Add ECDSA adaptor signature APIs

This commit adds the ECDSA adaptor signature APIs:

- Encrypted Signing

  Creates an adaptor signature, which includes a proof to verify the adaptor
  signature.

- Encryption Verification

  Verifies that the adaptor decryption key can be extracted from the adaptor
  signature and the completed ECDSA signature.

- Decrypt Signature

  Derives an ECDSA signature from an adaptor signature and an adaptor decryption
  key.

- Key Recovery

  Extracts the adaptor decryption key from the complete signature and the adaptor
  signature.

include/secp256k1_ecdsa_adaptor.h

@@ -51,6 +51,104 @@ typedef int (*secp256k1_nonce_function_hardened_ecdsa_adaptor)(
  */
 SECP256K1_API extern const secp256k1_nonce_function_hardened_ecdsa_adaptor secp256k1_nonce_function_ecdsa_adaptor;
 
+/** Encrypted Signing
+ *
+ *  Creates an adaptor signature, which includes a proof to verify the adaptor
+ *  signature.
+ *
+ *  Returns: 1 on success, 0 on failure
+ *  Args:             ctx: a secp256k1 context object, initialized for signing
+ *                         (cannot be NULL)
+ *  Out:   adaptor_sig162: pointer to 162 byte to store the returned signature
+ *                         (cannot be NULL)
+ *  In:          seckey32: pointer to 32 byte secret key that will be used for
+ *                         signing (cannot be NULL)
+ *                 enckey: pointer to the encryption public key (cannot be NULL)
+ *                  msg32: pointer to the 32-byte message to sign (cannot be NULL)
+ *                noncefp: pointer to a nonce generation function. If NULL,
+ *                         secp256k1_nonce_function_ecdsa_adaptor is used
+ *                  ndata: pointer to arbitrary data used by the nonce generation
+ *                         function (can be NULL). If it is non-NULL and
+ *                         secp256k1_nonce_function_ecdsa_adaptor is used, then
+ *                         ndata must be a pointer to 32-byte auxiliary randomness
+ *                         as per BIP-340.
+ */
+SECP256K1_API int secp256k1_ecdsa_adaptor_encrypt(
+    const secp256k1_context* ctx,
+    unsigned char *adaptor_sig162,
+    unsigned char *seckey32,
+    const secp256k1_pubkey *enckey,
+    const unsigned char *msg32,
+    secp256k1_nonce_function_hardened_ecdsa_adaptor noncefp,
+    void *ndata
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
+
+/** Encryption Verification
+ *
+ *  Verifies that the adaptor decryption key can be extracted from the adaptor signature
+ *  and the completed ECDSA signature.
+ *
+ *  Returns: 1 on success, 0 on failure
+ *  Args:            ctx: a secp256k1 context object, initialized for verification
+ *                        (cannot be NULL)
+ *  In:   adaptor_sig162: pointer to 162-byte signature to verify (cannot be NULL)
+ *                pubkey: pointer to the public key corresponding to the secret key
+ *                        used for signing (cannot be NULL)
+ *                 msg32: pointer to the 32-byte message (cannot be NULL)
+ *                enckey: pointer to the adaptor encryption public key (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_ecdsa_adaptor_sig_verify(
+    const secp256k1_context* ctx,
+    const unsigned char *adaptor_sig162,
+    const secp256k1_pubkey *pubkey,
+    const unsigned char *msg32,
+    const secp256k1_pubkey *enckey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
+
+/** Decrypt Signature
+ *
+ *  Derives an ECDSA signature from an adaptor signature and an adaptor decryption key.
+ *
+ *  Returns: 1 on success, 0 on failure
+ *  Args:              ctx: a secp256k1 context object (cannot be NULL)
+ *  Out:               sig: pointer to the ECDSA signature to create (cannot
+ *                          be NULL)
+ *  In:           deckey32: pointer to 32-byte decryption secret key for the adaptor
+ *                          encryption public key (cannot be NULL)
+ *          adaptor_sig162: pointer to 162-byte byte adaptor sig (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_ecdsa_adaptor_decrypt(
+    const secp256k1_context* ctx,
+    secp256k1_ecdsa_signature *sig,
+    const unsigned char *deckey32,
+    const unsigned char *adaptor_sig162
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+/** Decryption Key Recovery
+ *
+ *  Extracts the adaptor decryption key from the complete signature and the adaptor
+ *  signature.
+ *
+ *  Returns: 1 on success, 0 on failure
+ *  Args:             ctx: a secp256k1 context object, initialized for signing
+ *                         (cannot be NULL)
+ *  Out:         deckey32: pointer to 32-byte adaptor decryption key for the adaptor
+ *                         encryption public key (cannot be NULL)
+ *  In:               sig: pointer to ECDSA signature to recover the adaptor decryption
+ *                         key from (cannot be NULL)
+ *            adaptor_sig: pointer to adaptor signature to recover the adaptor
+ *                         decryption key from (cannot be NULL)
+ *                 enckey: pointer to the adaptor encryption public key
+ *                         (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_ecdsa_adaptor_recover(
+    const secp256k1_context* ctx,
+    unsigned char *deckey32,
+    const secp256k1_ecdsa_signature *sig,
+    const unsigned char *adaptor_sig162,
+    const secp256k1_pubkey *enckey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
+
 #ifdef __cplusplus
 }
 #endif