msg-->str or bytes
        Returns-->bytes

def sign(self, msg):
        '''
        msg-->str or bytes
        Returns-->bytes
        '''
        if self.__signingkey is None:
            raise KeyTypeError('Wrong key type for operation')
        sig = ffi.new(C_SIGNATURE)
        lib.ed25519_SignMessage(
            sig, self.__signingkey, ffi.NULL, msg, len(msg))
        return ffi.get_bytes(sig)