From 93d343bfc5323e56f6a60cb41d60b96368cc09c7 Mon Sep 17 00:00:00 2001 From: Tim Ruffing Date: Tue, 31 Mar 2020 14:28:48 +0200 Subject: [PATCH] Revert "ecdsa_impl: replace scalar if-checks with VERIFY_CHECKs in ecdsa_sig_sign" This reverts commit 25e3cfbf9b52d2f5afa543f967a73aa8850d2038. The reverted commit was probably based on the assumption that this is about the touched checks cover the secret nonce k instead of r, which is the x-coord of the public nonce. A signature with a zero r is invalid by the spec, so we should return 0 to make the caller retry with a different nonce. Overflow is not an issue. Fixes #720. --- src/ecdsa_impl.h | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/src/ecdsa_impl.h b/src/ecdsa_impl.h index 359c621b93c1f..a9442399fb8c1 100644 --- a/src/ecdsa_impl.h +++ b/src/ecdsa_impl.h @@ -288,10 +288,14 @@ 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 (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) { /* 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.