Merge bitcoin-core/secp256k1#1300: Avoid normalize conditional on VERIFY

97c63b9 Avoid normalize conditional on VERIFY (Pieter Wuille)

Pull request description:

  In the old code, `secp256k1_gej_rescale` requires a normalized input in VERIFY mode, but not otherwise. Its requirements shouldn't depend on this mode being enabled or not.

ACKs for top commit:
  real-or-random:
    utACK 97c63b9 I've also verified that the loop in secp256k1_ecmult_strauss_wnaf holds up the invariant that the magnitude of Z is 1, even with the normalization removed
  jonasnick:
    ACK 97c63b9

Tree-SHA512: 9598c133c6f4e488c74512089dabe0508529f20ca782be1c8fbeae9d7f132da9d570a061053acd3d245a9a187abf1f2581207441ce6aac8d0f8972cf357a349f

Author: sipa

src/ecmult_impl.h

@@ -279,9 +279,6 @@ static void secp256k1_ecmult_strauss_wnaf(const struct secp256k1_strauss_state *
          */
         tmp = a[np];
         if (no) {
-#ifdef VERIFY
-            secp256k1_fe_normalize_var(&Z);
-#endif
             secp256k1_gej_rescale(&tmp, &Z);
         }
         secp256k1_ecmult_odd_multiples_table(ECMULT_TABLE_SIZE(WINDOW_A), state->pre_a + no * ECMULT_TABLE_SIZE(WINDOW_A), state->aux + no * ECMULT_TABLE_SIZE(WINDOW_A), &Z, &tmp);

src/group_impl.h

@@ -748,7 +748,9 @@ static void secp256k1_gej_rescale(secp256k1_gej *r, const secp256k1_fe *s) {
     secp256k1_fe zz;
     secp256k1_gej_verify(r);
     secp256k1_fe_verify(s);
-    VERIFY_CHECK(!secp256k1_fe_is_zero(s));
+#ifdef VERIFY
+    VERIFY_CHECK(!secp256k1_fe_normalizes_to_zero_var(s));
+#endif
     secp256k1_fe_sqr(&zz, s);
     secp256k1_fe_mul(&r->x, &r->x, &zz);                /* r->x *= s^2 */
     secp256k1_fe_mul(&r->y, &r->y, &zz);