Make ecdsa_sig_sign constant-time again after reverting 25e3cfb

Author: real-or-random

src/ecdsa_impl.h

@@ -288,14 +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);
-    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.
@@ -314,7 +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 */