code: def encrypt(self, k, a, m):
          
          iv = _randombits(96)
          cipher = Cipher(algorithms.AES(k), modes.GCM(iv),
                          backend=self.backend)
          encryptor = cipher.encryptor()
          encryptor.authenticate_additional_data(a)
          e = encryptor.update(m) + encryptor.finalize()
  
          return (iv, e, encryptor.tag)
docs: Encrypt accoriding to the selected encryption and hashing
          functions.
  
          :param k: Encryption key (optional)
          :param a: Additional Authentication Data
          :param m: Plaintext
  
          Returns a dictionary with the computed data.