def decrypt(self, key, ciphertext, verify=False):
        """ decrypt data with convergence encryption.
        
            Args
                key: str, encryption key
                cipher: str, ciphertext
                verify: bool, verify decrypted data, default: False
        
            Returns
                the plain text
        """
        plain = aes(key, ciphertext)
        if verify:
            h = SHA256d(plain)
            if self.__convergence_secret:
                h.update(self.__convergence_secret)
            digest = h.digest()
            # can verify only if convergence secret is known!
            if self.__convergence_secret and not key == digest:
                msg = "Block verification error on %s." % SHA256d(key).hexdigest()
                log.error(msg)
                raise CryptError(msg)
        return plain