Python: Two-way Alphanumeric Encryption

Question

I am using Python 2.7. I have an alphanumeric string, on which I want to perform a encryption/decryption. Whatever I do should remain 2-way and the result should be alphanumeric too.

For example:

str = 'ma6546fbd'
encrypted_data = encrypt_function(str)
decrypted_data = decrypt_function(encrypted_data)
print decrypted_data # I get 'ma6546fbd'

What have I done:

I have written a function

def xor_crypt_string(data, key):
    return ''.join(chr(ord(x) ^ ord(y)) for (x,y) in izip(data, cycle(key)))

This takes the data and a key and returns the result, the problem is that it includes special characters too, which I want to avoid.

Answer 1

If you want serious encryption (read unbreakable) then I'd use AES from pycrypto something like this.

>>> from Crypto.Cipher import AES
>>> from Crypto import Random
>>> key = b'Sixteen byte key'
>>> iv = Random.new().read(AES.block_size)
>>> cipher = AES.new(key, AES.MODE_CFB, iv)
>>> msg = iv + cipher.encrypt(b'Attack at dawn')
>>> msg.encode("hex")
'e10e096aabff9db382abe8d704404995a7b64d72a4e1b9e5208912d206c4'

That is your ascii message. Now decode the message like this

>>> recv='e10e096aabff9db382abe8d704404995a7b64d72a4e1b9e5208912d206c4'
>>> cipher.decrypt(recv.decode("hex"))[len(iv):]
'Attack at dawn'
>>> 

Any encryption method you make up yourself will be easily breakable by an expert and the one you've shown above falls into that category.

Answer 2

How strict is the alphanumeric requirement?

How about base64 encoding the result of your existing encryption function? You might end up with a few stray '=' padding characters, but you could trim these and calculate and handle the extra padding.

def xor_crypt_string(plaintext, key):
    ciphertext = ''.join(chr(ord(x) ^ ord(y)) for (x,y) in izip(plaintext, cycle(key)))
    return ciphertext.encode('base64')

def xor_decrypt_string(ciphertext, key):
    ciphertext = ciphertext.decode('base64')
    return ''.join(chr(ord(x) ^ ord(y)) for (x,y) in izip(ciphertext, cycle(key)))