Add bip340 tagged hash functionality

include/wally_crypto.h

@@ -153,6 +153,20 @@ WALLY_CORE_API int wally_sha512(
     unsigned char *bytes_out,
     size_t len);
 
+/**
+ * BIP340 tagged hash: SHA-256(SHA-256(tag) || SHA-256(tag) || m)
+ *
+ * :param bytes: The message to hash.
+ * :param bytes_len: The length of ``bytes`` in bytes.
+ * :param tag: The BIP340 UTF-8 domain tag.
+ * :param bytes_out: Destination for the resulting 32-byte hash.
+ */
+WALLY_CORE_API int wally_tagged_hash(
+    const unsigned char *bytes,
+    size_t bytes_len,
+    const char *tag,
+    unsigned char *hash_out);
+
 /** Output length for `wally_ripemd160` */
 #define RIPEMD160_LEN 20
 

src/internal.c

@@ -103,6 +103,28 @@ int wally_bzero(void *bytes, size_t len)
     return WALLY_OK;
 }
 
+int wally_tagged_hash(const unsigned char *bytes, size_t bytes_len,
+                      const char *tag, unsigned char *hash_out)
+{
+    struct sha256 sha;
+    struct sha256_ctx ctx;
+
+    if ((!bytes && bytes_len != 0) || !hash_out)
+        return WALLY_EINVAL;
+
+    /* SHA256(SHA256(tag) || SHA256(tag) || msg) */
+    sha256(&sha, tag, strlen(tag));
+    sha256_init(&ctx);
+    sha256_update(&ctx, &sha, sizeof(sha));
+    sha256_update(&ctx, &sha, sizeof(sha));
+    sha256_update(&ctx, bytes, bytes_len);
+    sha256_done(&ctx, &sha);
+
+    memcpy(hash_out, &sha, sizeof(sha));
+    wally_clear(&sha, sizeof(sha));
+    return WALLY_OK;
+}
+
 int wally_sha256(const unsigned char *bytes, size_t bytes_len,
                  unsigned char *bytes_out, size_t len)
 {