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xxtea is a simple block cipher

Project description

XXTEA implemented as a Python extension module, licensed under 2-clause BSD.

The XXTEA algorithm takes a 128-bit key and operates on an array of 32-bit integers (at least 2 integers), but it doesn’t define the conversions between bytes and array. Due to this reason, many XXTEA implementations out there are not compatible with each other.

In this implementation, the conversions between bytes and array are taken care of by longs2bytes and bytes2longs. PKCS#7 padding is also used to make sure that the input bytes are padded to multiple of 4-byte (the size of a 32-bit integer) and at least 8-byte long (the size of two 32-bit integer, which is required by the XXTEA algorithm). As a result of these measures, you can encrypt not only texts, but also any binary bytes of any length.

Installation

$ pip install xxtea -U

Usage

This module provides four functions: encrypt(), decrypt(), encrypt_hex(), and decrypt_hex().

>>> import os
>>> import xxtea
>>> import binascii
>>>
>>> key = os.urandom(16)  # Key must be a 16-byte string.
>>> s = b"xxtea is good"
>>>
>>> enc = xxtea.encrypt(s, key)
>>> dec = xxtea.decrypt(enc, key)
>>> s == dec
True
>>>
>>> hexenc = xxtea.encrypt_hex(s, key)
>>> hexenc
b'7ad85672d770fb5cf636c49d57e732ae'
>>> s == xxtea.decrypt_hex(hexenc, key)
True
>>>
>>> binascii.hexlify(enc) == hexenc
True

encrypt_hex() and decrypt_hex() operate on ciphertext in a hexadecimal representation. They are exactly equivalent to:

>>> hexenc = binascii.hexlify(xxtea.encrypt(s, key))
>>> s == xxtea.decrypt(binascii.unhexlify(hexenc), key)
True

Padding

Padding is enabled by default, in this case you can encode any bytes of any length.

>>> xxtea.encrypt_hex('', key)
b'd63256eb59134f1f'
>>> xxtea.decrypt_hex(_, key)
b''
>>> xxtea.encrypt_hex(' ', key)
b'97009bd24074a7a5'
>>> xxtea.decrypt_hex(_, key)
b' '

You can disable padding by setting padding parameter to False. In this case data will not be padded, so data length must be a multiple of 4 bytes and must not be less than 8 bytes. Otherwise ValueError will be raised:

>>> xxtea.encrypt_hex('', key, padding=False)
ValueError: Data length must be a multiple of 4 bytes and must not be less than 8 bytes
>>> xxtea.encrypt_hex('xxtea is good', key, padding=False)
ValueError: Data length must be a multiple of 4 bytes and must not be less than 8 bytes
>>> xxtea.encrypt_hex('12345678', key, padding=False)
b'64f4e969ba90d386'
>>> xxtea.decrypt_hex(_, key, padding=False)
b'12345678'

Rounds

By default xxtea manipulates the input data for 6 + 52 / n rounds, where n denotes how many 32-bit integers the input data can fit in. We can change this by setting rounds parameter.

Do note that the more rounds it is, the more time will be consumed.

>>> import xxtea
>>> import string
>>> data = string.digits
>>> key = string.ascii_letters[:16]
>>> xxtea.encrypt_hex(data, key)
b'5b80b08a5d1923e4cd992dd5'
>>> 6 + 52 // ((len(data) + (4 - 1)) // 4)  # 4 means 4 bytes, size of a 32-bit integer
23
>>> xxtea.encrypt_hex(data, key, rounds=23)
b'5b80b08a5d1923e4cd992dd5'
>>> xxtea.encrypt_hex(data, key, rounds=1024)
b'1577bbf28c43ced93bd50720'

Catching Exceptions

When calling decrypt() and decrypt_hex(), it is possible that a ValueError or a TypeError was raised. Better to catch them, or your program will exit.

>>> from __future__ import print_function
>>> import xxtea
>>>
>>> def try_catch(func, *args, **kwargs):
...     try:
...         func(*args, **kwargs)
...     except Exception as e:
...         print(e.__class__.__name__, ':', e)
...
...
...
>>> try_catch(xxtea.decrypt, '', key='')
ValueError : Need a 16-byte key.
>>> try_catch(xxtea.decrypt, '', key=' '*16)
ValueError : Invalid data, data length is not a multiple of 4, or less than 8.
>>> try_catch(xxtea.decrypt, ' '*8, key=' '*16)
ValueError : Invalid data, illegal PKCS#7 padding. Could be using a wrong key.
>>> try_catch(xxtea.decrypt_hex, ' '*8, key=' '*16)
TypeError : Non-hexadecimal digit found
>>> try_catch(xxtea.decrypt_hex, 'abc', key=' '*16)
TypeError : Odd-length string
>>> try_catch(xxtea.decrypt_hex, 'abcd', key=' '*16)
ValueError : Invalid data, data length is not a multiple of 4, or less than 8.

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