fix(ecc): reduce ECDSA x-coordinate - #935
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Add a P-256 vector whose verification point has an x-coordinate larger than the subgroup order. The valid signature captures the interoperability failure reported by Wycheproof. Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>
Reduce the reconstructed verification point's x-coordinate modulo the curve order before comparing it with r, as required by ECDSA. This accepts valid signatures with x greater than or equal to n and lets the previously skipped Wycheproof cases run normally. Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>
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Summary
ECDSA verification compared the recomputed affine x-coordinate directly with
r. Valid ECDSA verification requires comparingx mod nwithr, wherenis the curve order.As a result, valid signatures with a recomputed x-coordinate greater than or equal to
nwere rejected. This is a correctness and interoperability issue. It causes false negatives but does not allow invalid signatures or forgeries.SEC 1: Elliptic Curve Cryptography, Version 2.0 Section 4.1.4, steps 7–8, specifies that ECDSA verification must compute
v = xR mod nand comparevwithr.Changes
r.k*G has a large x-coordinate.The regression vector is tcId 285 in the Wycheproof corpus bundled with PyCryptodome. The same vector is tcId 350 in the current C2SP Wycheproof corpus.