Merge bitcoin-core#629: Avoid calling _is_zero when _set_b32 fails.

cd473e0 Avoid calling secp256k1_*_is_zero when secp256k1_*_set_b32 fails. (Gregory Maxwell)

Pull request description:

  Most of the codebase correctly used short-cutting to avoid calling
   _is_zero on possibly incompletely initialized elements, but a few
   places were missed.

ACKs for commit cd473e:
  sipa:
    utACK cd473e0
  jonasnick:
    utACK cd473e0

Tree-SHA512: d6af2863f6795d2df26f2bd05a4e33085e88c45f7794601ea57e67238a2073ef1ee3ba0feab62a7fcbc0636c48dfd80eea07d0ca4f194414127f914b0478c732

Author: gmaxwell

src/ecmult_gen_impl.h

@@ -187,7 +187,7 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const
     do {
         secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
         retry = !secp256k1_fe_set_b32(&s, nonce32);
-        retry |= secp256k1_fe_is_zero(&s);
+        retry = retry || secp256k1_fe_is_zero(&s);
     } while (retry); /* This branch true is cryptographically unreachable. Requires sha256_hmac output > Fp. */
     /* Randomize the projection to defend against multiplier sidechannels. */
     secp256k1_gej_rescale(&ctx->initial, &s);
@@ -196,7 +196,7 @@ static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context *ctx, const
         secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);
         secp256k1_scalar_set_b32(&b, nonce32, &retry);
         /* A blinding value of 0 works, but would undermine the projection hardening. */
-        retry |= secp256k1_scalar_is_zero(&b);
+        retry = retry || secp256k1_scalar_is_zero(&b);
     } while (retry); /* This branch true is cryptographically unreachable. Requires sha256_hmac output > order. */
     secp256k1_rfc6979_hmac_sha256_finalize(&rng);
     memset(nonce32, 0, 32);

src/modules/recovery/main_impl.h

@@ -147,7 +147,7 @@ int secp256k1_ecdsa_sign_recoverable(const secp256k1_context* ctx, secp256k1_ecd
                 break;
             }
             secp256k1_scalar_set_b32(&non, nonce32, &overflow);
-            if (!secp256k1_scalar_is_zero(&non) && !overflow) {
+            if (!overflow && !secp256k1_scalar_is_zero(&non)) {
                 if (secp256k1_ecdsa_sig_sign(&ctx->ecmult_gen_ctx, &r, &s, &sec, &msg, &non, &recid)) {
                     break;
                 }

src/secp256k1.c

@@ -518,7 +518,7 @@ int secp256k1_ec_pubkey_create(const secp256k1_context* ctx, secp256k1_pubkey *p
     ARG_CHECK(seckey != NULL);
 
     secp256k1_scalar_set_b32(&sec, seckey, &overflow);
-    ret = (!overflow) & (!secp256k1_scalar_is_zero(&sec));
+    ret = !overflow && !secp256k1_scalar_is_zero(&sec);
     if (ret) {
         secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &pj, &sec);
         secp256k1_ge_set_gej(&p, &pj);