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Retry if r is zero during signing #732

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merged 2 commits into from
Apr 18, 2020
Merged

Retry if r is zero during signing #732

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93d343b
Revert "ecdsa_impl: replace scalar if-checks with VERIFY_CHECKs in ec…
real-or-random Mar 31, 2020
37ed51a
Make ecdsa_sig_sign constant-time again after reverting 25e3cfb
real-or-random Mar 31, 2020
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9 changes: 4 additions & 5 deletions src/ecdsa_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -288,10 +288,6 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
secp256k1_fe_normalize(&r.y);
secp256k1_fe_get_b32(b, &r.x);
secp256k1_scalar_set_b32(sigr, b, &overflow);
/* These two conditions should be checked before calling */
VERIFY_CHECK(!secp256k1_scalar_is_zero(sigr));
VERIFY_CHECK(overflow == 0);

if (recid) {
/* 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.
Expand All @@ -310,7 +306,10 @@ static int secp256k1_ecdsa_sig_sign(const secp256k1_ecmult_gen_context *ctx, sec
if (recid) {
*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 */