Add _prefix and _bip324 ellswift_xdh hash functions

include/secp256k1_ellswift.h

@@ -8,7 +8,7 @@ extern "C" {
 #endif
 
 /* This module provides an implementation of ElligatorSwift as well as a
- * version of x-only ECDH using it.
+ * version of x-only ECDH using it (including compatibility with BIP324).
  *
  * ElligatorSwift is described in https://eprint.iacr.org/2022/759 by
  * Chavez-Saab, Rodriguez-Henriquez, and Tibouchi. It permits encoding
@@ -67,6 +67,19 @@ typedef int (*secp256k1_ellswift_xdh_hash_function)(
     void *data
 );
 
+/** An implementation of an secp256k1_ellswift_xdh_hash_function which uses
+ *  SHA256(prefix64 || ell_a64 || ell_b64 || x32), where prefix64 is the 64-byte
+ *  array pointed to by data. */
+SECP256K1_API_VAR const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_prefix;
+
+/** An implementation of an secp256k1_ellswift_xdh_hash_function compatible with
+ *  BIP324. It returns H_tag(ell_a64 || ell_b64 || x32), where H_tag is the
+ *  BIP340 tagged hash function with tag "bip324_ellswift_xonly_ecdh". Equivalent
+ *  to secp256k1_ellswift_xdh_hash_function_prefix with prefix64 set to
+ *  SHA256("bip324_ellswift_xonly_ecdh")||SHA256("bip324_ellswift_xonly_ecdh").
+ *  The data argument is ignored. */
+SECP256K1_API_VAR const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_bip324;
+
 /** Construct a 64-byte ElligatorSwift encoding of a given pubkey.
  *
  *  Returns: 1 always.

src/modules/ellswift/main_impl.h

@@ -498,6 +498,51 @@ int secp256k1_ellswift_decode(const secp256k1_context *ctx, secp256k1_pubkey *pu
     return 1;
 }
 
+static int ellswift_xdh_hash_function_prefix(unsigned char *output, const unsigned char *x32, const unsigned char *ell_a64, const unsigned char *ell_b64, void *data) {
+    secp256k1_sha256 sha;
+
+    secp256k1_sha256_initialize(&sha);
+    secp256k1_sha256_write(&sha, data, 64);
+    secp256k1_sha256_write(&sha, ell_a64, 64);
+    secp256k1_sha256_write(&sha, ell_b64, 64);
+    secp256k1_sha256_write(&sha, x32, 32);
+    secp256k1_sha256_finalize(&sha, output);
+
+    return 1;
+}
+
+/** Set hash state to the BIP340 tagged hash midstate for "bip324_ellswift_xonly_ecdh". */
+static void secp256k1_ellswift_sha256_init_bip324(secp256k1_sha256* hash) {
+    secp256k1_sha256_initialize(hash);
+    hash->s[0] = 0x8c12d730ul;
+    hash->s[1] = 0x827bd392ul;
+    hash->s[2] = 0x9e4fb2eeul;
+    hash->s[3] = 0x207b373eul;
+    hash->s[4] = 0x2292bd7aul;
+    hash->s[5] = 0xaa5441bcul;
+    hash->s[6] = 0x15c3779ful;
+    hash->s[7] = 0xcfb52549ul;
+
+    hash->bytes = 64;
+}
+
+static int ellswift_xdh_hash_function_bip324(unsigned char* output, const unsigned char *x32, const unsigned char *ell_a64, const unsigned char *ell_b64, void *data) {
+    secp256k1_sha256 sha;
+
+    (void)data;
+
+    secp256k1_ellswift_sha256_init_bip324(&sha);
+    secp256k1_sha256_write(&sha, ell_a64, 64);
+    secp256k1_sha256_write(&sha, ell_b64, 64);
+    secp256k1_sha256_write(&sha, x32, 32);
+    secp256k1_sha256_finalize(&sha, output);
+
+    return 1;
+}
+
+const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_prefix = ellswift_xdh_hash_function_prefix;
+const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_bip324 = ellswift_xdh_hash_function_bip324;
+
 int secp256k1_ellswift_xdh(const secp256k1_context *ctx, unsigned char *output, const unsigned char *ell_a64, const unsigned char *ell_b64, const unsigned char *seckey32, int party, secp256k1_ellswift_xdh_hash_function hashfp, void *data) {
     int ret = 0;
     int overflow;