Identity quote failure when using TPM devices with manufacturer certificates signed by ECC keys

Environment

• OS / version: OpenBMC Yocto “phosphor”, Ubuntu 20.04, Ubuntu 22.04
• Processor architecture: ARM cortex A7, AMD x86_64 (mostly irrelevant)
• TPM Manufacturer: Nuvoton TPM signed by NUVO_1111.pem, but reproducible with swtpm.
• Keylime version: 6.4.0, but problem is reproducible with current master branch (Aug 8 2022)

Description

During the initial identity quote at the start of attestation the function tpm_main.verify_ek() is invoked by the keylime tenant to ensure that the TPM device on the agent is genuine. The process involves listing all the manufacturer certificates in the tpm_cert_store directory, finding one that has an Issuer that matches the EK cert’s, and then running verify().

When the manufacturer certificate signature algorithm is EC(*) the verify() function call fails, resulting in attestation failure.

Expected behavior vs. actual behavior

• Expected behavior: a Nuvoton device certified by the file keylime/tmp_cert_store/NUVO0_1111.pem should work.
• Actual behavior: said Nuvoton TPM device will never result in successful attestation, because the NUVO_1111.pem file is signed with an ECC key. The following error is generated during the identity quote phase by the tenant:

2022-08-09 14:43:15.313 - keylime.tpm - ERROR - verify() takes 4 positional arguments but 5 were given
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/keylime-6.4.0-py3.9.egg/keylime/tpm/tpm_main.py", line 857, in verify_ek
    signcert.public_key().verify(
TypeError: verify() takes 4 positional arguments but 5 were given

Steps to reproduce problem

1A. Install keylime attestation on any system with a Nuvoton chip that is certified by the NUVO_1111.pem file. 1B. Alternatively, create a swtpm instance in which the manufacturer cert is signed by an ECC key. 2. Start the keylime agent and let it register. Registration should execute normally. 3. Attempt to start attestation with the keylime_tenant -c add command. Expect the error message above to show up.

Root cause analysis

The problem is with the following sequence in the TPM code (keylime/tpm/tpm_main.py):

                    signcert.public_key().verify(
                        ek509.signature,
                        ek509.tbs_certificate_bytes,
                        padding.PKCS1v15(),
                        ek509.signature_hash_algorithm,
                    )

The call to the verify() method makes an assumption that (including “self”) 5 arguments are needed. But on elliptic curve public keys only 4 arguments are expected (there is no padding).

• Abstract method signature for RSA public keys: https://github.com/pyca/cryptography/blob/6b63cd56c746150ff49fdfe38c422523afaf21b8/src/cryptography/hazmat/primitives/asymmetric/rsa.py#L98-L104

• Abstract method signature for ECC public keys: https://github.com/pyca/cryptography/blob/6b63cd56c746150ff49fdfe38c422523afaf21b8/src/cryptography/hazmat/primitives/asymmetric/ec.py#L153-L158

This discrepancy causes the problem.

Proposed (tentative) fix

Insert an if statement to handle the different verify() method signatures in python cryptography.

[sergio-correia]

@THS-on: yep. @kkaarreell is adding a test for this in #1175

[THS-on]

@sergio-correia is this now fully fixed?