Merge bitcoin#732: Retry if r is zero during signing

37ed51a Make ecdsa_sig_sign constant-time again after reverting 25e3cfb (Tim Ruffing)
93d343b Revert "ecdsa_impl: replace scalar if-checks with VERIFY_CHECKs in ecdsa_sig_sign" (Tim Ruffing)

Pull request description:

ACKs for top commit:
  elichai:
    ACK 37ed51a makes sense.
  jonasnick:
    ACK 37ed51a

Tree-SHA512: 82b5b8e29f48e84fd7a0681b62923d3bd87d724b38ef18e8c7969b0dcc5a405ebb26c14b5c5f4c7ba0ccabd152d1531d217809d1daf40872fe0c1e079b55c64b

src/ecdsa_impl.h

@@ -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.
@@ -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 */