Use constant-time comparison for output prefix verification in MAC#73
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Use constant-time comparison for output prefix verification in MAC#73JACKURUVI99 wants to merge 1 commit into
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Arrays.equals() and util.Bytes.equals() exit on the first mismatching byte, leaking timing information in security-sensitive MAC verification paths. Replace with MessageDigest.isEqual() (constant-time) in: - ChunkedHmacImpl.createVerification() — output prefix check - ChunkedAesCmacImpl.createVerification() — output prefix check - LegacyFullMac.verifyMac() — prefix check before rawMac.verifyMac() Consistent with the fix already applied in ChunkedMacVerificationFromComputation (175df91). Fixes tink-crypto#72.
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The output prefix is public and does not need constant time verification. |
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Summary
Three places in MAC verification use non-constant-time comparisons for the output prefix check, inconsistent with the fix recently applied in
ChunkedMacVerificationFromComputation(175df91).ChunkedHmacImpl.createVerification()—util.Bytes.equals()→Arrays.equals()(non-CT)ChunkedAesCmacImpl.createVerification()— sameLegacyFullMac.verifyMac()—Arrays.equals()directly (non-CT)All three are replaced with
MessageDigest.isEqual()which is guaranteed constant-time by the JCA spec, consistent with the pattern already used inChunkedMacVerificationFromComputation.Files Changed
ChunkedHmacImpl.javaBytes.equals()→MessageDigest.isEqual()ChunkedAesCmacImpl.javaBytes.equals()→MessageDigest.isEqual()LegacyFullMac.javaArrays.equals()→MessageDigest.isEqual()Fixes #72.