The sign_xonly function incorrectly assumed even y-parity for all keys.
BIP-340 requires checking the actual y-parity of the derived pubkey and
negating the secret key when y is odd. Without this, signatures for keys
with odd-y pubkeys (roughly half of all keys) were invalid.
All operations now pass correctness tests:
- sign + verify for keys 1, 2, 3, 0xff, and 0xd217c1... (random)
- verify_fast (skip y-parity check)
- batch_verify (5 signatures from same pubkey)
C standalone benchmark (x86_64):
verifySchnorrFast: 52 µs (19,143 ops/s)
verifySchnorr: 60 µs (16,616 ops/s)
signSchnorr: 109 µs (9,177 ops/s)
pubkeyCreate: 54 µs (18,381 ops/s)
ecdhXOnly: 59 µs (16,958 ops/s)
https://claude.ai/code/session_011KVZhDcV2G7idNWEBz12GY
Claude
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