security: downgrade TPM attestation to software-only on SHA-1 fallback#319
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SHA-1 PCR measurements are cryptographically broken. When the TPM SHA-256 bank is unavailable and the provider falls back to SHA-1, the resulting measurement must not present as hardware-attested in the TRACE Claim. Set platform to software-only and emit a warning. Signed-off-by: Imran Siddique <imran.siddique@opaque.co> Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Summary
SHA-1 PCR bank measurements must not present as hardware attestation in the TRACE Claim. When the TPM SHA-256 bank is unavailable and the provider falls back to SHA-1, the resulting AttestationReport now sets platform to software-only rather than tpm. Adds a warning log at fallback time.
SHA-1 is cryptographically broken; an adversary with physical TPM access can manipulate PCR values in the SHA-1 bank. A sha1-based measurement that presents as hardware-attested gives false assurance to verifiers.
Test plan
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