Merge dashpay#463: Reduce usage of hardcoded size constants

c7680e5 Reduce usage of hardcoded size constants (Thomas Snider)

Pull request description:

  In particular the usage of keylen in nonce_function_rfc6979 seemed precarious - in one conditional it was unconditionally set, then in the next it was added to.  While it was clearly correct as written, I think this change makes it easier to reason about for new eyes and more resistant to breakage if there is any future change to what gets fed into the PRNG.

Tree-SHA512: 2241c183acc0f318f85a11ccff7fe28de7777bc53dea93ab8308bad15871047a268c6a2b36f77a599dce536fca48ab305ea746223840bc10953c893daffa0a50

src/hash_impl.h

@@ -133,9 +133,10 @@ static void secp256k1_sha256_write(secp256k1_sha256 *hash, const unsigned char *
     hash->bytes += len;
     while (bufsize + len >= 64) {
         /* Fill the buffer, and process it. */
-        memcpy(((unsigned char*)hash->buf) + bufsize, data, 64 - bufsize);
-        data += 64 - bufsize;
-        len -= 64 - bufsize;
+        size_t chunk_len = 64 - bufsize;
+        memcpy(((unsigned char*)hash->buf) + bufsize, data, chunk_len);
+        data += chunk_len;
+        len -= chunk_len;
         secp256k1_sha256_transform(hash->s, hash->buf);
         bufsize = 0;
     }
@@ -162,11 +163,11 @@ static void secp256k1_sha256_finalize(secp256k1_sha256 *hash, unsigned char *out
 }
 
 static void secp256k1_hmac_sha256_initialize(secp256k1_hmac_sha256 *hash, const unsigned char *key, size_t keylen) {
-    int n;
+    size_t n;
     unsigned char rkey[64];
-    if (keylen <= 64) {
+    if (keylen <= sizeof(rkey)) {
         memcpy(rkey, key, keylen);
-        memset(rkey + keylen, 0, 64 - keylen);
+        memset(rkey + keylen, 0, sizeof(rkey) - keylen);
     } else {
         secp256k1_sha256 sha256;
         secp256k1_sha256_initialize(&sha256);
@@ -176,17 +177,17 @@ static void secp256k1_hmac_sha256_initialize(secp256k1_hmac_sha256 *hash, const
     }
 
     secp256k1_sha256_initialize(&hash->outer);
-    for (n = 0; n < 64; n++) {
+    for (n = 0; n < sizeof(rkey); n++) {
         rkey[n] ^= 0x5c;
     }
-    secp256k1_sha256_write(&hash->outer, rkey, 64);
+    secp256k1_sha256_write(&hash->outer, rkey, sizeof(rkey));
 
     secp256k1_sha256_initialize(&hash->inner);
-    for (n = 0; n < 64; n++) {
+    for (n = 0; n < sizeof(rkey); n++) {
         rkey[n] ^= 0x5c ^ 0x36;
     }
-    secp256k1_sha256_write(&hash->inner, rkey, 64);
-    memset(rkey, 0, 64);
+    secp256k1_sha256_write(&hash->inner, rkey, sizeof(rkey));
+    memset(rkey, 0, sizeof(rkey));
 }
 
 static void secp256k1_hmac_sha256_write(secp256k1_hmac_sha256 *hash, const unsigned char *data, size_t size) {

src/secp256k1.c

@@ -132,7 +132,7 @@ static int secp256k1_pubkey_load(const secp256k1_context* ctx, secp256k1_ge* ge,
          * representation inside secp256k1_pubkey, as conversion is very fast.
          * Note that secp256k1_pubkey_save must use the same representation. */
         secp256k1_ge_storage s;
-        memcpy(&s, &pubkey->data[0], 64);
+        memcpy(&s, &pubkey->data[0], sizeof(s));
         secp256k1_ge_from_storage(ge, &s);
     } else {
         /* Otherwise, fall back to 32-byte big endian for X and Y. */
@@ -149,7 +149,7 @@ static void secp256k1_pubkey_save(secp256k1_pubkey* pubkey, secp256k1_ge* ge) {
     if (sizeof(secp256k1_ge_storage) == 64) {
         secp256k1_ge_storage s;
         secp256k1_ge_to_storage(&s, ge);
-        memcpy(&pubkey->data[0], &s, 64);
+        memcpy(&pubkey->data[0], &s, sizeof(s));
     } else {
         VERIFY_CHECK(!secp256k1_ge_is_infinity(ge));
         secp256k1_fe_normalize_var(&ge->x);
@@ -319,9 +319,14 @@ int secp256k1_ecdsa_verify(const secp256k1_context* ctx, const secp256k1_ecdsa_s
             secp256k1_ecdsa_sig_verify(&ctx->ecmult_ctx, &r, &s, &q, &m));
 }
 
+static SECP256K1_INLINE void buffer_append(unsigned char *buf, unsigned int *offset, const void *data, unsigned int len) {
+    memcpy(buf + *offset, data, len);
+    *offset += len;
+}
+
 static int nonce_function_rfc6979(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *algo16, void *data, unsigned int counter) {
    unsigned char keydata[112];
-   int keylen = 64;
+   unsigned int offset = 0;
    secp256k1_rfc6979_hmac_sha256 rng;
    unsigned int i;
    /* We feed a byte array to the PRNG as input, consisting of:
@@ -332,17 +337,15 @@ static int nonce_function_rfc6979(unsigned char *nonce32, const unsigned char *m
     *  different argument mixtures to emulate each other and result in the same
     *  nonces.
     */
-   memcpy(keydata, key32, 32);
-   memcpy(keydata + 32, msg32, 32);
+   buffer_append(keydata, &offset, key32, 32);
+   buffer_append(keydata, &offset, msg32, 32);
    if (data != NULL) {
-       memcpy(keydata + 64, data, 32);
-       keylen = 96;
+       buffer_append(keydata, &offset, data, 32);
    }
    if (algo16 != NULL) {
-       memcpy(keydata + keylen, algo16, 16);
-       keylen += 16;
+       buffer_append(keydata, &offset, algo16, 16);
    }
-   secp256k1_rfc6979_hmac_sha256_initialize(&rng, keydata, keylen);
+   secp256k1_rfc6979_hmac_sha256_initialize(&rng, keydata, offset);
    memset(keydata, 0, sizeof(keydata));
    for (i = 0; i <= counter; i++) {
        secp256k1_rfc6979_hmac_sha256_generate(&rng, nonce32, 32);