docs: def asn1_from_ecdsap256_private_key(priv_key, pub_key):
      """
      Returns the ASN.1 encoding of a ECDSAP256 private ket.
  
      :priv_key: an ECDSAP256PrivateKey instance
      :returns: the ASN.1 encoding as a byte string
      """
      encoded_len = _lib.xtt_asn1_private_key_length()
      encoded = _ffi.new('unsigned char[]', encoded_len)
      rc = _lib.xtt_asn1_from_ecdsap256_private_key(priv_key.native, pub_key.native, encoded, len(encoded))
      if rc == RC.SUCCESS:
          return _ffi.buffer(encoded)[:]
      else:
          raise error_from_code(rc)