test: cross-check P2TR signing against an independent implementation - #29
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test: cross-check P2TR signing against an independent implementation#29BitHighlander wants to merge 1 commit into
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Spends a P2TR input on the emulator and compares the 64-byte witness byte for byte against a signature computed outside the firmware. The expected value is not a round trip through our own verifier -- that would pass even if the device committed to the wrong transaction. It comes from a standalone Python implementation of BIP-340/341 written from the specs, keyed from BIP-86's own published xprv for m/86'/0'/0'/0/0, and self-checked against BIP-86's published internal and output keys before being used. BIP-340 signing is deterministic given aux_rand, so equality is meaningful. This caught a real bug. BIP-143 hashes prevouts/sequences/outputs with DOUBLE sha256 (hasher_sign is HASHER_SHA2D for Bitcoin) while BIP-341 specifies SINGLE sha256, so reusing the BIP-143 accumulators produced a cryptographically valid signature over the wrong commitment -- exactly the failure a self-consistent test cannot see. Includes a synthetic prev-tx fixture in txcache so the test runs offline.
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Stacked on #28 — base is
test/getaddress-taproot.Spends a P2TR input on the emulator and compares the 64-byte witness byte for byte against a signature computed outside the firmware.
Why not a round trip
Verifying the device's signature with our own verifier would pass even if the device committed to the wrong transaction. So the expected value comes from a standalone Python implementation of BIP-340/341 written from the specs, keyed from BIP-86's own published xprv for
m/86'/0'/0'/0/0, and self-checked against BIP-86's published internal and output keys before being trusted. BIP-340 signing is deterministic givenaux_rand, so equality is meaningful rather than a smoke test.It caught a real bug
BIP-143 hashes prevouts/sequences/outputs with double sha256 (
hasher_signisHASHER_SHA2Dfor Bitcoin); BIP-341 specifies single sha256. The firmware was reusing the BIP-143 accumulators, so it produced a cryptographically valid signature over a commitment nobody authorised — invisible to any self-consistent check. Fixed in the companion firmware branch.A synthetic prev-tx fixture ships in
txcacheso this runs offline.Same gating caveat as #28:
TAPROOT_FIRMWARE_VERSION = "7.16.0"whileCMakeLists.txtis7.15.0, so it SKIPS until the project version bumps.