Skip to content

Retry if r is zero during signing #732

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 2 commits into from
Apr 18, 2020
Merged

Retry if r is zero during signing #732

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
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
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
9 changes: 4 additions & 5 deletions src/ecdsa_impl.h
View file
Open in desktop
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 */