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Fix TPM2_ASN_TrimZeros skipping bytes - #644
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The loop advanced both idx and the in pointer, so each check after the first read two bytes past the intended one. An ECDSA R or S starting 00 <0x80 00 <0x80 lost a significant byte, and wolfTPM2_PK_EccSign then DER-encoded a wrong signature that the TLS peer rejected (-330). Only advance idx and return in + idx. Add a unit test using the S bytes from the failing CI run.
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🟢 Approval recommended
The localized fix corrects the indexing defect, with regression coverage and no identified blockers.
No open issues 🎉
What changed in this PR
Fixes leading-zero trimming in ASN.1 integers to prevent corruption of ECDSA signatures.
Changes:
- Advances only the index while trimming, keeping the returned pointer and length consistent.
- Adds regression tests for the reported failure, required sign padding, and consecutive leading zeros.
| File | Description |
|---|---|
| tests/unit_tests.c | Adds and registers trimming regression tests. |
| src/tpm2_asn.c | Removes double advancement that skipped significant bytes. |
Review effort: Balanced
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Okay to test. Contributor agreement on file. |
aidangarske
approved these changes
Oct 21, 2026
aidangarske
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aidangarske
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Thanks for the contribution @deadcaf3
dgarske
approved these changes
Oct 21, 2026
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Summary
TPM2_ASN_TrimZerosincremented bothidxandin, so after the first leading zero it checked bytes two positions ahead. An ECDSA R or S starting00 <0x80 00 <0x80lost a real byte, andwolfTPM2_PK_EccSignsent a bad signature.This caused a rare flake in the PQC TLS examples job (
tls client ecc -pk -aes failed!, client error-330). There, S padded to00 71 00 4f ...was trimmed to00 4f ..., giving a 67-byte signature instead of 69.Fix: only advance
idxand returnin + idx.Testing
test_TPM2_ASN_TrimZeroswith the bytes from the failed run. It fails without the fix.wc_ecc_verify_hash. The fix gives 69 bytes and verifies.make check(fwTPM + swtpm + v1.85) passes.