Add a function to extract the secretkey from a keypair

include/secp256k1_extrakeys.h

@@ -165,6 +165,19 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_create(
     const unsigned char *seckey
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
+/** Get the secret key from a keypair.
+ *
+ *  Returns: 0 if the arguments are invalid. 1 otherwise.
+ *  Args:   ctx: pointer to a context object (cannot be NULL)
+ *  Out: seckey: pointer to a 32-byte buffer for the secret key (cannot be NULL)
+ *  In: keypair: pointer to a keypair (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_sec(
+    const secp256k1_context* ctx,
+    unsigned char *seckey,
+    const secp256k1_keypair *keypair
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
 /** Get the public key from a keypair.
  *
  *  Returns: 0 if the arguments are invalid. 1 otherwise.

src/modules/extrakeys/main_impl.h

@@ -186,6 +186,16 @@ int secp256k1_keypair_create(const secp256k1_context* ctx, secp256k1_keypair *ke
     return ret;
 }
 
+int secp256k1_keypair_sec(const secp256k1_context* ctx, unsigned char *seckey, const secp256k1_keypair *keypair) {
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(seckey != NULL);
+    memset(seckey, 0, 32);
+    ARG_CHECK(keypair != NULL);
+
+    memcpy(seckey, &keypair->data[0], 32);
+    return 1;
+}
+
 int secp256k1_keypair_pub(const secp256k1_context* ctx, secp256k1_pubkey *pubkey, const secp256k1_keypair *keypair) {
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(pubkey != NULL);