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draft-ietf-emu-aka-pfs-latest.xml
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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<rfc ipr="trust200902" docName="draft-ietf-emu-aka-pfs-latest"
category="std" updates="5448,9048" consensus="true">
<?rfc toc="yes"?>
<?rfc symrefs="yes"?>
<?rfc autobreaks="yes"?>
<?rfc tocindent="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<front>
<title abbrev="EAP-AKA' FS">Forward Secrecy for the
Extensible Authentication Protocol Method for Authentication and Key Agreement (EAP-AKA' FS)</title>
<author initials="J" surname="Arkko" fullname="Jari Arkko">
<organization>Ericsson</organization>
<address>
<postal>
<street/>
<city>Jorvas</city> <code>02420</code>
<country>Finland</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author initials="K" surname="Norrman" fullname="Karl Norrman">
<organization>Ericsson</organization>
<address>
<postal>
<street/>
<city>Stockholm</city> <code>16483</code>
<country>Sweden</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author initials="J" surname="Preuß Mattsson" fullname="John Preuß Mattsson">
<organization>Ericsson</organization>
<address>
<postal>
<street/>
<city>Kista</city> <code>164 40</code>
<country>Sweden</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<keyword>EAP</keyword>
<keyword>AKA</keyword>
<keyword>AKA'</keyword>
<keyword>EAP-AKA'</keyword>
<keyword>EAP-AKA' FS</keyword>
<keyword>3GPP</keyword>
<abstract>
<t>This document updates RFC 9048, the improved Extensible
Authentication Protocol Method for 3GPP Mobile Network Authentication
and Key Agreement (EAP-AKA'), with an optional extension providing ephemeral key exchange.
Similarly, this document also updates the earlier version
of the EAP-AKA' specification in RFC 5448. The extension EAP-AKA' Forward Secrecy (EAP-AKA' FS), when
negotiated, provides forward secrecy for the session keys
generated as a part of the authentication run in EAP-AKA'. This
prevents an attacker who has gained access to the long-term
key from obtaining session keys established in the past, assuming
these have been properly deleted. In addition, EAP-AKA' FS mitigates
passive attacks (e.g., large scale pervasive monitoring)
against future sessions. This forces attackers to use active attacks instead.</t>
</abstract>
</front>
<middle>
<section anchor="sec:intro" title="Introduction">
<t>Many different attacks have been reported as part of revelations
associated with pervasive surveillance. Some of the reported attacks
involved compromising the Universal Subscriber Identity Module
(USIM) card supply chain. Attacks revealing the AKA long-term key may occur for
instance, during the manufacturing process of USIM cards, during the
transfer of the cards and associated information to
the operator, and when a system is running. Since
the publication of reports about such attacks
<xref target="Heist2015"/>, manufacturing and provisioning
processes have gained much scrutiny and have improved.</t>
<t>However, the danger of resourceful attackers attempting to gain
information about long-term keys is still a concern because these keys are high-value targets.
Note that
the attacks are largely independent of the used authentication
technology; the issue is not vulnerabilities in algorithms or
protocols, but rather the possibility of someone gaining unauthorized
access to key material. Furthermore, an explicit goal of the IETF is to ensure
that we understand the surveillance concerns related to IETF
protocols and take appropriate countermeasures <xref target="RFC7258"/>.</t>
<t>While strong protection of manufacturing and other processes is
essential in mitigating surveillance and other risks associated with
AKA long-term keys, there are also protocol mechanisms that can
help.</t>
<t>This document updates <xref target="RFC9048"/>, the Improved 3GPP
Mobile Network Authentication and Key Agreement (EAP-AKA') method,
with an optional extension providing ephemeral key exchange
minimizing the impact of long-term key compromise and strengthens
the identity privacy requirements. This is important, given the
large number of users of AKA in mobile networks.</t>
<t>The extension, when
negotiated, provides Forward Secrecy (FS) <xref target="DOW1992"/> for the session key
generated as a part of the authentication run in EAP-AKA'. This
prevents an attacker who has gained access to the long-term
key in a USIM card from getting access to past session
keys. In addition to FS, the included Diffie-Hellman exchange, forces
attackers to be active if they want access to future session keys even
if they have access to the long-term key. This is beneficial, because
active attacks demand much more resources to launch, and are easier to
detect. As
with other protocols, an active attacker with access to the
long-term key material will of course be able to attack all future
communications, but risks detection, particularly if done at
scale.</t>
<t>It should also be noted that 5G network architecture <xref target="TS.33.501"/>
includes the
use of the EAP framework for authentication. While any methods can
be run, the default authentication method within that context will
be EAP-AKA'. As a result, improvements in EAP-AKA' security have a
potential to improve security for many users.</t>
</section>
<section title="Requirements Language">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only
when, they appear in all capitals, as shown here.</t>
</section>
<section title="Protocol Design and Deployment Objectives">
<t>The extension specified here re-uses large portions of the
current structure of 3GPP interfaces and functions, with the
rationale that this will make the construction more easily adopted.
In particular, the construction keeps the interface between the
USIM and the mobile
terminal intact. As a consequence, there is no need to roll out new
credentials to existing subscribers. The work is based on an earlier
paper <xref target="TrustCom2015"/>, and uses much of the same
material, but applied to EAP rather than the underlying AKA
method.</t>
<t>It has been a goal to implement this change as an extension
of the widely supported EAP-AKA' method, rather than a completely new
authentication method. The extension is implemented as a set of
new, optional attributes, that are provided alongside the
base attributes in EAP-AKA'. Old implementations can ignore
these attributes, but their presence will nevertheless be verified
as part of base EAP-AKA' integrity verification process, helping
protect against bidding down attacks. This extension does
not increase the number of rounds necessary to complete the
protocol.</t>
<t>The use of this extension is at the discretion of the
authenticating parties. It should be noted that FS and defenses
against passive attacks do not solve all problems, but they can
provide a partial defense that increases the cost and risk
associated with pervasive surveillance.</t>
<t>While adding forward secrecy to the existing mobile
network infrastructure can be done in multiple different ways, this
document specifies a solution that is relatively easily
deployable. In particular:
<list style="symbols">
<t>As noted above, no new credentials are needed; there is no
change to USIM cards.</t>
<t>FS property can be incorporated into any current or future
system that supports EAP, without changing
any network functions beyond the EAP endpoints.</t>
<t>Key generation happens at the endpoints, enabling highest grade
key material to be used both by the endpoints and the intermediate
systems (such as access points that are given access to specific
keys).</t>
<t>While EAP-AKA' is just one EAP method, for practical purposes
forward secrecy being available for both EAP-TLS <xref
target="RFC5216"/> <xref target="RFC9190"/> and
EAP-AKA' ensures that for many practical systems forward
secrecy can be enabled for either all or significant fraction of
users.</t>
</list></t>
</section>
<section title="Background">
<t>The reader is assumed to
have basic understanding of the EAP framework <xref target="RFC3748"/>.</t>
<section title="AKA">
<t>We use the term Authentication and Key Agreement (AKA) for the
main authentication and key agreement protocol used by 3GPP mobile
networks from the third generation (3G) and onward. Later
generations adds new features to AKA, but the core remains the
same. It is based on challenge-response mechanisms and symmetric
cryptography. In contrast to its earlier GSM counterparts, AKA
provides long key lengths and mutual authentication. The phone
typically executes AKA in a USIM. USIM is technically just an
application that can reside on a removable UICC (Universal
Integrated Circuit Card), an embedded UICC, or integrated in a
Trusted Execution Environment (TEE). In this document we use the
term "USIM card" to refer to any Subscriber Identity Module
capable of running AKA.</t>
<t>The goal of AKA is to mutually authenticate the USIM and the so-called
home environment, which is the authentication server in the subscribers
home operator's network.</t>
<t>AKA works in the following manner:
<list style="symbols">
<t>The USIM and the home environment have agreed on a
long-term symmetric key beforehand.</t>
<t>The actual authentication process starts by having the home
environment produce an authentication vector, based on the long-term
key and a sequence number. The authentication vector contains a
random part RAND, an authenticator part AUTN used for
authenticating the network to the USIM, an expected
result part XRES, a 128-bit session key for integrity check IK,
and a 128-bit session key for encryption CK.</t>
<t>The authentication vector is passed to the serving network, which
uses it to authenticate the device.</t>
<t>The RAND and the AUTN are delivered to the USIM.</t>
<t>The USIM verifies the AUTN, again based on the long-term
key and the sequence number. If this process is successful (the
AUTN is valid and the sequence number used to generate AUTN is
within the correct range), the USIM produces an
authentication result RES and sends it to the serving network.</t>
<t>The serving network verifies that the result from the USIM
matches the expected value in the authentication vector.
If it does, the USIM is considered authenticated,
and IK and CK can be used to
protect further communications between the USIM and the
home environment.</t>
</list></t>
</section>
<section title="EAP-AKA' Protocol">
<t>When AKA is embedded into EAP, the authentication processing on
the network side is moved to the home environment. The 3GPP authentication
database (AD) generates authentication vectors. The 3GPP authentication
server takes the role of EAP server. The USIM combined with
the mobile phone takes the role of the client.
The difference between EAP-AKA <xref target="RFC4187"/> and
EAP-AKA' <xref target="RFC9048"/> is that EAP-AKA'
binds the derived keys to the name of access network.
<xref target="figaka"/> describes the basic flow in the EAP-AKA'
authentication process. The definition of the full protocol
behavior, along with the definition of attributes AT_RAND,
AT_AUTN, AT_MAC, and AT_RES can be found in <xref
target="RFC9048"/> and <xref target="RFC4187"/>.
Note the use of EAP-terminology from hereon. That is, the 3GPP
serving network takes on the role of an EAP access network.
</t>
<figure title="EAP-AKA' Authentication Process" anchor="figaka">
<artset>
<artwork type="ascii-art"><![CDATA[
Peer Server
| |
| EAP-Request/Identity |
|<-----------------------------------------------------------+
| |
| EAP-Response/Identity |
| (Includes user's Network Access Identifier, NAI) |
+----------------------------------------------------------->|
| +-----------------------------------------------------+--+
| | Server determines the network name and ensures that |
| | the given access network is authorized to use the |
| | claimed name. The server then runs the AKA' algorithms |
| | generating RAND and AUTN, derives session keys from |
| | CK' and IK'. RAND and AUTN are sent as AT_RAND and |
| | AT_AUTN attributes, whereas the network name is |
| | transported in the AT_KDF_INPUT attribute. AT_KDF |
| | signals the used key derivation function. The session |
| | keys are used to create the AT_MAC attribute. |
| +-----------------------------------------------------+--+
| |
| EAP-Request/AKA'-Challenge |
| (AT_RAND, AT_AUTN, AT_KDF, AT_KDF_INPUT, AT_MAC) |
|<-----------------------------------------------------------+
+--+-----------------------------------------------------+ |
| The peer determines what the network name should be, | |
| based on, e.g., what access technology it is using. | |
| The peer also retrieves the network name sent by the | |
| network from the AT_KDF_INPUT attribute. The two names | |
| are compared for discrepancies, and if they do not | |
| match, the authentication is aborted. Otherwise, the | |
| network name from AT_KDF_INPUT attribute is used in | |
| running the AKA' algorithms, verifying AUTN from | |
| AT_AUTN and MAC from AT_MAC attributes. The peer then | |
| generates RES. The peer also derives session keys from | |
| CK'/IK'. The AT_RES and AT_MAC attributes are | |
| constructed. | |
+--+-----------------------------------------------------+ |
| |
| EAP-Response/AKA'-Challenge |
| (AT_RES, AT_MAC) |
+----------------------------------------------------------->|
| +-----------------------------------------------------+--+
| | Server checks the RES and MAC values received in |
| | AT_RES and AT_MAC, respectively. Success requires both |
| | compared values match, respectively. |
| +-----------------------------------------------------+--+
| |
| EAP-Success |
|<-----------------------------------------------------------+
| |
]]></artwork>
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<text x="28" y="36">Peer</text>
<text x="516" y="36">Server</text>
<text x="428" y="68">EAP-Request/Identity</text>
<text x="128" y="116">EAP-Response/Identity</text>
<text x="80" y="132">(Includes</text>
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<text x="356" y="180">name</text>
<text x="392" y="180">and</text>
<text x="440" y="180">ensures</text>
<text x="492" y="180">that</text>
<text x="112" y="196">the</text>
<text x="152" y="196">given</text>
<text x="204" y="196">access</text>
<text x="264" y="196">network</text>
<text x="308" y="196">is</text>
<text x="364" y="196">authorized</text>
<text x="420" y="196">to</text>
<text x="448" y="196">use</text>
<text x="480" y="196">the</text>
<text x="128" y="212">claimed</text>
<text x="184" y="212">name.</text>
<text x="224" y="212">The</text>
<text x="268" y="212">server</text>
<text x="316" y="212">then</text>
<text x="356" y="212">runs</text>
<text x="392" y="212">the</text>
<text x="428" y="212">AKA'</text>
<text x="492" y="212">algorithms</text>
<text x="140" y="228">generating</text>
<text x="204" y="228">RAND</text>
<text x="240" y="228">and</text>
<text x="280" y="228">AUTN,</text>
<text x="336" y="228">derives</text>
<text x="400" y="228">session</text>
<text x="452" y="228">keys</text>
<text x="492" y="228">from</text>
<text x="112" y="244">CK'</text>
<text x="144" y="244">and</text>
<text x="180" y="244">IK'.</text>
<text x="220" y="244">RAND</text>
<text x="256" y="244">and</text>
<text x="292" y="244">AUTN</text>
<text x="328" y="244">are</text>
<text x="364" y="244">sent</text>
<text x="396" y="244">as</text>
<text x="440" y="244">AT_RAND</text>
<text x="488" y="244">and</text>
<text x="128" y="260">AT_AUTN</text>
<text x="208" y="260">attributes,</text>
<text x="288" y="260">whereas</text>
<text x="336" y="260">the</text>
<text x="384" y="260">network</text>
<text x="436" y="260">name</text>
<text x="468" y="260">is</text>
<text x="144" y="276">transported</text>
<text x="204" y="276">in</text>
<text x="232" y="276">the</text>
<text x="300" y="276">AT_KDF_INPUT</text>
<text x="396" y="276">attribute.</text>
<text x="468" y="276">AT_KDF</text>
<text x="128" y="292">signals</text>
<text x="176" y="292">the</text>
<text x="212" y="292">used</text>
<text x="248" y="292">key</text>
<text x="308" y="292">derivation</text>
<text x="392" y="292">function.</text>
<text x="448" y="292">The</text>
<text x="496" y="292">session</text>
<text x="116" y="308">keys</text>
<text x="152" y="308">are</text>
<text x="188" y="308">used</text>
<text x="220" y="308">to</text>
<text x="260" y="308">create</text>
<text x="304" y="308">the</text>
<text x="348" y="308">AT_MAC</text>
<text x="420" y="308">attribute.</text>
<text x="404" y="356">EAP-Request/AKA'-Challenge</text>
<text x="160" y="372">(AT_RAND,</text>
<text x="236" y="372">AT_AUTN,</text>
<text x="304" y="372">AT_KDF,</text>
<text x="392" y="372">AT_KDF_INPUT,</text>
<text x="480" y="372">AT_MAC)</text>
<text x="32" y="420">The</text>
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<text x="132" y="420">determines</text>
<text x="196" y="420">what</text>
<text x="232" y="420">the</text>
<text x="280" y="420">network</text>
<text x="332" y="420">name</text>
<text x="380" y="420">should</text>
<text x="424" y="420">be,</text>
<text x="40" y="436">based</text>
<text x="80" y="436">on,</text>
<text x="120" y="436">e.g.,</text>
<text x="164" y="436">what</text>
<text x="212" y="436">access</text>
<text x="284" y="436">technology</text>
<text x="340" y="436">it</text>
<text x="364" y="436">is</text>
<text x="404" y="436">using.</text>
<text x="32" y="452">The</text>
<text x="68" y="452">peer</text>
<text x="108" y="452">also</text>
<text x="168" y="452">retrieves</text>
<text x="224" y="452">the</text>
<text x="272" y="452">network</text>
<text x="324" y="452">name</text>
<text x="364" y="452">sent</text>
<text x="396" y="452">by</text>
<text x="424" y="452">the</text>
<text x="48" y="468">network</text>
<text x="100" y="468">from</text>
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<text x="204" y="468">AT_KDF_INPUT</text>
<text x="300" y="468">attribute.</text>
<text x="360" y="468">The</text>
<text x="392" y="468">two</text>
<text x="432" y="468">names</text>
<text x="32" y="484">are</text>
<text x="84" y="484">compared</text>
<text x="136" y="484">for</text>
<text x="212" y="484">discrepancies,</text>
<text x="288" y="484">and</text>
<text x="316" y="484">if</text>
<text x="348" y="484">they</text>
<text x="380" y="484">do</text>
<text x="408" y="484">not</text>
<text x="44" y="500">match,</text>
<text x="88" y="500">the</text>
<text x="164" y="500">authentication</text>
<text x="236" y="500">is</text>
<text x="284" y="500">aborted.</text>
<text x="364" y="500">Otherwise,</text>
<text x="424" y="500">the</text>
<text x="48" y="516">network</text>
<text x="100" y="516">name</text>
<text x="140" y="516">from</text>
<text x="212" y="516">AT_KDF_INPUT</text>
<text x="304" y="516">attribute</text>
<text x="356" y="516">is</text>
<text x="388" y="516">used</text>
<text x="420" y="516">in</text>
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<text x="348" y="532">AUTN</text>
<text x="388" y="532">from</text>
<text x="48" y="548">AT_AUTN</text>
<text x="96" y="548">and</text>
<text x="128" y="548">MAC</text>
<text x="164" y="548">from</text>
<text x="212" y="548">AT_MAC</text>
<text x="288" y="548">attributes.</text>
<text x="352" y="548">The</text>
<text x="388" y="548">peer</text>
<text x="428" y="548">then</text>
<text x="56" y="564">generates</text>
<text x="116" y="564">RES.</text>
<text x="152" y="564">The</text>
<text x="188" y="564">peer</text>
<text x="228" y="564">also</text>
<text x="280" y="564">derives</text>
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<text x="396" y="564">keys</text>
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<text x="52" y="580">CK'/IK'.</text>
<text x="104" y="580">The</text>
<text x="148" y="580">AT_RES</text>
<text x="192" y="580">and</text>
<text x="236" y="580">AT_MAC</text>
<text x="308" y="580">attributes</text>
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<text x="92" y="644">EAP-Response</text>
<text x="204" y="644">AKA'-Challenge</text>
<text x="76" y="660">(AT_RES,</text>
<text x="144" y="660">AT_MAC)</text>
<text x="124" y="708">Server</text>
<text x="180" y="708">checks</text>
<text x="224" y="708">the</text>
<text x="256" y="708">RES</text>
<text x="288" y="708">and</text>
<text x="320" y="708">MAC</text>
<text x="364" y="708">values</text>
<text x="428" y="708">received</text>
<text x="476" y="708">in</text>
<text x="124" y="724">AT_RES</text>
<text x="168" y="724">and</text>
<text x="216" y="724">AT_MAC,</text>
<text x="304" y="724">respectively.</text>
<text x="392" y="724">Success</text>
<text x="460" y="724">requires</text>
<text x="516" y="724">both</text>
<text x="132" y="740">compared</text>
<text x="196" y="740">values</text>
<text x="252" y="740">match,</text>
<text x="336" y="740">respectively.</text>
<text x="464" y="788">EAP-Success</text>
</g>
</svg>
</artwork>
</artset>
</figure>
</section>
<section anchor="attacks" title="Attacks Against Long-Term Keys in Smart Cards">
<t>The general security properties and potential
vulnerabilities of AKA and EAP-AKA' are discussed in <xref
target="RFC9048"/>.</t>
<t>An important question in that discussion relates to the
potential compromise of long-term keys, as discussed earlier.
Attacks on long-term keys are not specific to
AKA or EAP-AKA', and all security systems fail at least to some
extent if key material is stolen. However, it would be preferable
to retain some security even in
the face of such attacks. This document specifies a mechanism
that reduces risks to compromise of key material belonging to
previous sessions, before the long-term keys were compromised. It
also forces attackers to be active even after the compromise.</t>
</section>
</section>
<section title="Protocol Overview">
<t>Forward secrecy for EAP-AKA' is achieved by using an Elliptic
Curve Diffie-Hellman (ECDH) exchange <xref target="RFC7748"/>. To provide
FS, the exchange must be run in an ephemeral manner, i.e.,
both sides generate temporary keys according to the negotiated ciphersuite,
e.g., for X25519 this is done as specified in <xref target="RFC7748"/>.
This method is referred to as ECDHE, where the last 'E' stands
for Ephemeral. The two initially registered elliptic curves and their
wire formats are chosen to align with the elliptic curves and formats
specified for Subscription Concealed Identifier (SUCI) encryption in
Appendix C.3.4 of 3GPP TS 33.501 <xref target="TS.33.501"/>.</t>
<t>The enhancements in the EAP-AKA' FS protocol are compatible
with the signaling flow and other basic structures of both AKA and
EAP-AKA'. The intent is to implement the enhancement as optional
attributes that legacy implementations ignore.</t>
<t>The purpose of the protocol is to achieve mutual authentication
between the EAP server and peer, and to establish keying material
for secure communication between the two. This document specifies
the calculation of key material, providing new properties that are
not present in key material provided by EAP-AKA' in its original
form.</t>
<t><xref target="figakafs"/> below describes the overall process. Since the goal
has been to not require new infrastructure or credentials, the
flow diagrams also show the conceptual interaction with the USIM card
and the home environment. Recall that the home environment represent
the 3GPP Authentication Database (AD) and server.
The details of those interactions
are outside the scope of this document, however, and the reader
is referred to the 3GPP specifications. For 5G this is
specified in 3GPP TS 33.501 <xref target="TS.33.501"/></t>
<figure title="EAP-AKA' FS Authentication Process" anchor="figakafs">
<artset>
<artwork type="ascii-art"><![CDATA[
USIM Peer Server AD
| | | |
| | EAP-Req/Identity | |
| |<---------------------------+ |
| | | |
| | EAP-Resp/Identity | |
| | (Privacy-Friendly) | |
| +--------------------------->| |
| +-------+----------------------------+----------------+--+
| | Server now has an identity for the peer. The server |
| | then asks the help of AD to run AKA algorithms, |
| | generating RAND, AUTN, XRES, CK, IK. Typically, the |
| | AD performs the first part of key derivations so that |
| | the authentication server gets the CK' and IK' keys |
| | already tied to a particular network name. |
| +-------+----------------------------+----------------+--+
| | | |
| | | ID, key deriv. |
| | | function, |
| | | network name |
| | +--------------->|
| | | |
| | | RAND, AUTN, |
| | | XRES, CK', IK' |
| | |<---------------+
| +-------+----------------------------+----------------+--+
| | Server now has the needed authentication vector. It |
| | generates an ephemeral key pair, sends the public key |
| | of that key pair and the first EAP method message to |
| | the peer. In the message the AT_PUB_ECDHE attribute |
| | carries the public key and the AT_KDF_FS attribute |
| | carries other FS-related parameters. Both of these are |
| | skippable attributes that can be ignored if the peer |
| | does not support this extension. |
| +-------+----------------------------+----------------+--+
| | | |
| | EAP-Req/AKA'-Challenge | |
| | AT_RAND, AT_AUTN, AT_KDF, | |
| | AT_KDF_FS, AT_KDF_INPUT, | |
| | AT_PUB_ECDHE, AT_MAC | |
| |<---------------------------+ |
+--+--------------+----------------------------+---------+ |
| The peer checks if it wants to do the FS extension. If | |
| yes, it will eventually respond with AT_PUB_ECDHE and | |
| AT_MAC. If not, it will ignore AT_PUB_ECDHE and | |
| AT_KDF_FS and base all calculations on basic EAP-AKA' | |
| attributes, continuing just as in EAP-AKA' per RFC | |
| 9048 rules. In any case, the peer needs to query the | |
| auth parameters from the USIM card. | |
+--+--------------+----------------------------+---------+ |
| | | |
| RAND, AUTN | | |
|<-------------+ | |
| | | |
| CK, IK, RES | | |
+------------->| | |
+--+--------------+----------------------------+---------+ |
| The peer now has everything to respond. If it wants to | |
| participate in the FS extension, it will then generate | |
| its key pair, calculate a shared key based on its key | |
| pair and the server's public key. Finally, it proceeds | |
| to derive all EAP-AKA' key values and constructs a | |
| full response. | |
+--+--------------+----------------------------+---------+ |
| | | |
| | EAP-Resp/AKA'-Challenge | |
| | AT_RES, AT_PUB_ECDHE, | |
| | AT_MAC | |
| +--------------------------->| |
| +-------+----------------------------+----------------+--+
| | The server now has all the necessary values. It |
| | generates the ECDHE shared secret and checks the RES |
| | and MAC values received in AT_RES and AT_MAC, |
| | respectively. Success requires both to be found |
| | correct. Note that when this document is used, |
| | the keys generated from EAP-AKA' are based on CK, IK, |
| | and the ECDHE value. Even if there was an attacker who |
| | held the long-term key, only an active attacker could |
| | have determined the generated session keys; in basic |
| | EAP-AKA' the generated keys are only based on CK and |
| | IK. |
| +-------+----------------------------+----------------+--+
| | | |
| | EAP-Success | |
| |<---------------------------+ |
| | | |
]]></artwork>
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<text x="28" y="36">USIM</text>
<text x="148" y="36">Peer</text>
<text x="380" y="36">Server</text>
<text x="524" y="36">AD</text>
<text x="308" y="68">EAP-Req/Identity</text>
<text x="232" y="116">EAP-Resp/Identity</text>
<text x="236" y="132">(Privacy-Friendly)</text>
<text x="124" y="180">Server</text>
<text x="168" y="180">now</text>
<text x="200" y="180">has</text>
<text x="228" y="180">an</text>
<text x="276" y="180">identity</text>
<text x="328" y="180">for</text>
<text x="360" y="180">the</text>
<text x="400" y="180">peer.</text>
<text x="440" y="180">The</text>
<text x="484" y="180">server</text>
<text x="116" y="196">then</text>
<text x="156" y="196">asks</text>
<text x="192" y="196">the</text>
<text x="228" y="196">help</text>
<text x="260" y="196">of</text>
<text x="284" y="196">AD</text>
<text x="308" y="196">to</text>
<text x="336" y="196">run</text>
<text x="368" y="196">AKA</text>
<text x="432" y="196">algorithms,</text>
<text x="140" y="212">generating</text>
<text x="208" y="212">RAND,</text>
<text x="256" y="212">AUTN,</text>
<text x="304" y="212">XRES,</text>
<text x="344" y="212">CK,</text>
<text x="376" y="212">IK.</text>
<text x="436" y="212">Typically,</text>
<text x="496" y="212">the</text>
<text x="108" y="228">AD</text>
<text x="156" y="228">performs</text>
<text x="208" y="228">the</text>
<text x="248" y="228">first</text>
<text x="292" y="228">part</text>
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<text x="352" y="228">key</text>
<text x="416" y="228">derivations</text>
<text x="476" y="228">so</text>
<text x="508" y="228">that</text>
<text x="112" y="244">the</text>
<text x="188" y="244">authentication</text>
<text x="276" y="244">server</text>
<text x="324" y="244">gets</text>
<text x="360" y="244">the</text>
<text x="392" y="244">CK'</text>
<text x="424" y="244">and</text>
<text x="456" y="244">IK'</text>
<text x="492" y="244">keys</text>
<text x="128" y="260">already</text>
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<text x="232" y="260">a</text>
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