decrypt a message

def decrypt(self, msg):
        """decrypt a message"""
        error = False
        signature = msg[0:SHA256.digest_size]
        iv = msg[SHA256.digest_size:SHA256.digest_size + AES.block_size]
        cipher_text = msg[SHA256.digest_size + AES.block_size:]
        if self.sign(iv + cipher_text) != signature:
            error = True
        ctr = Counter.new(AES.block_size * 8, initial_value=self.bin2long(iv))
        cipher = AES.AESCipher(self._cipherkey, AES.MODE_CTR, counter=ctr)
        plain_text = cipher.decrypt(cipher_text)
        if error:
            raise DecryptionError
        return plain_text