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test(brc105): fix ~1/256 flaky nonce-tamper test (red 3.12 shard) #2

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Merged
kryp2 merged 1 commit into from
Jun 1, 2026
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test(brc105): fix ~1/256 flaky nonce-tamper test (red 3.12 shard) #2

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8 changes: 6 additions & 2 deletions tests/test_brc105.py
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Original file line number Diff line number Diff line change
Expand Up @@ -40,8 +40,12 @@ def test_unknown_nonce_rejected(self):

def test_tampered_nonce_rejected(self):
nonce = self.manager.create()
# Flip a byte in the random part
tampered = "ff" + nonce[2:]
# Flip the first byte of the random part. XOR with 0xff guarantees a
# change regardless of the original value — a plain "ff" + nonce[2:]
# would be a no-op (and falsely pass) ~1/256 of the time, when the
# random first byte already happened to be 0xff.
flipped = f"{int(nonce[:2], 16) ^ 0xFF:02x}"
tampered = flipped + nonce[2:]
# Remove from store so we test the HMAC check path
self.manager._nonces[tampered] = self.manager._nonces.pop(nonce)
assert not self.manager.verify(tampered)
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