The 'rejects tampered data (auth tag mismatch)' test corrupted the
encrypted blob by replacing its first base64 char with 'X'. When the
random 16-byte IV's first base64 char was already 'X' (~1/64 chance),
the replacement was a no-op and decryption succeeded — causing the test
to flake ~1.6% of runs.
Fix: parse the iv:authTag:ciphertext format, XOR the first authTag byte
with 0xFF (guaranteed to change the value), reassemble. This reliably
triggers the AES-256-GCM integrity failure every time.
Verified: 30/30 isolated runs + 8/8 full-suite runs (1036/1036), zero
failures. The production encryptBackup/decryptBackup (AES-256-GCM)
code is correct and unchanged.
Hermes
1f887725fb