Allow initializing tagged sha256

This will be used by the schnorrsig module

diff --git a/src/hash_impl.h b/src/hash_impl.h
index 1985a07836daf2938332ab0588c614d7f3dab880..409772587bebb3a88976e129508165fba18593c5 100644 (file)
--- a/src/hash_impl.h
+++ b/src/hash_impl.h
@@ -164,6 +164,19 @@ static void secp256k1_sha256_finalize(secp256k1_sha256 *hash, unsigned char *out
     memcpy(out32, (const unsigned char*)out, 32);
 }
 
+/* Initializes a sha256 struct and writes the 64 byte string
+ * SHA256(tag)||SHA256(tag) into it. */
+static void secp256k1_sha256_initialize_tagged(secp256k1_sha256 *hash, const unsigned char *tag, size_t taglen) {
+    unsigned char buf[32];
+    secp256k1_sha256_initialize(hash);
+    secp256k1_sha256_write(hash, tag, taglen);
+    secp256k1_sha256_finalize(hash, buf);
+
+    secp256k1_sha256_initialize(hash);
+    secp256k1_sha256_write(hash, buf, 32);
+    secp256k1_sha256_write(hash, buf, 32);
+}
+
 static void secp256k1_hmac_sha256_initialize(secp256k1_hmac_sha256 *hash, const unsigned char *key, size_t keylen) {
     size_t n;
     unsigned char rkey[64];