test(identity): make ciphertext tampering deterministic - #61
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Both tamper-detection tests built their payload as `'ff' + ct.slice(2)`, which is a no-op the 1-in-256 times the ciphertext already starts with `ff` — decryption then succeeds and the assertion fails for reasons unrelated to the code under test. Measured at 0.36% over 5000 samples, and it took down CI on the 1.0.0 release merge. Flipping the byte instead is a no-op for none of the 256 possible values.
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CI on
masterwent red on the 1.0.0 release merge (run 30407887853) attests/core/identity/crypto.test.ts— "Received function did not throw".It's a test bug, not a crypto bug. Both tamper-detection tests built their tampered payload as:
When the ciphertext already starts with
ff, that assignment changes nothing. The payload is byte-identical, AES-GCM decrypts it correctly, and.toThrow()fails. 1 in 256.Measured on the real code path: 18/5000 = 0.36%, against a predicted 1/256 = 0.39%.
Fixed by flipping the byte rather than assigning it, via a shared helper used at both sites. Verified exhaustively over all 256 possible first-byte values: the old form is a no-op for exactly 1, the new form for 0.
Targets
masterdirectly because that is the branch that is red and the release is gated behind it.