code: def sign_message(message, private_key, public_key=None):
      
  
      if public_key is None:
          public_key = private_to_public_key(private_key)
  
      return ed25519_blake2.signature_unsafe(message, private_key, public_key)
docs: Signs a `message` using `private_key` and `public_key`
  
      .. warning:: Not safe to use with secret keys or secret data. See module
                   docstring.  This function should be used for testing only.
  
      :param message: the message to sign
      :type message: bytes
  
      :param private_key: private key used to sign message
      :type private_key: bytes
  
      :param public_key: public key used to sign message
      :type public_key: bytes
  
      :return: the signature of the signed message
      :rtype: bytes