Formatting Preserving Encryption in Cython

An implementation of format-preserving Feistel-based encryption (FFX) in Cython, heavily inspired by pyffx.

Intended to work with Python 3.7 and Cython 0.29.

Example

>>> import cypyffx

>>> e = cypyffx.IntegerFFX(b'secret-key', length=4)
>>> e.encrypt(1234)
5648
>>> e.decrypt(5648)
1234

The above shows usage of the IntegerFFX class for encrypting numbers. More generally, the module can work on any bytes object and returns another bytes object...

>>> ffx = cypyffx.cFFX(b'secret-key', radix=255, rounds=10, digestmod='sha1')
>>> ffx.encrypt(b'hello world')
b'\x10H(U\x8fR=\x8e\x9b\xbd\xf4'

Limitations

The maximum radix (alphabet length) is 255 because the method operates on arrays of bytes. What the values of those bytes mean is up to you. The IntegerFFX class shows how to map bytes 0-9 with the decimal digits for encrypting and decrypting integers. A similar class could be constructed for any alphabet.

Note that because the alphabet is limited to 255 characters, this method will not work with arbitrary Unicode strings. You can pass any bytes to cFFX.encrypt that you want, but are not guaranteed to get back a sequence of bytes that represents a valid string for a given multi-byte character encoding.

The length of encoded values is limited by the digest method and length of the alphabet. By default, sha1 is used and the maximum integer "length" is 48 decimal digits. With sha256, the maximum length is 77 decimal digits.

This module is really intended for fast encryption of short messages. For example, it is well-suited to encrypting credit card and social security numbers.