extrakeys: Add xonly_pubkey with serialize, parse and from_pubkey

include/secp256k1_extrakeys.h

@@ -7,6 +7,75 @@
 extern "C" {
 #endif
 
+/** Opaque data structure that holds a parsed and valid "x-only" public key.
+ *  An x-only pubkey encodes a point whose Y coordinate is even. It is
+ *  serialized using only its X coordinate (32 bytes). See BIP-340 for more
+ *  information about x-only pubkeys.
+ *
+ *  The exact representation of data inside is implementation defined and not
+ *  guaranteed to be portable between different platforms or versions. It is
+ *  however guaranteed to be 64 bytes in size, and can be safely copied/moved.
+ *  If you need to convert to a format suitable for storage, transmission, or
+ *  comparison, use secp256k1_xonly_pubkey_serialize and
+ *  secp256k1_xonly_pubkey_parse.
+ */
+typedef struct {
+    unsigned char data[64];
+} secp256k1_xonly_pubkey;
+
+/** Parse a 32-byte sequence into a xonly_pubkey object.
+ *
+ *  Returns: 1 if the public key was fully valid.
+ *           0 if the public key could not be parsed or is invalid.
+ *
+ *  Args:   ctx: a secp256k1 context object (cannot be NULL).
+ *  Out: pubkey: pointer to a pubkey object. If 1 is returned, it is set to a
+ *               parsed version of input. If not, it's set to an invalid value.
+ *               (cannot be NULL).
+ *  In: input32: pointer to a serialized xonly_pubkey (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_parse(
+    const secp256k1_context* ctx,
+    secp256k1_xonly_pubkey* pubkey,
+    const unsigned char *input32
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Serialize an xonly_pubkey object into a 32-byte sequence.
+ *
+ *  Returns: 1 always.
+ *
+ *  Args:     ctx: a secp256k1 context object (cannot be NULL).
+ *  Out: output32: a pointer to a 32-byte array to place the serialized key in
+ *                 (cannot be NULL).
+ *  In:    pubkey: a pointer to a secp256k1_xonly_pubkey containing an
+ *                 initialized public key (cannot be NULL).
+ */
+SECP256K1_API int secp256k1_xonly_pubkey_serialize(
+    const secp256k1_context* ctx,
+    unsigned char *output32,
+    const secp256k1_xonly_pubkey* pubkey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Converts a secp256k1_pubkey into a secp256k1_xonly_pubkey.
+ *
+ *  Returns: 1 if the public key was successfully converted
+ *           0 otherwise
+ *
+ *  Args:         ctx: pointer to a context object (cannot be NULL)
+ *  Out: xonly_pubkey: pointer to an x-only public key object for placing the
+ *                     converted public key (cannot be NULL)
+ *          pk_parity: pointer to an integer that will be set to 1 if the point
+ *                     encoded by xonly_pubkey is the negation of the pubkey and
+ *                     set to 0 otherwise. (can be NULL)
+ *  In:        pubkey: pointer to a public key that is converted (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_from_pubkey(
+    const secp256k1_context* ctx,
+    secp256k1_xonly_pubkey *xonly_pubkey,
+    int *pk_parity,
+    const secp256k1_pubkey *pubkey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(4);
+
 #ifdef __cplusplus
 }
 #endif