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Make ecdsa_sig_sign constant-time again after reverting 25e3cfb

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This commit is contained in:
Tim Ruffing 2020-03-31 14:57:19 +02:00
parent 93d343bfc5
commit 37ed51a7ea
1 changed files with 4 additions and 9 deletions
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13
src/ecdsa_impl.h
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@ -288,14 +288,6 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
secp256k1_fe_normalize(&r.y); secp256k1_fe_normalize(&r.y);
secp256k1_fe_get_b32(b, &r.x); secp256k1_fe_get_b32(b, &r.x);
secp256k1_scalar_set_b32(sigr, b, &overflow); secp256k1_scalar_set_b32(sigr, b, &overflow);
if (secp256k1_scalar_is_zero(sigr)) {
/* P.x = order is on the curve, so technically sig->r could end up zero, which would be an invalid signature.
* This branch is cryptographically unreachable as hitting it requires finding the discrete log of P.x = N.
*/
secp256k1_gej_clear(&rp);
secp256k1_ge_clear(&r);
return 0;
}
if (recid) { if (recid) {
/* The overflow condition is cryptographically unreachable as hitting it requires finding the discrete log /* The overflow condition is cryptographically unreachable as hitting it requires finding the discrete log
* of some P where P.x >= order, and only 1 in about 2^127 points meet this criteria. * of some P where P.x >= order, and only 1 in about 2^127 points meet this criteria.
@ -314,7 +306,10 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
if (recid) { if (recid) {
*recid ^= high; *recid ^= high;
} }
return !secp256k1_scalar_is_zero(sigs); /* P.x = order is on the curve, so technically sig->r could end up being zero, which would be an invalid signature.
* This is cryptographically unreachable as hitting it requires finding the discrete log of P.x = N.
*/
return !secp256k1_scalar_is_zero(sigr) & !secp256k1_scalar_is_zero(sigs);
} }
#endif /* SECP256K1_ECDSA_IMPL_H */ #endif /* SECP256K1_ECDSA_IMPL_H */