gistfile1.txt

import hmac
from Crypto.Util import Counter
from Crypto import Random
from Crypto.Cipher import AES


SIGN_LENGTH = 4


class Encryptor(object):
    def __init__(self, key):
        self.key = key
        self.counter = Counter.new(128, initial_value=0)
        
    def process(self, data):
        tag = hmac.new(self.key, data).digest()[:SIGN_LENGTH]
        cipher = AES.new(self.key, AES.MODE_CTR, counter=self.counter)
        return tag + cipher.encrypt(data)
        

class Decryptor(object):
    def __init__(self, key):
        self.key = key
        self.counter = Counter.new(128, initial_value=0)
        
    def process(self, data):
        tag = data[:SIGN_LENGTH]
        ciphertext = data[SIGN_LENGTH:]
        cipher = AES.new(self.key, AES.MODE_CTR, counter=self.counter)
        plaintext = cipher.decrypt(ciphertext)
        assert(hmac.new(self.key, plaintext).digest()[:SIGN_LENGTH] == tag)
        return plaintext


if __name__ == '__main__':
    secret_key = Random.new().read(16)
    e = Encryptor(secret_key)
    plain_text = 'The Magic Words are Squeamish Ossifrage'
    cipher = e.process(plain_text)
    
    d = Decryptor(secret_key)
    assert(d.process(cipher) == plain_text)