sm2: reject point at infinity in signature verification - #1946
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Step B6 of the verification algorithm fails if [s']G + [t]PA is the point at infinity. Without the check its x coordinate is taken as 0 and a signature with r = e passes B7.
tarcieri
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Oct 2, 2026
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In
verify_prehash, step B6 computes[s']G + [t]PAand goes straight to.to_affine().x(). draft-shen-sm2-ecdsa 5.3.1 (and GB/T 32918.2) say verification fails if that point is the point at infinity, but here the identity's x is just 0, so B7 turns intor == eand passes.Producing such a signature needs the private key (
r = e,s = -r*d / (1 + d)), so it's a conformance issue rather than a forgery, but other implementations reject it and this one shouldn't accept it either.This adds the identity check before taking x, plus a test in
tests/sm2dsa.rsthat builds that signature and fails on master.