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Secure Boot Root-of-Trust (ROT)

Design for storage of public key based root of trust into TPM:

  1. Use AES-CFB parameter encryption for all communication (salted and bound)
  2. Derive a password based on unique device parameters used as "auth" to load NV (authenticate)
  3. The NV contains a hash of the public key (hash matches .config setting)
  4. WolfBoot still has the public key internally and programs the TPM with the NV if not populated.
  5. The NV is locked and created under the platform hierarchy

Example:

$ ./examples/boot/secure_rot -write=../wolfBoot/wolfboot_signing_public_key.der -lock
TPM2: Caps 0x00000000, Did 0x0000, Vid 0x0000, Rid 0x 0
TPM2_Startup pass
TPM2_SelfTest pass
NV Auth (32)
	19 3f bf 0c bb 90 ca a1 40 96 a6 ee 8e fc 7c 3f | .?......@.....|?
	c1 c2 7f 1d c3 e0 a2 5e c7 72 5a a1 94 76 63 53 | .......^.rZ..vcS
Parameter Encryption: Enabled. (AES CFB)

TPM2_StartAuthSession: handle 0x2000000, algorithm AES
TPM2_StartAuthSession: sessionHandle 0x2000000
Storing hash of public key file ../wolfBoot/wolfboot_signing_public_key.der to NV index 0x1400200 with password protection

Public Key Hash (32)
	e3 29 f9 9e 56 93 6e 24 02 34 13 81 0f 7c 73 4d | .)..V.n$.4...|sM
	8f 9d 63 b8 8f 43 39 7b e5 46 93 dd 77 58 77 29 | ..c..C9{.F..wXw)
TPM2_NV_ReadPublic: Sz 14, Idx 0x1400200, nameAlg 11, Attr 0x42072005, authPol 0, dataSz 32, name 34
TPM2_NV_DefineSpace: Auth 0x4000000c, Idx 0x1400200, Attribs 0x1107763205, Size 32
TPM2_NV_Write: Auth 0x1400200, Idx 0x1400200, Offset 0, Size 32
Wrote 32 bytes to NV 0x1400200
Reading NV 0x1400200 public key hash
TPM2_NV_ReadPublic: Sz 14, Idx 0x1400200, nameAlg 11, Attr 0x62072005, authPol 0, dataSz 32, name 34
TPM2_NV_Read: Auth 0x1400200, Idx 0x1400200, Offset 0, Size 32
Read Public Key Hash (32)
	e3 29 f9 9e 56 93 6e 24 02 34 13 81 0f 7c 73 4d | .)..V.n$.4...|sM
	8f 9d 63 b8 8f 43 39 7b e5 46 93 dd 77 58 77 29 | ..c..C9{.F..wXw)
Locking NV index 0x1400200
NV 0x1400200 locked
TPM2_FlushContext: Closed handle 0x2000000

Secure Boot Encryption Key Storage

To seal a secret to PCR's without having the "brittleness" issue we use an external key to sign the state of the PCR(s).

Tools:

  • ./examples/pcr/policy_sign: Signs a digest for a PCR policy. Outputs the signature and also generates a policy authorization digest for the public key (-outpolicy)
  • ./examples/boot/secret_seal: Seals a secret using the authorization policy digest for the public key. If no secret provided a random value is generated and sealed.
  • ./examples/boot/secret_unseal: Unseals a secret using the sign authorization policy and the public key.

Example for created a signed PCR policy:

# Extend "aaa" to test PCR 16
echo aaa > aaa.bin
./examples/pcr/reset 16
./examples/pcr/extend 16 aaa.bin

# RSA Sign this PCR (result to pcrsig.bin) - also creates policyauth.bin based on public key
./examples/pcr/policy_sign -pcr=16 -rsa -key=./certs/example-rsa2048-key.der -out=pcrsig.bin -outpolicy=policyauth.bin
# OR
# ECC Sign
./examples/pcr/policy_sign -pcr=16 -ecc -key=./certs/example-ecc256-key.der -out=pcrsig.bin -outpolicy=policyauth.bin

Example for creating a sealed secret using that signed policy based on public key:

# Create a keyed hash sealed object using the policy authorization for the public key
./examples/boot/secret_seal -rsa -policy=policyauth.bin -out=sealblob.bin
./examples/boot/secret_seal -ecc -policy=policyauth.bin -out=sealblob.bin
# OR
# Provide the public key for policy authorization (instead of -policy=)
./examples/boot/secret_seal -rsa -publickey=./certs/example-rsa2048-key-pub.der -out=sealblob.bin
./examples/boot/secret_seal -ecc -publickey=./certs/example-ecc256-key-pub.der -out=sealblob.bin

Example for unsealing:

# Unseal using public key
./examples/boot/secret_unseal -pcr=16 -pcrsig=pcrsig.bin -rsa -publickey=./certs/example-rsa2048-key-pub.der -seal=sealblob.bin
./examples/boot/secret_unseal -pcr=16 -pcrsig=pcrsig.bin -ecc -publickey=./certs/example-ecc256-key-pub.der -seal=sealblob.bin