULD dash-escape lines commencing "From"
   followed by a space, and MUST dash-escape any line commencing in a
   dash.  The message digest is computed using the cleartext itself, not
   the dash-escaped form.

   As with binary signatures on text documents, a cleartext signature is
   calculated on the text using canonical <CR><LF> line endings.  The
   line ending (i.e., the <CR><LF>) before the '-----BEGIN PGP
   SIGNATURE-----' line that terminates the signed text is not
   considered part of the signed text.

   When reversing dash-escaping, an implementation MUST strip the string
   "- " if it occurs at the beginning of a line, and SHOULD warn on "-"
   and any character other than a space at the beginning of a line.

   Also, any trailing whitespace -- spaces (0x20), tabs (0x09) or
   carriage returns (0x0d) -- at the end of any line is removed when the
   cleartext signature is generated and verified.

8.  {8} Regular Expressions

   A regular expression is zero or more branches, separated by '|'.  It
   matches anything that matches one of the branches.

   A branch is zero or more pieces, concatenated.  It matches a match
   for the first, followed by a match for the second, etc.

   A piece is an atom possibly followed by '_', '+', or '?'.  An atom
   followed by '_' matches a sequence of 0 or more matches of the atom.
   An atom followed by '+' matches a sequence of 1 or more matches of
   the atom.  An atom followed by '?' matches a match of the atom, or
   the null string.

   An atom is a regular expression in parentheses (matching a match for
   the regular expression), a range (see below), '.' (matching any
   single character), '^' (matching the null string at the beginning of
   the input string), '$' (matching the null string at the end of the
   input string), a '\' followed by a single character (matching that
   character), or a single character with no other significance
   (matching that character).

   A range is a sequence of characters enclosed in '[]'.  It normally
   matches any single character from the sequence.  If the sequence
   begins with '^', it matches any single character not from the rest of
   the sequence.  If two characters in the sequence are separated by
   '-', this is shorthand for the full list of ASCII characters between
   them (e.g., '[0-9]' matches any decimal digit).  To include a literal



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   ']' in the sequence, make it the first character (following a
   possible '^').  To include a literal '-', make it the first or last
   character.

9.  {9} Constants

   This section describes the constants used in OpenPGP.

   Note that these tables are not exhaustive lists; an implementation
   MAY implement an algorithm not on these lists, so long as the
   algorithm numbers are chosen from the private or experimental
   algorithm range.

   See the section "Notes on Algorithms" below for more discussion of
   the algorithms.

9.1.  {9.1} Public-Key Algorithms

    +-----------+----------------------------------------------------+
    |        ID | Algorithm                                          |
    +-----------+----------------------------------------------------+
    |         1 | RSA (Encrypt or Sign) [HAC]                        |
    |         2 | RSA Encrypt-Only [HAC]                             |
    |         3 | RSA Sign-Only [HAC]                                |
    |        16 | Elgamal (Encrypt-Only) [ELGAMAL] [HAC]             |
    |        17 | DSA (Digital Signature Algorithm) [FIPS186] [HAC]  |
    |        18 | ECDH public key algorithm                          |
    |        19 | ECDSA public key algorithm [FIPS186]               |
    |        20 | Reserved (formerly Elgamal Encrypt or Sign)        |
    |        21 | Reserved for Diffie-Hellman                        |
    |           | (X9.42, as defined for IETF-S/MIME)                |
    |        22 | EdDSA [I-D.irtf-cfrg-eddsa]                        |
    |  100--110 | Private/Experimental algorithm                     |
    +-----------+----------------------------------------------------+

   Implementations MUST implement DSA and ECDSA for signatures, and
   Elgamal and ECDH for encryption.  Implementations SHOULD implement
   RSA keys (1).  RSA Encrypt-Only (2) and RSA Sign-Only are deprecated
   and SHOULD NOT be generated, but may be interpreted.  See
   Section 13.5.  See Section 13.8 for notes on Elgamal Encrypt or Sign
   (20), and X9.42 (21).  Implementations MAY implement any other
   algorithm.

   A compatible specification of ECDSA is given in [RFC6090] as "KT-I
   Signatures" and in [SEC1]; ECDH is defined in Section 13.5 this
   document.





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9.2.  ECC Curve OID

   The parameter curve OID is an array of octets that define a named
   curve.  The table below specifies the exact sequence of bytes for
   each named curve referenced in this document:

   +------------------------+-----+------------------+-----------------+
   | ASN.1 Object           | OID | Curve OID bytes  | Curve name      |
   | Identifier             | len | in hexadecimal   |                 |
   |                        |     | representation   |                 |
   +------------------------+-----+------------------+-----------------+
   | 1.2.840.10045.3.1.7    | 8   | 2A 86 48 CE 3D   | NIST P-256      |
   |                        |     | 03 01 07         |                 |
   | 1.3.132.0.34           | 5   | 2B 81 04 00 22   | NIST P-384      |
   | 1.3.132.0.35           | 5   | 2B 81 04 00 23   | NIST P-521      |
   | 1.3.36.3.3.2.8.1.1.7   | 9   | 2B 24 03 03 02   | brainpoolP256r1 |
   |                        |     | 08 01 01 07      |                 |
   | 1.3.36.3.3.2.8.1.1.13  | 9   | 2B 24 03 03 02   | brainpoolP512r1 |
   |                        |     | 08 01 01 0D      |                 |
   | 1.3.6.1.4.1.11591.15.1 | 9   | 2B 06 01 04 01   | Ed25519         |
   |                        |     | DA 47 0F 01      |                 |
   | 1.3.6.1.4.1.3029.1.5.1 | 10  | 2B 06 01 04 01   | Curve25519      |
   |                        |     | 97 55 01 05 01   |                 |
   +------------------------+-----+------------------+-----------------+

   The sequence of octets in the third column is the result of applying
   the Distinguished Encoding Rules (DER) to the ASN.1 Object Identifier
   with subsequent truncation.  The truncation removes the two fields of
   encoded Object Identifier.  The first omitted field is one octet
   representing the Object Identifier tag, and the second omitted field
   is the length of the Object Identifier body.  For example, the
   complete ASN.1 DER encoding for the NIST P-256 curve OID is "06 08 2A
   86 48 CE 3D 03 01 07", from which the first entry in the table above
   is constructed by omitting the first two octets.  Only the truncated
   sequence of octets is the valid representation of a curve OID.

9.3.  {9.2} Symmetric-Key Algorithms














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       +-----------+-----------------------------------------------+
       |        ID | Algorithm                                     |
       +-----------+-----------------------------------------------+
       |         0 | Plaintext or unencrypted data                 |
       |         1 | IDEA [IDEA]                                   |
       |         2 | TripleDES (DES-EDE, [SCHNEIER] [HAC]          |
       |           | - 168 bit key derived from 192)               |
       |         3 | CAST5 (128 bit key, as per [RFC2144])         |
       |         4 | Blowfish (128 bit key, 16 rounds) [BLOWFISH]  |
       |         5 | Reserved                                      |
       |         6 | Reserved                                      |
       |         7 | AES with 128-bit key [AES]                    |
       |         8 | AES with 192-bit key                          |
       |         9 | AES with 256-bit key                          |
       |        10 | Twofish with 256-bit key [TWOFISH]            |
       |        11 | Camellia with 128-bit key [RFC3713]           |
       |        12 | Camellia with 192-bit key                     |
       |        13 | Camellia with 256-bit key                     |
       |  100--110 | Private/Experimental algorithm                |
       +-----------+-----------------------------------------------+

   Implementations MUST implement TripleDES.  Implementations SHOULD
   implement AES-128 and CAST5.  Implementations that interoperate with
   PGP 2.6 or earlier need to support IDEA, as that is the only
   symmetric cipher those versions use.  Implementations MAY implement
   any other algorithm.

9.4.  {9.3} Compression Algorithms

              +-----------+---------------------------------+
              |        ID | Algorithm                       |
              +-----------+---------------------------------+
              |         0 | Uncompressed                    |
              |         1 | ZIP [RFC1951]                   |
              |         2 | ZLIB [RFC1950]                  |
              |         3 | BZip2 [BZ2]                     |
              |  100--110 | Private/Experimental algorithm  |
              +-----------+---------------------------------+

   Implementations MUST implement uncompressed data.  Implementations
   SHOULD implement ZIP.  Implementations MAY implement any other
   algorithm.

9.5.  {9.4} Hash Algorithms







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      +-----------+---------------------------------+--------------+
      |        ID | Algorithm                       | Text Name    |
      +-----------+---------------------------------+--------------+
      |         1 | MD5 [HAC]                       | "MD5"        |
      |         2 | SHA-1 [FIPS180]                 | "SHA1"       |
      |         3 | RIPE-MD/160 [HAC]               | "RIPEMD160"  |
      |         4 | Reserved                        |              |
      |         5 | Reserved                        |              |
      |         6 | Reserved                        |              |
      |         7 | Reserved                        |              |
      |         8 | SHA2-256 [FIPS180]              | "SHA256"     |
      |         9 | SHA2-384 [FIPS180]              | "SHA384"     |
      |        10 | SHA2-512 [FIPS180]              | "SHA512"     |
      |        11 | SHA2-224 [FIPS180]              | "SHA224"     |
      |        12 | SHA3-256 [FIPS202]              | "SHA3-256"   |
      |        13 | Reserved                        |              |
      |        14 | SHA3-512 [FIPS202]              | "SHA3-512"   |
      |  100--110 | Private/Experimental algorithm  |              |
      +-----------+---------------------------------+--------------+

   Implementations MUST implement SHA2-256.  Implementations MAY
   implement other algorithms.  Implementations SHOULD NOT create
   messages which require the use of SHA-1 with the exception of
   computing version 4 key fingerprints and for purposes of the MDC
   packet.  Implementations SHOULD NOT use MD5 or RIPE-MD/160.

9.6.  AEAD Algorithms

              +-----------+---------------------------------+
              |        ID | Algorithm                       |
              +-----------+---------------------------------+
              |         1 | EAX [EAX]                       |
              |         2 | OCB [RFC7253]                   |
              |  100--110 | Private/Experimental algorithm  |
              +-----------+---------------------------------+

   Implementations MUST implement EAX.  Implementations MAY implement
   OCB and other algorithms.

10.  {10} IANA Considerations

   OpenPGP is highly parameterized, and consequently there are a number
   of considerations for allocating parameters for extensions.  This
   section describes how IANA should look at extensions to the protocol
   as described in this document.






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10.1.  {10.1} New String-to-Key Specifier Types

   OpenPGP S2K specifiers contain a mechanism for new algorithms to turn
   a string into a key.  This specification creates a registry of S2K
   specifier types.  The registry includes the S2K type, the name of the
   S2K, and a reference to the defining specification.  The initial
   values for this registry can be found in Section 3.7.1.  Adding a new
   S2K specifier MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

10.2.  {10.2} New Packets

   Major new features of OpenPGP are defined through new packet types.
   This specification creates a registry of packet types.  The registry
   includes the packet type, the name of the packet, and a reference to
   the defining specification.  The initial values for this registry can
   be found in Section 4.3.  Adding a new packet type MUST be done
   through the IETF CONSENSUS method, as described in [RFC2434].

10.2.1.  {10.2.1} User Attribute Types

   The User Attribute packet permits an extensible mechanism for other
   types of certificate identification.  This specification creates a
   registry of User Attribute types.  The registry includes the User
   Attribute type, the name of the User Attribute, and a reference to
   the defining specification.  The initial values for this registry can
   be found in Section 5.12.  Adding a new User Attribute type MUST be
   done through the IETF CONSENSUS method, as described in [RFC2434].

   This document requests that IANA register the User ID Attribute Type
   found in Section 5.12.2:

           +--------+------------+-----------------------------+
           | Value  | Attribute  | Reference                   |
           +--------+------------+-----------------------------+
           | TBD1   | User ID    | This Document Section 5.12  |
           +--------+------------+-----------------------------+

10.2.2.  {10.2.1.1} Image Format Subpacket Types

   Within User Attribute packets, there is an extensible mechanism for
   other types of image-based user attributes.  This specification
   creates a registry of Image Attribute subpacket types.  The registry
   includes the Image Attribute subpacket type, the name of the Image
   Attribute subpacket, and a reference to the defining specification.
   The initial values for this registry can be found in Section 5.12.1.
   Adding a new Image Attribute subpacket type MUST be done through the
   IETF CONSENSUS method, as described in [RFC2434].



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10.2.3.  {10.2.2} New Signature Subpackets

   OpenPGP signatures contain a mechanism for signed (or unsigned) data
   to be added to them for a variety of purposes in the Signature
   subpackets as discussed in Section 5.2.3.1.  This specification
   creates a registry of Signature subpacket types.  The registry
   includes the Signature subpacket type, the name of the subpacket, and
   a reference to the defining specification.  The initial values for
   this registry can be found in Section 5.2.3.1.  Adding a new
   Signature subpacket MUST be done through the IETF CONSENSUS method,
   as described in [RFC2434].

10.2.3.1.  {10.2.2.1} Signature Notation Data Subpackets

   OpenPGP signatures further contain a mechanism for extensions in
   signatures.  These are the Notation Data subpackets, which contain a
   key/value pair.  Notations contain a user space that is completely
   unmanaged and an IETF space.

   This specification creates a registry of Signature Notation Data
   types.  The registry includes the Signature Notation Data type, the
   name of the Signature Notation Data, its allowed values, and a
   reference to the defining specification.  The initial values for this
   registry can be found in Section 5.2.3.16.  Adding a new Signature
   Notation Data subpacket MUST be done through the EXPERT REVIEW
   method, as described in [RFC2434].

   This document requests IANA register the following Signature Notation
   Data types:






















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   +---------------+--------+--------------------+---------------------+
   | Allowed       | Name   | Type               | Reference           |
   | Values        |        |                    |                     |
   +---------------+--------+--------------------+---------------------+
   | Any String    | manu   | Manufacturer Name  | This Doc Section    |
   |               |        |                    | 5.2.3.16.1          |
   | Any String    | make   | Product Make       | This Doc Section    |
   |               |        |                    | 5.2.3.16.2          |
   | Any String    | model  | Product Model      | This Doc Section    |
   |               |        |                    | 5.2.3.16.3          |
   | Any String    | prodid | Product ID         | This Doc Section    |
   |               |        |                    | 5.2.3.16.4          |
   | Any String    | pvers  | Product Version    | This Doc Section    |
   |               |        |                    | 5.2.3.16.5          |
   | Any String    | lot    | Product Lot Number | This Doc Section    |
   |               |        |                    | 5.2.3.16.6          |
   | Decimal       | qty    | Package Quantity   | This Doc Section    |
   | Integer       |        |                    | 5.2.3.16.7          |
   | String        |        |                    |                     |
   | A geo: URI    | loc    | Current Geo-       | This Doc Section    |
   |               |        |                    | 5.2.3.16.8          |
   | without the   |        | location           |                     |
   | "geo:"        |        | Latitude/Longitude |                     |
   | A geo: URI    | dest   | Destination Geo-   | This Doc Section    |
   |               |        |                    | 5.2.3.16.8          |
   | without the   |        | location           |                     |
   | "geo:"        |        | Latitude/Longitude |                     |
   | Hash Notation | hash   | The Hash of        | This Doc Section    |
   |               |        |                    | 5.2.3.16.9          |
   | data          |        | external data      |                     |
   +---------------+--------+--------------------+---------------------+

10.2.3.2.  {10.2.2.2} Key Server Preference Extensions

   OpenPGP signatures contain a mechanism for preferences to be
   specified about key servers.  This specification creates a registry
   of key server preferences.  The registry includes the key server
   preference, the name of the preference, and a reference to the
   defining specification.  The initial values for this registry can be
   found in Section 5.2.3.17.  Adding a new key server preference MUST
   be done through the IETF CONSENSUS method, as described in [RFC2434].

10.2.3.3.  {10.2.2.3} Key Flags Extensions

   OpenPGP signatures contain a mechanism for flags to be specified
   about key usage.  This specification creates a registry of key usage
   flags.  The registry includes the key flags value, the name of the
   flag, and a reference to the defining specification.  The initial



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   values for this registry can be found in Section 5.2.3.21.  Adding a
   new key usage flag MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

10.2.3.4.  {10.2.2.4} Reason for Revocation Extensions

   OpenPGP signatures contain a mechanism for flags to be specified
   about why a key was revoked.  This specification creates a registry
   of "Reason for Revocation" flags.  The registry includes the "Reason
   for Revocation" flags value, the name of the flag, and a reference to
   the defining specification.  The initial values for this registry can
   be found in Section 5.2.3.23.  Adding a new feature flag MUST be done
   through the IETF CONSENSUS method, as described in [RFC2434].

10.2.3.5.  {10.2.2.5} Implementation Features

   OpenPGP signatures contain a mechanism for flags to be specified
   stating which optional features an implementation supports.  This
   specification creates a registry of feature-implementation flags.
   The registry includes the feature-implementation flags value, the
   name of the flag, and a reference to the defining specification.  The
   initial values for this registry can be found in Section 5.2.3.24.
   Adding a new feature-implementation flag MUST be done through the
   IETF CONSENSUS method, as described in [RFC2434].

   Also see Section 13.12 for more information about when feature flags
   are needed.

10.2.4.  {10.2.3} New Packet Versions

   The core OpenPGP packets all have version numbers, and can be revised
   by introducing a new version of an existing packet.  This
   specification creates a registry of packet types.  The registry
   includes the packet type, the number of the version, and a reference
   to the defining specification.  The initial values for this registry
   can be found in Section 5.  Adding a new packet version MUST be done
   through the IETF CONSENSUS method, as described in [RFC2434].

10.3.  {10.3} New Algorithms

   Section 9 lists the core algorithms that OpenPGP uses.  Adding in a
   new algorithm is usually simple.  For example, adding in a new
   symmetric cipher usually would not need anything more than allocating
   a constant for that cipher.  If that cipher had other than a 64-bit
   or 128-bit block size, there might need to be additional
   documentation describing how OpenPGP-CFB mode would be adjusted.
   Similarly, when DSA was expanded from a maximum of 1024-bit public
   keys to 3072-bit public keys, the revision of FIPS 186 contained



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   enough information itself to allow implementation.  Changes to this
   document were made mainly for emphasis.

10.3.1.  {10.3.1} Public-Key Algorithms

   OpenPGP specifies a number of public-key algorithms.  This
   specification creates a registry of public-key algorithm identifiers.
   The registry includes the algorithm name, its key sizes and
   parameters, and a reference to the defining specification.  The
   initial values for this registry can be found in Section 9.  Adding a
   new public-key algorithm MUST be done through the IETF CONSENSUS
   method, as described in [RFC2434].

   This document requests IANA register the following public-key
   algorithm:

            +-----+-----------------------------+------------+
            | ID  | Algorithm                   | Reference  |
            +-----+-----------------------------+------------+
            | 22  | EdDSA public key algorithm  | This doc   |
            +-----+-----------------------------+------------+

   [Notes to RFC-Editor: Please remove the table above on publication.
   It is desirable not to reuse old or reserved algorithms because some
   existing tools might print a wrong description.  A higher number is
   also an indication for a newer algorithm.  As of now 22 is the next
   free number.]

10.3.2.  {10.3.2} Symmetric-Key Algorithms

   OpenPGP specifies a number of symmetric-key algorithms.  This
   specification creates a registry of symmetric-key algorithm
   identifiers.  The registry includes the algorithm name, its key sizes
   and block size, and a reference to the defining specification.  The
   initial values for this registry can be found in Section 9.  Adding a
   new symmetric-key algorithm MUST be done through the IETF CONSENSUS
   method, as described in [RFC2434].

10.3.3.  {10.3.3} Hash Algorithms

   OpenPGP specifies a number of hash algorithms.  This specification
   creates a registry of hash algorithm identifiers.  The registry
   includes the algorithm name, a text representation of that name, its
   block size, an OID hash prefix, and a reference to the defining
   specification.  The initial values for this registry can be found in
   Section 9 for the algorithm identifiers and text names, and
   Section 5.2.2 for the OIDs and expanded signature prefixes.  Adding a




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   new hash algorithm MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

   This document requests IANA register the following hash algorithms:

                     +-----+------------+------------+
                     | ID  | Algorithm  | Reference  |
                     +-----+------------+------------+
                     | 12  | SHA3-256   | This doc   |
                     | 13  | Reserved   |            |
                     | 14  | SHA3-512   | This doc   |
                     +-----+------------+------------+

   [Notes to RFC-Editor: Please remove the table above on publication.
   It is desirable not to reuse old or reserved algorithms because some
   existing tools might print a wrong description.  The ID 13 has been
   reserved so that the SHA3 algorithm IDs align nicely with their SHA2
   counterparts.]

10.3.4.  {10.3.4} Compression Algorithms

   OpenPGP specifies a number of compression algorithms.  This
   specification creates a registry of compression algorithm
   identifiers.  The registry includes the algorithm name and a
   reference to the defining specification.  The initial values for this
   registry can be found in Section 9.3.  Adding a new compression key
   algorithm MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

11.  {11} Packet Composition

   OpenPGP packets are assembled into sequences in order to create
   messages and to transfer keys.  Not all possible packet sequences are
   meaningful and correct.  This section describes the rules for how
   packets should be placed into sequences.

11.1.  {11.1} Transferable Public Keys

   OpenPGP users may transfer public keys.  The essential elements of a
   transferable public key are as follows:

   o  One Public-Key packet

   o  Zero or more revocation signatures

   o  Zero or more User ID packets





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   o  After each User ID packet, zero or more Signature packets
      (certifications)

   o  Zero or more User Attribute packets

   o  After each User Attribute packet, zero or more Signature packets
      (certifications)

   o  Zero or more Subkey packets

   o  After each Subkey packet, one Signature packet, plus optionally a
      revocation

   The Public-Key packet occurs first.  Each of the following User ID
   packets provides the identity of the owner of this public key.  If
   there are multiple User ID packets, this corresponds to multiple
   means of identifying the same unique individual user; for example, a
   user may have more than one email address, and construct a User ID
   for each one.

   Immediately following each User ID packet, there are zero or more
   Signature packets.  Each Signature packet is calculated on the
   immediately preceding User ID packet and the initial Public-Key
   packet.  The signature serves to certify the corresponding public key
   and User ID.  In effect, the signer is testifying to his or her
   belief that this public key belongs to the user identified by this
   User ID.

   Within the same section as the User ID packets, there are zero or
   more User Attribute packets.  Like the User ID packets, a User
   Attribute packet is followed by zero or more Signature packets
   calculated on the immediately preceding User Attribute packet and the
   initial Public-Key packet.

   User Attribute packets and User ID packets may be freely intermixed
   in this section, so long as the signatures that follow them are
   maintained on the proper User Attribute or User ID packet.

   After the User ID packet or Attribute packet, there may be zero or
   more Subkey packets.  In general, subkeys are provided in cases where
   the top-level public key is a signature-only key.  However, any V4
   key may have subkeys, and the subkeys may be encryption-only keys,
   signature-only keys, or general-purpose keys.  V3 keys MUST NOT have
   subkeys.

   Each Subkey packet MUST be followed by one Signature packet, which
   should be a subkey binding signature issued by the top-level key.
   For subkeys that can issue signatures, the subkey binding signature



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   MUST contain an Embedded Signature subpacket with a primary key
   binding signature (0x19) issued by the subkey on the top-level key.

   Subkey and Key packets may each be followed by a revocation Signature
   packet to indicate that the key is revoked.  Revocation signatures
   are only accepted if they are issued by the key itself, or by a key
   that is authorized to issue revocations via a Revocation Key
   subpacket in a self-signature by the top-level key.

   Transferable public-key packet sequences may be concatenated to allow
   transferring multiple public keys in one operation.

11.2.  {11.2} Transferable Secret Keys

   OpenPGP users may transfer secret keys.  The format of a transferable
   secret key is the same as a transferable public key except that
   secret-key and secret-subkey packets are used instead of the public
   key and public-subkey packets.  Implementations SHOULD include self-
   signatures on any user IDs and subkeys, as this allows for a complete
   public key to be automatically extracted from the transferable secret
   key.  Implementations MAY choose to omit the self-signatures,
   especially if a transferable public key accompanies the transferable
   secret key.

11.3.  {11.3} OpenPGP Messages

   An OpenPGP message is a packet or sequence of packets that
   corresponds to the following grammatical rules (comma represents
   sequential composition, and vertical bar separates alternatives):






















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   OpenPGP Message :- Encrypted Message | Signed Message |
                      Compressed Message | Literal Message.

   Compressed Message :- Compressed Data Packet.

   Literal Message :- Literal Data Packet.

   ESK :- Public-Key Encrypted Session Key Packet |
          Symmetric-Key Encrypted Session Key Packet.

   ESK Sequence :- ESK | ESK Sequence, ESK.

   Encrypted Data :- Symmetrically Encrypted Data Packet |
       Symmetrically Encrypted Integrity Protected Data Packet

   Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.

   One-Pass Signed Message :- One-Pass Signature Packet,
               OpenPGP Message, Corresponding Signature Packet.

   Signed Message :- Signature Packet, OpenPGP Message |
                     One-Pass Signed Message.

   In addition, decrypting a Symmetrically Encrypted Data packet or a
   Symmetrically Encrypted Integrity Protected Data packet as well as
   decompressing a Compressed Data packet must yield a valid OpenPGP
   Message.

11.4.  {11.4} Detached Signatures

   Some OpenPGP applications use so-called "detached signatures".  For
   example, a program bundle may contain a file, and with it a second
   file that is a detached signature of the first file.  These detached
   signatures are simply a Signature packet stored separately from the
   data for which they are a signature.

12.  {12} Enhanced Key Formats

12.1.  {12.1} Key Structures

   The format of an OpenPGP V3 key is as follows.  Entries in square
   brackets are optional and ellipses indicate repetition.

   RSA Public Key
      [Revocation Self Signature]
       User ID [Signature ...]
      [User ID [Signature ...] ...]




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   Each signature certifies the RSA public key and the preceding User
   ID.  The RSA public key can have many User IDs and each User ID can
   have many signatures.  V3 keys are deprecated.  Implementations MUST
   NOT generate new V3 keys, but MAY continue to use existing ones.

   The format of an OpenPGP V4 key that uses multiple public keys is
   similar except that the other keys are added to the end as "subkeys"
   of the primary key.

   Primary-Key
      [Revocation Self Signature]
      [Direct Key Signature...]
      [User ID [Signature ...] ...]
      [User Attribute [Signature ...] ...]
      [[Subkey [Binding-Signature-Revocation]
              Primary-Key-Binding-Signature] ...]

   A subkey always has a single signature after it that is issued using
   the primary key to tie the two keys together.  This binding signature
   may be in either V3 or V4 format, but SHOULD be V4.  Subkeys that can
   issue signatures MUST have a V4 binding signature due to the REQUIRED
   embedded primary key binding signature.

   In the above diagram, if the binding signature of a subkey has been
   revoked, the revoked key may be removed, leaving only one key.

   In a V4 key, the primary key SHOULD be a key capable of
   certification.  There are cases, such as device certificates, where
   the primary key may not be capable of certification.  A primary key
   capable of making signatures SHOULD be accompanied by either a
   certification signature (on a User ID or User Attribute) or a
   signature directly on the key.

   Implementations SHOULD accept encryption-only primary keys without a
   signature.  It also SHOULD allow importing any key accompanied either
   by a certification signature or a signature on itself.  It MAY accept
   signature-capable primary keys without an accompanying signature.

   The subkeys may be keys of any other type.  There may be other
   constructions of V4 keys, too.  For example, there may be a single-
   key RSA key in V4 format, a DSA primary key with an RSA encryption
   key, or RSA primary key with an Elgamal subkey, etc.

   It is also possible to have a signature-only subkey.  This permits a
   primary key that collects certifications (key signatures), but is
   used only for certifying subkeys that are used for encryption and
   signatures.




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12.2.  {12.2} Key IDs and Fingerprints

   For a V3 key, the eight-octet Key ID consists of the low 64 bits of
   the public modulus of the RSA key.

   The fingerprint of a V3 key is formed by hashing the body (but not
   the two-octet length) of the MPIs that form the key material (public
   modulus n, followed by exponent e) with MD5.  Note that both V3 keys
   and MD5 are deprecated.

   A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
   followed by the two-octet packet length, followed by the entire
   Public-Key packet starting with the version field.  The Key ID is the
   low-order 64 bits of the fingerprint.  Here are the fields of the
   hash material, with the example of a DSA key:

  a.1) 0x99 (1 octet)

  a.2) two-octet scalar octet count of (b)-(e)

  b) version number = 4 (1 octet);

  c) timestamp of key creation (4 octets);

  d) algorithm (1 octet): 17 = DSA (example);

  e) Algorithm-specific fields.

  Algorithm-Specific Fields for DSA keys (example):

  e.1) MPI of DSA prime p;

  e.2) MPI of DSA group order q (q is a prime divisor of p-1);

  e.3) MPI of DSA group generator g;

  e.4) MPI of DSA public-key value y (= g\*\*x mod p where x is secret).

   A V5 fingerprint is the 256-bit SHA2-256 hash of the octet 0x99,
   followed by the four-octet packet length, followed by the entire
   Public-Key packet starting with the version field.  The Key ID is the
   high-order 64 bits of the fingerprint.  Here are the fields of the
   hash material, with the example of a DSA key:








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  a.1) 0x9A (1 octet)

  a.2) four-octet scalar octet count of (b)-(f)

  b) version number = 5 (1 octet);

  c) timestamp of key creation (4 octets);

  d) algorithm (1 octet): 17 = DSA (example);

  e) four-octet scalar octet count for the following key material;

  f) algorithm-specific fields.

  Algorithm-Specific Fields for DSA keys (example):

  f.1) MPI of DSA prime p;

  f.2) MPI of DSA group order q (q is a prime divisor of p-1);

  f.3) MPI of DSA group generator g;

  f.4) MPI of DSA public-key value y (= g\*\*x mod p where x is secret).

   Note that it is possible for there to be collisions of Key IDs -- two
   different keys with the same Key ID.  Note that there is a much
   smaller, but still non-zero, probability that two different keys have
   the same fingerprint.

   Also note that if V3, V4, and V5 format keys share the same RSA key
   material, they will have different Key IDs as well as different
   fingerprints.

   Finally, the Key ID and fingerprint of a subkey are calculated in the
   same way as for a primary key, including the 0x99 (V3 and V4 key) or
   0x9A (V5 key) as the first octet (even though this is not a valid
   packet ID for a public subkey).

13.  Elliptic Curve Cryptography

   This section descripes algorithms and parameters used with Elliptic
   Curve Cryptography (ECC) keys.  A thorough introduction to ECC can be
   found in [KOBLITZ].








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13.1.  Supported ECC Curves

   This document references five named prime field curves, defined in
   [FIPS186] as "Curve P-256", "Curve P-384", and "Curve P-521"; and
   defined in [RFC5639] as "brainpoolP256r1", and "brainpoolP512r1".
   Further curve "Curve25519", defined in [RFC7748] is referenced for
   use with Ed25519 (EdDSA signing) and X25519 (encryption).

   The named curves are referenced as a sequence of bytes in this
   document, called throughout, curve OID.  Section 9.2 describes in
   detail how this sequence of bytes is formed.

13.2.  ECDSA and ECDH Conversion Primitives

   This document defines the uncompressed point format for ECDSA and
   ECDH and a custom compression format for certain curves.  The point
   is encoded in the Multiprecision Integer (MPI) format.

   For an uncompressed point the content of the MPI is:

   B = 04 || x || y

   where x and y are coordinates of the point P = (x, y), each encoded
   in the big-endian format and zero-padded to the adjusted underlying
   field size.  The adjusted underlying field size is the underlying
   field size that is rounded up to the nearest 8-bit boundary.  This
   encoding is compatible with the definition given in [SEC1].

   For a custom compressed point the content of the MPI is:

   B = 40 || x

   where x is the x coordinate of the point P encoded to the rules
   defined for the specified curve.  This format is used for ECDH keys
   based on curves expressed in Montgomery form.

   Therefore, the exact size of the MPI payload is 515 bits for "Curve
   P-256", 771 for "Curve P-384", 1059 for "Curve P-521", and 263 for
   Curve25519.

   Even though the zero point, also called the point at infinity, may
   occur as a result of arithmetic operations on points of an elliptic
   curve, it SHALL NOT appear in data structures defined in this
   document.

   If other conversion methods are defined in the future, a compliant
   application MUST NOT use a new format when in doubt that any
   recipient can support it.  Consider, for example, that while both the



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   public key and the per-recipient ECDH data structure, respectively
   defined in Sections 9{FIXME} and 10{FIXME}, contain an encoded point
   field, the format changes to the field in Section 10{FIXME} only
   affect a given recipient of a given message.

13.3.  EdDSA Point Format

   The EdDSA algorithm defines a specific point compression format.  To
   indicate the use of this compression format and to make sure that the
   key can be represented in the Multiprecision Integer (MPI) format the
   octet string specifying the point is prefixed with the octet 0x40.
   This encoding is an extension of the encoding given in [SEC1] which
   uses 0x04 to indicate an uncompressed point.

   For example, the length of a public key for the curve Ed25519 is 263
   bit: 7 bit to represent the 0x40 prefix octet and 32 octets for the
   native value of the public key.

13.4.  Key Derivation Function

   A key derivation function (KDF) is necessary to implement the EC
   encryption.  The Concatenation Key Derivation Function (Approved
   Alternative 1) [SP800-56A] with the KDF hash function that is
   SHA2-256 [FIPS180] or stronger is REQUIRED.  See Section 12{FIXME}
   for the details regarding the choice of the hash function.

   For convenience, the synopsis of the encoding method is given below
   with significant simplifications attributable to the restricted
   choice of hash functions in this document.  However, [SP800-56A] is
   the normative source of the definition.

   //   Implements KDF( X, oBits, Param );
   //   Input: point X = (x,y)
   //   oBits - the desired size of output
   //   hBits - the size of output of hash function Hash
   //   Param - octets representing the parameters
   //   Assumes that oBits <= hBits
   // Convert the point X to the octet string, see section 6{FIXME}:
   //   ZB' = 04 || x || y
   // and extract the x portion from ZB'
   ZB = x;
   MB = Hash ( 00 || 00 || 00 || 01 || ZB || Param );
   return oBits leftmost bits of MB.

   Note that ZB in the KDF description above is the compact
   representation of X, defined in Section 4.2 of [RFC6090].





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13.5.  EC DH Algorithm (ECDH)

   The method is a combination of an ECC Diffie-Hellman method to
   establish a shared secret, a key derivation method to process the
   shared secret into a derived key, and a key wrapping method that uses
   the derived key to protect a session key used to encrypt a message.

   The One-Pass Diffie-Hellman method C(1, 1, ECC CDH) [SP800-56A] MUST
   be implemented with the following restrictions: the ECC CDH primitive
   employed by this method is modified to always assume the cofactor as
   1, the KDF specified in Section 7 is used, and the KDF parameters
   specified below are used.

   The KDF parameters are encoded as a concatenation of the following 5
   variable-length and fixed-length fields, compatible with the
   definition of the OtherInfo bitstring [SP800-56A]:

   o  a variable-length field containing a curve OID, formatted as
      follows:

      *  a one-octet size of the following field

      *  the octets representing a curve OID, defined in Section 11

   o  a one-octet public key algorithm ID defined in Section 5

   o  a variable-length field containing KDF parameters, identical to
      the corresponding field in the ECDH public key, formatted as
      follows:

      *  a one-octet size of the following fields; values 0 and 0xff are
         reserved for future extensions

      *  a one-octet value 01, reserved for future extensions

      *  a one-octet hash function ID used with the KDF

      *  a one-octet algorithm ID for the symmetric algorithm used to
         wrap the symmetric key for message encryption; see Section 8
         for details

   o  20 octets representing the UTF-8 encoding of the string "Anonymous
      Sender ", which is the octet sequence 41 6E 6F 6E 79 6D 6F 75 73
      20 53 65 6E 64 65 72 20 20 20 20

   o  20 octets representing a recipient encryption subkey or a master
      key fingerprint, identifying the key material that is needed for




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      the decryption.  For version 5 keys the 20 leftmost octets of the
      fingerprint are used.

   The size of the KDF parameters sequence, defined above, is either 54
   for the NIST curve P-256, 51 for the curves P-384 and P-521, or 56
   for Curve25519.

   The key wrapping method is described in [RFC3394].  KDF produces a
   symmetric key that is used as a key-encryption key (KEK) as specified
   in [RFC3394].  Refer to Section 13{FIXME} for the details regarding
   the choice of the KEK algorithm, which SHOULD be one of three AES
   algorithms.  Key wrapping and unwrapping is performed with the
   default initial value of [RFC3394].

   The input to the key wrapping method is the value "m" derived from
   the session key, as described in Section 5.1{FIXME}, "Public-Key
   Encrypted Session Key Packets (Tag 1)", except that the PKCS #1.5
   padding step is omitted.  The result is padded using the method
   described in [PKCS5] to the 8-byte granularity.  For example, the
   following AES-256 session key, in which 32 octets are denoted from k0
   to k31, is composed to form the following 40 octet sequence:

   09 k0 k1 ... k31 c0 c1 05 05 05 05 05

   The octets c0 and c1 above denote the checksum.  This encoding allows
   the sender to obfuscate the size of the symmetric encryption key used
   to encrypt the data.  For example, assuming that an AES algorithm is
   used for the session key, the sender MAY use 21, 13, and 5 bytes of
   padding for AES-128, AES-192, and AES-256, respectively, to provide
   the same number of octets, 40 total, as an input to the key wrapping
   method.

   The output of the method consists of two fields.  The first field is
   the MPI containing the ephemeral key used to establish the shared
   secret.  The second field is composed of the following two fields:

   o  a one-octet encoding the size in octets of the result of the key
      wrapping method; the value 255 is reserved for future extensions;

   o  up to 254 octets representing the result of the key wrapping
      method, applied to the 8-byte padded session key, as described
      above.

   Note that for session key sizes 128, 192, and 256 bits, the size of
   the result of the key wrapping method is, respectively, 32, 40, and
   48 octets, unless the size obfuscation is used.





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   For convenience, the synopsis of the encoding method is given below;
   however, this section, [SP800-56A], and [RFC3394] are the normative
   sources of the definition.

   Obtain the authenticated recipient public key R
   Generate an ephemeral key pair {v, V=vG}
   Compute the shared point S = vR;
   m = symm_alg_ID || session key || checksum || pkcs5_padding;
   curve_OID_len = (byte)len(curve_OID);
   Param = curve_OID_len || curve_OID || public_key_alg_ID || 03
   || 01 || KDF_hash_ID || KEK_alg_ID for AESKeyWrap || "Anonymous
   Sender    " || recipient_fingerprint;
   Z_len = the key size for the KEK_alg_ID used with AESKeyWrap
   Compute Z = KDF( S, Z_len, Param );
   Compute C = AESKeyWrap( Z, m ) as per [RFC3394]
   VB = convert point V to the octet string
   Output (MPI(VB) || len(C) || C).

   The decryption is the inverse of the method given.  Note that the
   recipient obtains the shared secret by calculating

   S = rV = rvG, where (r,R) is the recipient's key pair.

   Consistent with Section 5.13{FIXME}, "Sym.  Encrypted Integrity
   Protected Data Packet (Tag 18)", a Modification Detection Code (MDC)
   MUST be used anytime the symmetric key is protected by ECDH.

14.  {13} Notes on Algorithms

14.1.  {13.1} PKCS#1 Encoding in OpenPGP

   This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and
   EMSA-PKCS1-v1_5.  However, the calling conventions of these functions
   has changed in the past.  To avoid potential confusion and
   interoperability problems, we are including local copies in this
   document, adapted from those in PKCS#1 v2.1 [RFC3447].  RFC 3447
   should be treated as the ultimate authority on PKCS#1 for OpenPGP.
   Nonetheless, we believe that there is value in having a self-
   contained document that avoids problems in the future with needed
   changes in the conventions.

14.1.1.  {13.1.1} EME-PKCS1-v1_5-ENCODE









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   Input:

   k = the length in octets of the key modulus

   M = message to be encoded, an octet string of length mLen, where mLen
   \<= k - 11

   Output:

   EM = encoded message, an octet string of length k

   Error: "message too long"

    1. Length checking: If mLen > k - 11, output "message too long"
       and stop.

    2. Generate an octet string PS of length k - mLen - 3 consisting
       of pseudo-randomly generated nonzero octets.  The length of PS
       will be at least eight octets.

    3. Concatenate PS, the message M, and other padding to form an
       encoded message EM of length k octets as

       EM = 0x00 || 0x02 || PS || 0x00 || M.

    4. Output EM.

14.1.2.  {13.1.2} EME-PKCS1-v1_5-DECODE

   Input:

   EM = encoded message, an octet string

   Output:

   M = message, an octet string

   Error: "decryption error"

   To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
   into an octet string PS consisting of nonzero octets and a message M
   as follows

    EM = 0x00 || 0x02 || PS || 0x00 || M.

   If the first octet of EM does not have hexadecimal value 0x00, if the
   second octet of EM does not have hexadecimal value 0x02, if there is
   no octet with hexadecimal value 0x00 to separate PS from M, or if the



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   length of PS is less than 8 octets, output "decryption error" and
   stop.  See also the security note in Section 14 regarding differences
   in reporting between a decryption error and a padding error.

14.1.3.  {13.1.3} EMSA-PKCS1-v1_5

   This encoding method is deterministic and only has an encoding
   operation.

   Option:

   Hash - a hash function in which hLen denotes the length in octets
          of the hash function output

   Input:

   M = message to be encoded

   emLen = intended length in octets of the encoded message, at least
        tLen + 11, where tLen is the octet length of the DER encoding
        T of a certain value computed during the encoding operation

   Output:

   EM = encoded message, an octet string of length emLen

   Errors: "message too long";
           "intended encoded message length too short"

   Steps:





















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    1. Apply the hash function to the message M to produce a hash
       value H:

       H = Hash(M).

       If the hash function outputs "message too long," output
       "message too long" and stop.

    2. Using the list in Section 5.2.2, produce an ASN.1 DER value
       for the hash function used.  Let T be the full hash prefix
       from Section 5.2.2, and let tLen be the length in octets of T.

    3. If emLen < tLen + 11, output "intended encoded message length
       too short" and stop.

    4. Generate an octet string PS consisting of emLen - tLen - 3
       octets with hexadecimal value 0xFF.  The length of PS will be
       at least 8 octets.

    5. Concatenate PS, the hash prefix T, and other padding to form
       the encoded message EM as

           EM = 0x00 || 0x01 || PS || 0x00 || T.

    6. Output EM.

14.2.  {13.2} Symmetric Algorithm Preferences

   The symmetric algorithm preference is an ordered list of algorithms
   that the keyholder accepts.  Since it is found on a self-signature,
   it is possible that a keyholder may have multiple, different
   preferences.  For example, Alice may have TripleDES only specified
   for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for
   "alice@home.org".  Note that it is also possible for preferences to
   be in a subkey's binding signature.

   Since TripleDES is the MUST-implement algorithm, if it is not
   explicitly in the list, it is tacitly at the end.  However, it is
   good form to place it there explicitly.  Note also that if an
   implementation does not implement the preference, then it is
   implicitly a TripleDES-only implementation.

   An implementation MUST NOT use a symmetric algorithm that is not in
   the recipient's preference list.  When encrypting to more than one
   recipient, the implementation finds a suitable algorithm by taking
   the intersection of the preferences of the recipients.  Note that the
   MUST-implement algorithm, TripleDES, ensures that the intersection is




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   not null.  The implementation may use any mechanism to pick an
   algorithm in the intersection.

   If an implementation can decrypt a message that a keyholder doesn't
   have in their preferences, the implementation SHOULD decrypt the
   message anyway, but MUST warn the keyholder that the protocol has
   been violated.  For example, suppose that Alice, above, has software
   that implements all algorithms in this specification.  Nonetheless,
   she prefers subsets for work or home.  If she is sent a message
   encrypted with IDEA, which is not in her preferences, the software
   warns her that someone sent her an IDEA-encrypted message, but it
   would ideally decrypt it anyway.

14.3.  {13.3} Other Algorithm Preferences

   Other algorithm preferences work similarly to the symmetric algorithm
   preference, in that they specify which algorithms the keyholder
   accepts.  There are two interesting cases that other comments need to
   be made about, though, the compression preferences and the hash
   preferences.

14.3.1.  {13.3.1} Compression Preferences

   Compression has been an integral part of PGP since its first days.
   OpenPGP and all previous versions of PGP have offered compression.
   In this specification, the default is for messages to be compressed,
   although an implementation is not required to do so.  Consequently,
   the compression preference gives a way for a keyholder to request
   that messages not be compressed, presumably because they are using a
   minimal implementation that does not include compression.
   Additionally, this gives a keyholder a way to state that it can
   support alternate algorithms.

   Like the algorithm preferences, an implementation MUST NOT use an
   algorithm that is not in the preference vector.  If the preferences
   are not present, then they are assumed to be [ZIP(1),
   Uncompressed(0)].

   Additionally, an implementation MUST implement this preference to the
   degree of recognizing when to send an uncompressed message.  A robust
   implementation would satisfy this requirement by looking at the
   recipient's preference and acting accordingly.  A minimal
   implementation can satisfy this requirement by never generating a
   compressed message, since all implementations can handle messages
   that have not been compressed.






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14.3.2.  {13.3.2} Hash Algorithm Preferences

   Typically, the choice of a hash algorithm is something the signer
   does, rather than the verifier, because a signer rarely knows who is
   going to be verifying the signature.  This preference, though, allows
   a protocol based upon digital signatures ease in negotiation.

   Thus, if Alice is authenticating herself to Bob with a signature, it
   makes sense for her to use a hash algorithm that Bob's software uses.
   This preference allows Bob to state in his key which algorithms Alice
   may use.

   Since SHA2-256 is the MUST-implement hash algorithm, if it is not
   explicitly in the list, it is tacitly at the end.  However, it is
   good form to place it there explicitly.

14.4.  {13.4} Plaintext

   Algorithm 0, "plaintext", may only be used to denote secret keys that
   are stored in the clear.  Implementations MUST NOT use plaintext in
   Symmetrically Encrypted Data packets; they must use Literal Data
   packets to encode unencrypted or literal data.

14.5.  {13.5} RSA

   There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only
   keys.  These types are deprecated.  The "key flags" subpacket in a
   signature is a much better way to express the same idea, and
   generalizes it to all algorithms.  An implementation SHOULD NOT
   create such a key, but MAY interpret it.

   An implementation SHOULD NOT implement RSA keys of size less than
   1024 bits.

14.6.  {13.6} DSA

   An implementation SHOULD NOT implement DSA keys of size less than
   1024 bits.  It MUST NOT implement a DSA key with a q size of less
   than 160 bits.  DSA keys MUST also be a multiple of 64 bits, and the
   q size MUST be a multiple of 8 bits.  The Digital Signature Standard
   (DSS) [FIPS186] specifies that DSA be used in one of the following
   ways:

   o  1024-bit key, 160-bit q, SHA-1, SHA2--224, SHA2-256, SHA2-384, or
      SHA2-512 hash

   o  2048-bit key, 224-bit q, SHA2-224, SHA2-256, SHA2-384, or SHA2-512
      hash



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   o  2048-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 hash

   o  3072-bit key, 256-bit q, SHA2-256, SHA2-384, or SHA2-512 hash

   The above key and q size pairs were chosen to best balance the
   strength of the key with the strength of the hash.  Implementations
   SHOULD use one of the above key and q size pairs when generating DSA
   keys.  If DSS compliance is desired, one of the specified SHA hashes
   must be used as well.  [FIPS186] is the ultimate authority on DSS,
   and should be consulted for all questions of DSS compliance.

   Note that earlier versions of this standard only allowed a 160-bit q
   with no truncation allowed, so earlier implementations may not be
   able to handle signatures with a different q size or a truncated
   hash.

14.7.  {13.7} Elgamal

   An implementation SHOULD NOT implement Elgamal keys of size less than
   1024 bits.

14.8.  EdDSA

   Although the EdDSA algorithm allows arbitrary data as input, its use
   with OpenPGP requires that a digest of the message is used as input
   (pre-hashed).  See section XXXXX, "Computing Signatures" for details.
   Truncation of the resulting digest is never applied; the resulting
   digest value is used verbatim as input to the EdDSA algorithm.

14.9.  {13.8} Reserved Algorithm Numbers

   A number of algorithm IDs have been reserved for algorithms that
   would be useful to use in an OpenPGP implementation, yet there are
   issues that prevent an implementer from actually implementing the
   algorithm.  These are marked in Section 9.1, "Public-Key Algorithms",
   as "reserved for".

   The reserved public-key algorithm X9.42 (21) does not have the
   necessary parameters, parameter order, or semantics defined.

   Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
   with a public-key identifier of 20.  These are no longer permitted.
   An implementation MUST NOT generate such keys.  An implementation
   MUST NOT generate Elgamal signatures.  See [BLEICHENBACHER].







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14.10.  {13.9} OpenPGP CFB Mode

   OpenPGP does symmetric encryption using a variant of Cipher Feedback
   mode (CFB mode).  This section describes the procedure it uses in
   detail.  This mode is what is used for Symmetrically Encrypted Data
   Packets; the mechanism used for encrypting secret-key material is
   similar, and is described in the sections above.

   In the description below, the value BS is the block size in octets of
   the cipher.  Most ciphers have a block size of 8 octets.  The AES and
   Twofish have a block size of 16 octets.  Also note that the
   description below assumes that the IV and CFB arrays start with an
   index of 1 (unlike the C language, which assumes arrays start with a
   zero index).

   OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and
   prefixes the plaintext with BS+2 octets of random data, such that
   octets BS+1 and BS+2 match octets BS-1 and BS.  It does a CFB
   resynchronization after encrypting those BS+2 octets.

   Thus, for an algorithm that has a block size of 8 octets (64 bits),
   the IV is 10 octets long and octets 7 and 8 of the IV are the same as
   octets 9 and 10.  For an algorithm with a block size of 16 octets
   (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
   octets 15 and 16.  Those extra two octets are an easy check for a
   correct key.

   Step by step, here is the procedure:

   1.   The feedback register (FR) is set to the IV, which is all zeros.

   2.   FR is encrypted to produce FRE (FR Encrypted).  This is the
        encryption of an all-zero value.

   3.   FRE is xored with the first BS octets of random data prefixed to
        the plaintext to produce C[1] through C[BS], the first BS octets
        of ciphertext.

   4.   FR is loaded with C[1] through C[BS].

   5.   FR is encrypted to produce FRE, the encryption of the first BS
        octets of ciphertext.

   6.   The left two octets of FRE get xored with the next two octets of
        data that were prefixed to the plaintext.  This produces C[BS+1]
        and C[BS+2], the next two octets of ciphertext.





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   7.   (The resynchronization step) FR is loaded with C[3] through
        C[BS+2].

   8.   FRE is xored with the first BS octets of the given plaintext,
        now that we have finished encrypting the BS+2 octets of prefixed
        data.  This produces C[BS+3] through C[BS+(BS+2)], the next BS
        octets of ciphertext.

   9.   FR is encrypted to produce FRE.

   10.  FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18
        for an 8-octet block).

   11.  FR is encrypted to produce FRE.

   12.  FRE is xored with the next BS octets of plaintext, to produce
        the next BS octets of ciphertext.  These are loaded into FR, and
        the process is repeated until the plaintext is used up.

14.11.  {13.10} Private or Experimental Parameters

   S2K specifiers, Signature subpacket types, user attribute types,
   image format types, and algorithms described in Section 9 all reserve
   the range 100 to 110 for private and experimental use.  Packet types
   reserve the range 60 to 63 for private and experimental use.  These
   are intentionally managed with the PRIVATE USE method, as described
   in [RFC2434].

   However, implementations need to be careful with these and promote
   them to full IANA-managed parameters when they grow beyond the
   original, limited system.

14.12.  {13.12} Meta-Considerations for Expansion

   If OpenPGP is extended in a way that is not backwards-compatible,
   meaning that old implementations will not gracefully handle their
   absence of a new feature, the extension proposal can be declared in
   the key holder's self-signature as part of the Features signature
   subpacket.

   We cannot state definitively what extensions will not be upwards-
   compatible, but typically new algorithms are upwards-compatible,
   whereas new packets are not.

   If an extension proposal does not update the Features system, it
   SHOULD include an explanation of why this is unnecessary.  If the
   proposal contains neither an extension to the Features system nor an




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   explanation of why such an extension is unnecessary, the proposal
   SHOULD be rejected.

15.  {14} Security Considerations

   o  As with any technology involving cryptography, you should check
      the current literature to determine if any algorithms used here
      have been found to be vulnerable to attack.

   o  This specification uses Public-Key Cryptography technologies.  It
      is assumed that the private key portion of a public-private key
      pair is controlled and secured by the proper party or parties.

   o  Certain operations in this specification involve the use of random
      numbers.  An appropriate entropy source should be used to generate
      these numbers (see [RFC4086]).

   o  The MD5 hash algorithm has been found to have weaknesses, with
      collisions found in a number of cases.  MD5 is deprecated for use
      in OpenPGP.  Implementations MUST NOT generate new signatures
      using MD5 as a hash function.  They MAY continue to consider old
      signatures that used MD5 as valid.

   o  SHA2-224 and SHA2-384 require the same work as SHA2-256 and
      SHA2-512, respectively.  In general, there are few reasons to use
      them outside of DSS compatibility.  You need a situation where one
      needs more security than smaller hashes, but does not want to have
      the full 256-bit or 512-bit data length.

   o  Many security protocol designers think that it is a bad idea to
      use a single key for both privacy (encryption) and integrity
      (signatures).  In fact, this was one of the motivating forces
      behind the V4 key format with separate signature and encryption
      keys.  If you as an implementer promote dual-use keys, you should
      at least be aware of this controversy.

   o  The DSA algorithm will work with any hash, but is sensitive to the
      quality of the hash algorithm.  Verifiers should be aware that
      even if the signer used a strong hash, an attacker could have
      modified the signature to use a weak one.  Only signatures using
      acceptably strong hash algorithms should be accepted as valid.

   o  As OpenPGP combines many different asymmetric, symmetric, and hash
      algorithms, each with different measures of strength, care should
      be taken that the weakest element of an OpenPGP message is still
      sufficiently strong for the purpose at hand.  While consensus
      about the strength of a given algorithm may evolve, NIST Special




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      Publication 800-57 [SP800-57] recommends the following list of
      equivalent strengths:

        Asymmetric  |  Hash  |  Symmetric
         key size   |  size  |   key size
        ------------+--------+-----------
           1024        160         80
           2048        224        112
           3072        256        128
           7680        384        192
          15360        512        256

   o  There is a somewhat-related potential security problem in
      signatures.  If an attacker can find a message that hashes to the
      same hash with a different algorithm, a bogus signature structure
      can be constructed that evaluates correctly.

      For example, suppose Alice DSA signs message M using hash
      algorithm H.  Suppose that Mallet finds a message M' that has the
      same hash value as M with H'.  Mallet can then construct a
      signature block that verifies as Alice's signature of M' with H'.
      However, this would also constitute a weakness in either H or H'
      or both.  Should this ever occur, a revision will have to be made
      to this document to revise the allowed hash algorithms.

   o  If you are building an authentication system, the recipient may
      specify a preferred signing algorithm.  However, the signer would
      be foolish to use a weak algorithm simply because the recipient
      requests it.

   o  Some of the encryption algorithms mentioned in this document have
      been analyzed less than others.  For example, although CAST5 is
      presently considered strong, it has been analyzed less than
      TripleDES.  Other algorithms may have other controversies
      surrounding them.

   o  In late summer 2002, Jallad, Katz, and Schneier published an
      interesting attack on the OpenPGP protocol and some of its
      implementations [JKS02].  In this attack, the attacker modifies a
      message and sends it to a user who then returns the erroneously
      decrypted message to the attacker.  The attacker is thus using the
      user as a random oracle, and can often decrypt the message.

      Compressing data can ameliorate this attack.  The incorrectly
      decrypted data nearly always decompresses in ways that defeat the
      attack.  However, this is not a rigorous fix, and leaves open some
      small vulnerabilities.  For example, if an implementation does not
      compress a message before encryption (perhaps because it knows it



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      was already compressed), then that message is vulnerable.  Because
      of this happenstance -- that modification attacks can be thwarted
      by decompression errors -- an implementation SHOULD treat a
      decompression error as a security problem, not merely a data
      problem.

      This attack can be defeated by the use of Modification Detection,
      provided that the implementation does not let the user naively
      return the data to the attacker.  An implementation MUST treat an
      MDC failure as a security problem, not merely a data problem.

      In either case, the implementation MAY allow the user access to
      the erroneous data, but MUST warn the user as to potential
      security problems should that data be returned to the sender.

      While this attack is somewhat obscure, requiring a special set of
      circumstances to create it, it is nonetheless quite serious as it
      permits someone to trick a user to decrypt a message.
      Consequently, it is important that:

      1.  Implementers treat MDC errors and decompression failures as
          security problems.

      2.  Implementers implement Modification Detection with all due
          speed and encourage its spread.

      3.  Users migrate to implementations that support Modification
          Detection with all due speed.

   o  PKCS#1 has been found to be vulnerable to attacks in which a
      system that reports errors in padding differently from errors in
      decryption becomes a random oracle that can leak the private key
      in mere millions of queries.  Implementations must be aware of
      this attack and prevent it from happening.  The simplest solution
      is to report a single error code for all variants of decryption
      errors so as not to leak information to an attacker.

   o  Some technologies mentioned here may be subject to government
      control in some countries.

   o  In winter 2005, Serge Mister and Robert Zuccherato from Entrust
      released a paper describing a way that the "quick check" in
      OpenPGP CFB mode can be used with a random oracle to decrypt two
      octets of every cipher block [MZ05].  They recommend as prevention
      not using the quick check at all.

      Many implementers have taken this advice to heart for any data
      that is symmetrically encrypted and for which the session key is



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      public-key encrypted.  In this case, the quick check is not needed
      as the public-key encryption of the session key should guarantee
      that it is the right session key.  In other cases, the
      implementation should use the quick check with care.

      On the one hand, there is a danger to using it if there is a
      random oracle that can leak information to an attacker.  In
      plainer language, there is a danger to using the quick check if
      timing information about the check can be exposed to an attacker,
      particularly via an automated service that allows rapidly repeated
      queries.

      On the other hand, it is inconvenient to the user to be informed
      that they typed in the wrong passphrase only after a petabyte of
      data is decrypted.  There are many cases in cryptographic
      engineering where the implementer must use care and wisdom, and
      this is one.

   o  Refer to [FIPS186], B.4.1, for the method to generate a uniformly
      distributed ECC private key.

   o  The curves proposed in this document correspond to the symmetric
      key sizes 128 bits, 192 bits, and 256 bits, as described in the
      table below.  This allows a compliant application to offer
      balanced public key security, which is compatible with the
      symmetric key strength for each AES algorithm defined here.

      The following table defines the hash and the symmetric encryption
      algorithm that SHOULD be used with a given curve for ECDSA or
      ECDH.  A stronger hash algorithm or a symmetric key algorithm MAY
      be used for a given ECC curve.  However, note that the increase in
      the strength of the hash algorithm or the symmetric key algorithm
      may not increase the overall security offered by the given ECC
      key.

       Curve name | ECC      | RSA         | Hash size | Symmetric
                  | strength | strength,   |           | key size
                  |          | informative |           |
       -----------+----------+-------------+-----------+-----------
       NIST P-256      256         3072         256         128
       NIST P-384      384         7680         384         192
       NIST P-521      521        15360         512         256

      Requirement levels indicated elsewhere in this document lead to
      the following combinations of algorithms in the OpenPGP profile:
      MUST implement NIST curve P-256 / SHA2-256 / AES-128, SHOULD
      implement NIST curve P-521 / SHA2-512 / AES-256, MAY implement




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      NIST curve P-384 / SHA2-384 / AES-256, among other allowed
      combinations.

      Consistent with the table above, the following table defines the
      KDF hash algorithm and the AES KEK encryption algorithm that
      SHOULD be used with a given curve for ECDH.  A stronger KDF hash
      algorithm or AES KEK algorithm MAY be used for a given ECC curve.

       Curve name | Recommended KDF | Recommended KEK
                  | hash algorithm  | encryption algorithm
       -----------+-----------------+-----------------------
       NIST P-256      SHA2-256            AES-128
       NIST P-384      SHA2-384            AES-192
       NIST P-521      SHA2-512            AES-256

      This document explicitly discourages the use of algorithms other
      than AES as a KEK algorithm because backward compatibility of the
      ECDH format is not a concern.  The KEK algorithm is only used
      within the scope of a Public-Key Encrypted Session Key Packet,
      which represents an ECDH key recipient of a message.  Compare this
      with the algorithm used for the session key of the message, which
      MAY be different from a KEK algorithm.

      Compliant applications SHOULD implement, advertise through key
      preferences, and use the strongest algorithms specified in this
      document.

      Note that the symmetric algorithm preference list may make it
      impossible to use the balanced strength of symmetric key
      algorithms for a corresponding public key.  For example, the
      presence of the symmetric key algorithm IDs and their order in the
      key preference list affects the algorithm choices available to the
      encoding side, which in turn may make the adherence to the table
      above infeasible.  Therefore, compliance with this specification
      is a concern throughout the life of the key, starting immediately
      after the key generation when the key preferences are first added
      to a key.  It is generally advisable to position a symmetric
      algorithm ID of strength matching the public key at the head of
      the key preference list.

      Encryption to multiple recipients often results in an unordered
      intersection subset.  For example, if the first recipient's set is
      {A, B} and the second's is {B, A}, the intersection is an
      unordered set of two algorithms, A and B.  In this case, a
      compliant application SHOULD choose the stronger encryption
      algorithm.





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      Resource constraints, such as limited computational power, is a
      likely reason why an application might prefer to use the weakest
      algorithm.  On the other side of the spectrum are applications
      that can implement every algorithm defined in this document.  Most
      applications are expected to fall into either of two categories.
      A compliant application in the second, or strongest, category
      SHOULD prefer AES-256 to AES-192.

      SHA-1 MUST NOT be used with the ECDSA or the KDF in the ECDH
      method.

      MDC MUST be used when a symmetric encryption key is protected by
      ECDH.  None of the ECC methods described in this document are
      allowed with deprecated V3 keys.  A compliant application MUST
      only use iterated and salted S2K to protect private keys, as
      defined in Section 3.7.1.3{FIXME}, "Iterated and Salted S2K".

      Side channel attacks are a concern when a compliant application's
      use of the OpenPGP format can be modeled by a decryption or
      signing oracle model, for example, when an application is a
      network service performing decryption to unauthenticated remote
      users.  ECC scalar multiplication operations used in ECDSA and
      ECDH are vulnerable to side channel attacks.  Countermeasures can
      often be taken at the higher protocol level, such as limiting the
      number of allowed failures or time-blinding of the operations
      associated with each network interface.  Mitigations at the scalar
      multiplication level seek to eliminate any measurable distinction
      between the ECC point addition and doubling operations.

   o  Although technically possible, the EdDSA algorithm MUST NOT be
      used with a digest algorithms weaker than SHA2-256.

   OpenPGP was designed with security in mind, with many smart,
   intelligent people spending a lot of time thinking about the
   ramifications of their decisions.  Removing the requirement for self-
   certifying User ID (and User Attribute) packets on a key means that
   someone could surreptitiously add an unwanted ID to a key and sign
   it.  If enough "trusted" people sign that surreptitious identity then
   other people might believe it.  The attack could wind up sending
   encrypted mail destined for alice to some other target, bob, because
   someone added "alice" to bob's key without bob's consent.

   In the case of device certificates the device itself does not have
   any consent.  It is given an identity by the device manufacturer and
   the manufacturer can insert that ID on the device certificate,
   signing it with the manufacturer's key.  If another people wants to
   label the device by another name, they can do so.  There is no harm




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   in multiple IDs, because the verification is all done based on who
   has signed those IDs.

   When a key can self-sign, it is still suggested to self-certify IDs,
   even if it no longer required by this modification to OpenPGP.  This
   at least signals to recipients of keys that yes, the owner of this
   key asserts that this identity belongs to herself.  Note, however,
   that mallet could still assert that he is 'alice' and could even
   self-certify that.  So the attack is not truly different.  Moreover,
   in the case of device certificates, it's more the manufacturer than
   the device that wants to assert an identity (even if the device could
   self-certify).

   There is no signaling whether a key is using this looser-requirement
   key format.  An attacker could therefore just remove the self-
   signature off a published key.  However one would hope that wide
   publication would result in another copy still having that signature
   and it being returned quickly.  However, the lack of signaling also
   means that a user with an application following RFC 4880 directly
   would see a key following this specification as "broken" and may not
   accept it.

   On a different note, including the "geo" notation could leak
   information about where a signer is located.  However it is just an
   assertion (albeit a signed assertion) so there is no verifiable truth
   to the location information released.  Similarly, all the rest of the
   signature notations are pure assertions, so they should be taken with
   the trustworthiness of the signer.

   Combining the User ID with the User Attribute means that an ID and
   image would not be separable.  For a person this is probably not
   good, but for a device it's unlikely the image will change so it
   makes sense to combine the ID and image into a single signed packet
   with a single signature.

16.  Compatibility Profiles

16.1.  OpenPGP ECC Profile

   A compliant application MUST implement NIST curve P-256, SHOULD
   implement NIST curve P-521, SHOULD implemend Ed25519, SHOULD
   implement Curve25519, MAY implement NIST curve P-384, MAY implement
   brainpoolP256r1, MAY implement brainpoolP512r1, and MAY implement
   Curve25519, as defined in Section 11.  A compliant application MUST
   implement SHA2-256 and SHOULD implement SHA2-384 and SHA2-512.  A
   compliant application MUST implement AES-128 and SHOULD implement
   AES-256.




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   A compliant application SHOULD follow Section 13{FIXME} regarding the
   choice of the following algorithms for each curve:

   o  the KDF hash algorithm,

   o  the KEK algorithm,

   o  the message digest algorithm and the hash algorithm used in the
      key certifications,

   o  the symmetric algorithm used for message encryption.

   It is recommended that the chosen symmetric algorithm for message
   encryption be no less secure than the KEK algorithm.

16.2.  Suite-B Profile

   A subset of algorithms allowed by this document can be used to
   achieve [SuiteB] compatibility.  The references to [SuiteB] in this
   document are informative.  This document is primarily concerned with
   format specification, leaving additional security restrictions
   unspecified, such as matching the assigned security level of
   information to authorized recipients or interoperability concerns
   arising from fewer allowed algorithms in [SuiteB] than allowed by
   this document.

16.3.  Security Strength at 192 Bits

   To achieve the security strength of 192 bits, [SuiteB] requires NIST
   curve P-384, AES-256, and SHA2-384.  The symmetric algorithm
   restriction means that the algorithm of KEK used for key wrapping in
   Section 8 and an OpenPGP session key used for message encryption must
   be AES-256.  The hash algorithm restriction means that the hash
   algorithms of KDF and the OpenPGP message digest calculation must be
   SHA2-384.

16.4.  Security Strength at 128 Bits

   The set of algorithms in Section 12.2.1{FIXME} is extended to allow
   NIST curve P-256, AES-128, and SHA2-256.

17.  {15} Implementation Nits

   This section is a collection of comments to help an implementer,
   particularly with an eye to backward compatibility.  Previous
   implementations of PGP are not OpenPGP compliant.  Often the
   differences are small, but small differences are frequently more
   vexing than large differences.  Thus, this is a non-comprehensive



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   list of potential problems and gotchas for a developer who is trying
   to be backward-compatible.

   o  The IDEA algorithm is patented, and yet it is required for PGP 2.x
      interoperability.  It is also the de-facto preferred algorithm for
      a V3 key with a V3 self-signature (or no self- signature).

   o  When exporting a private key, PGP 2.x generates the header "BEGIN
      PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK".
      All previous versions ignore the implied data type, and look
      directly at the packet data type.

   o  PGP 2.0 through 2.5 generated V2 Public-Key packets.  These are
      identical to the deprecated V3 keys except for the version number.
      An implementation MUST NOT generate them and may accept or reject
      them as it sees fit.  Some older PGP versions generated V2 PKESK
      packets (Tag 1) as well.  An implementation may accept or reject
      V2 PKESK packets as it sees fit, and MUST NOT generate them.

   o  PGP 2.6.x will not accept key-material packets with versions
      greater than 3.

   o  There are many ways possible for two keys to have the same key
      material, but different fingerprints (and thus Key IDs).  Perhaps
      the most interesting is an RSA key that has been "upgraded" to V4
      format, but since a V4 fingerprint is constructed by hashing the
      key creation time along with other things, two V4 keys created at
      different times, yet with the same key material will have
      different fingerprints.

   o  If an implementation is using zlib to interoperate with PGP 2.x,
      then the "windowBits" parameter should be set to -13.

   o  The 0x19 back signatures were not required for signing subkeys
      until relatively recently.  Consequently, there may be keys in the
      wild that do not have these back signatures.  Implementing
      software may handle these keys as it sees fit.

   o  OpenPGP does not put limits on the size of public keys.  However,
      larger keys are not necessarily better keys.  Larger keys take
      more computation time to use, and this can quickly become
      impractical.  Different OpenPGP implementations may also use
      different upper bounds for public key sizes, and so care should be
      taken when choosing sizes to maintain interoperability.  As of
      2007 most implementations have an upper bound of 4096 bits.

   o  ASCII armor is an optional feature of OpenPGP.  The OpenPGP
      working group strives for a minimal set of mandatory-to-implement



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      features, and since there could be useful implementations that
      only use binary object formats, this is not a "MUST" feature for
      an implementation.  For example, an implementation that is using
      OpenPGP as a mechanism for file signatures may find ASCII armor
      unnecessary.  OpenPGP permits an implementation to declare what
      features it does and does not support, but ASCII armor is not one
      of these.  Since most implementations allow binary and armored
      objects to be used indiscriminately, an implementation that does
      not implement ASCII armor may find itself with compatibility
      issues with general-purpose implementations.  Moreover,
      implementations of OpenPGP-MIME [RFC3156] already have a
      requirement for ASCII armor so those implementations will
      necessarily have support.

   o  The OCB mode is patented and a debate is still underway on whether
      it can be included in RFC4880bis or needs to be moved to a
      separate document.  For the sole purpose of experimenting with the
      Preferred AEAD Algorithms signature subpacket it is has been
      included in this I-D.

18.  References

18.1.  Normative References

   [AES]      NIST, "FIPS PUB 197, Advanced Encryption Standard (AES)",
              November 2001,
              <http://csrc.nist.gov/publications/fips/fips197/
              fips-197.{ps,pdf}>.

   [BLOWFISH]
              Schneier, B., "Description of a New Variable-Length Key,
              64-Bit Block Cipher (Blowfish)", Fast Software Encryption,
              Cambridge Security Workshop Proceedings, Springer-Verlag,
              1994, pp191-204, December 1993,
              <http://www.counterpane.com/bfsverlag.html>.

   [BZ2]      Seward, J., "The Bzip2 and libbzip2 home page",
              <http://www.bzip.org/>.

   [EAX]      Bellare, M., Rogaway, P., and D. Wagner, "A Conventional
              Authenticated-Encryption Mode", April 2003.

   [ELGAMAL]  Elgamal, T., "A Public-Key Cryptosystem and a Signature
              Scheme Based on Discrete Logarithms,", IEEE Transactions
              on Information Theory v. IT-31, n. 4, 1985, pp. 469-472, .






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   [FIPS180]  National Institute of Standards and Technology, U.S.
              Department of Commerce, "Secure Hash Standard (SHS), FIPS
              180-4", August 2015,
              <http://dx.doi.org/10.6028/NIST.FIPS.180-4>.

   [FIPS186]  National Institute of Standards and Technology, U.S.
              Department of Commerce, "Digital Signature Standard (DSS),
              FIPS 186-4", July 2013,
              <http://dx.doi.org/10.6028/NIST.FIPS.186-4>.

   [FIPS202]  National Institute of Standards and Technology, U.S.
              Department of Commerce, "SHA-3 Standard: Permutation-Based
              Hash and Extendable-Output Functions, FIPS 202", August
              2015, <http://dx.doi.org/10.6028/NIST.FIPS.202>.

   [HAC]      Menezes, A., Oorschot, P., and S. Vanstone, "Handbook of
              Applied Cryptography", 1996.

   [I-D.irtf-cfrg-eddsa]
              Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
              Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-02
              (work in progress), January 2016.

   [IDEA]     Lai, X., "On the design and security of block ciphers",
              ETH Series in Information Processing, J.L.  Massey
              (editor), Vol. 1, Hartung-Gorre Verlag Konstanz,
              Technische Hochschule (Zurich), 1992.

   [ISO10646]
              International Organization for Standardization,
              "Information Technology - Universal Multiple-octet coded
              Character Set (UCS) - Part 1: Architecture and Basic
              Multilingual Plane", ISO Standard 10646-1, May 1993.

   [JFIF]     Eric Hamilton, C-Cube Microsystems, Milpitas, CA, "JPEG
              File Interchange Format (Version 1.02).", September 1996.

   [OCB1]     Rogaway, P., Bellare, M., Black, J., and T. Krovetz, "OCB:
              A Block-Cipher Mode of Operation for Efficient
              Authenticated Encryption", April 2001.

   [OCB2]     Rogaway, P., "Efficient Instantiations of Tweakable
              Blockciphers and Refinements to Modes OCB and PMAC.",
              April 2004.

   [OCB3]     Krovetz, T. and P. Rogaway, "The Software Performance of
              Authenticated-Encryption Modes", April 2011.




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   [PKCS5]    RSA Laboratories, "PKCS #5 v2.0: Password-Based
              Cryptography Standard", March 1999.

   [RFC1950]  Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950, May 1996.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, May 1996.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2144]  Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
              May 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April
              2001.

   [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
              "MIME Security with OpenPGP", RFC 3156, August 2001.

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, September 2002.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3713]  Matsui, M., Nakajima, J., and S. Moriai, "A Description of
              the Camellia Encryption Algorithm", RFC 3713, April 2004.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.







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   [RFC5639]  Lochter, M. and J. Merkle, "Elliptic Curve Cryptography
              (ECC) Brainpool Standard Curves and Curve Generation", RFC
              5639, DOI 10.17487/RFC5639, March 2010,
              <http://www.rfc-editor.org/info/rfc5639>.

   [RFC5870]  Mayrhofer, A. and C. Spanring, "A Uniform Resource
              Identifier for Geographic Locations ('geo' URI)", RFC
              5870, DOI 10.17487/RFC5870, June 2010,
              <http://www.rfc-editor.org/info/rfc5870>.

   [RFC7253]  Krovetz, T. and P. Rogaway, "The OCB Authenticated-
              Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May
              2014, <https://www.rfc-editor.org/info/rfc7253>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [SCHNEIER]
              Schneier, B., "Applied Cryptography Second Edition:
              protocols, algorithms, and source code in C", 1996.

   [SP800-56A]
              Barker, E., Johnson, D., and M. Smid, "Recommendation for
              Pair-Wise Key Establishment Schemes Using Discrete
              Logarithm Cryptography", NIST Special Publication 800-56A
              Revision 1, March 2007.

   [SuiteB]   National Security Agency, "NSA Suite B Cryptography",
              March 2010,
              <http://www.nsa.gov/ia/programs/suiteb_cryptography/>.

   [TWOFISH]  Schneier, B., Kelsey, J., Whiting, D., Wagner, D., Hall,
              C., and N. Ferguson, "The Twofish Encryption Algorithm",
              1999.

18.2.  Informative References

   [KOBLITZ]  Koblitz, N., "A course in number theory and cryptography,
              Chapter VI. Elliptic Curves, ISBN: 0-387-96576-9,
              Springer-Verlag", 1997.

   [RFC1423]  Balenson, D., "Privacy Enhancement for Internet Electronic
              Mail: Part III: Algorithms, Modes, and Identifiers", RFC
              1423, February 1993.

   [RFC1991]  Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
              Exchange Formats", RFC 1991, August 1996.



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   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
              "OpenPGP Message Format", RFC 2440, November 1998.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011.

   [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
              Elliptic Curve Cryptography", September 2000.

   [SP800-57]
              NIST, "Recommendation on Key Management", NIST Special
              Publication 800-57, March 2007,
              <http://csrc.nist.gov/publications/nistpubs/800-57/
              SP800-57-Part{1,2}.pdf>.

18.3.  URIs

   [1] AEAD%20Algorithms

Appendix A.  Test vectors

   To help implementing this specification a non-normative example for
   the EdDSA algorithm is given.

A.1.  Sample EdDSA key

   The secret key used for this example is:

   D: 1a8b1ff05ded48e18bf50166c664ab023ea70003d78d9e41f5758a91d850f8d2

   Note that this is the raw secret key used as input to the EdDSA
   signing operation.  The key was created on 2014-08-19 14:28:27 and
   thus the fingerprint of the OpenPGP key is:

      C959 BDBA FA32 A2F8 9A15  3B67 8CFD E121 9796 5A9A

   The algorithm specific input parameters without the MPI length
   headers are:

   oid: 2b06010401da470f01

   q: 403f098994bdd916ed4053197934e4a87c80733a1280d62f8010992e43ee3b2406

   The entire public key packet is thus:







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      98 33 04 53 f3 5f 0b 16  09 2b 06 01 04 01 da 47
      0f 01 01 07 40 3f 09 89  94 bd d9 16 ed 40 53 19
      79 34 e4 a8 7c 80 73 3a  12 80 d6 2f 80 10 99 2e
      43 ee 3b 24 06

A.2.  Sample EdDSA signature

   The signature is created using the sample key over the input data
   "OpenPGP" on 2015-09-16 12:24:53 and thus the input to the hash
   function is:

   m: 4f70656e504750040016080006050255f95f9504ff0000000c

   Using the SHA2-256 hash algorithm yields the digest:

   d: f6220a3f757814f4c2176ffbb68b00249cd4ccdc059c4b34ad871f30b1740280

   Which is fed into the EdDSA signature function and yields this
   signature:

   r: 56f90cca98e2102637bd983fdb16c131dfd27ed82bf4dde5606e0d756aed3366

   s: d09c4fa11527f038e0f57f2201d82f2ea2c9033265fa6ceb489e854bae61b404

   The entire signature packet is thus:

      88 5e 04 00 16 08 00 06  05 02 55 f9 5f 95 00 0a
      09 10 8c fd e1 21 97 96  5a 9a f6 22 01 00 56 f9
      0c ca 98 e2 10 26 37 bd  98 3f db 16 c1 31 df d2
      7e d8 2b f4 dd e5 60 6e  0d 75 6a ed 33 66 01 00
      d0 9c 4f a1 15 27 f0 38  e0 f5 7f 22 01 d8 2f 2e
      a2 c9 03 32 65 fa 6c eb  48 9e 85 4b ae 61 b4 04

Appendix B.  ECC Point compression flag bytes

   This specification introduces the new flag byte 0x40 to indicate the
   point compression format.  The value has been chosen so that the high
   bit is not cleared and thus to avoid accidental sign extension.  Two
   other values might also be interesting for other ECC specifications:

     Flag  Description
     ----  -----------
     0x04  Standard flag for uncompressed format
     0x40  Native point format of the curve follows
     0x41  Only X coordinate follows.
     0x42  Only Y coordinate follows.





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Appendix C.  Changes since RFC-4880

   o  Applied errata 2270, 2271, 2242, 3298.

   o  Added Camellia cipher from RFC 5581.

   o  Incorporated RFC 6637 (ECC for OpenPGP)

   o  Added draft-atkins-openpgp-device-certificates

   o  Added draft-koch-eddsa-for-openpgp-04

   o  Added Issuer Fingerprint signature subpacket.

   o  Added a v5 key and fingerprint format.

   o  Added OIDs for brainpool curves and Curve25519.

   o  Marked SHA2-256 as MUST implement.

   o  Marked Curve25519 and Ed25519 as SHOULD implement.

   o  Marked SHA-1 as SHOULD NOT be used to create messages.

   o  Marked MD5 as SHOULD NOT implement.

   o  Changed v5 key fingerprint format to full 32 octets.

   o  Added Literal Data Packet format octet 'm'.

   o  Added Feature Flag for v5 key support.

   o  Added AEAD Encrypted Data Packet.

   o  Removed notes on extending the MDC packet.

   { Informational rfcs: [RFC1423] }

Appendix D.  The principal authors of RFC-4880 are as follows:












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     Jon Callas
     EMail: jon@callas.org

     Lutz Donnerhacke
     EMail: lutz@iks-jena.de

     Hal Finney

     David Shaw
     EMail: dshaw@jabberwocky.com

     Rodney Thayer
     EMail: rodney@canola-jones.com

Author's Address

   Werner Koch

   Email: wk@gnupg.org
































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