Handle incompatibility between parameters and unique in TPublic

[nicolastemciuc]

Summary

This PR addresses an issue where the t_public.parameters and t_public.unique are incompatible by preventing the gem from raising an exception when this happens and just returning nil as result of TPM:KeyAttestation#key.

Why?

We discovered this issue after upgrading tpm-key_attestation to 0.13.1 in cedarcode/webauthn-ruby#449. During this upgrade, a test failure in the WebAuthn test suite revealed an unsupported edge case: calling the key method of KeyAttestation raises an exception when t_public.parameters and t_public.unique are incompatible.

The exception occurs in the following code:

tpm-key_attestation/lib/tpm/t_public.rb

Lines 82 to 85 in 8ea5976

	point = OpenSSL::PKey::EC::Point.new(
	group,
	bn(ECC_UNCOMPRESSED_POINT_INDICATOR + unique.x.buffer.value + unique.y.buffer.value)
	)

This issue went unnoticed before because key_attestation.valid? used to return false (because it failed to validate the signature), preventing key_attestation.key from being called. However, after changes to the signature validation process in #29, valid? now returns true for this scenario, which led the test to fail because the pubArea for this scenario was set up with incompatible parameters and unique attributes (because the coordinates x and y are inconsistent with its curve).

Although we haven’t encountered this scenario in real-world applications, the WebAuthn test suite helped uncover this validation gap.

[nicolastemciuc]

It's worth discussing (and perhaps checking the TPM 2.0 documentation) whether the valid? method in KeyAttestation should return true when key is nil

Previously, valid? returned false in this case:

tpm-key_attestation/lib/tpm/key_attestation.rb

Lines 46 to 48 in c92d696

	def valid?
	!!key
	end