Four Bites AES

Author: Mark Percival
Email: mark@mpercival.com
Copyright: Mark Percival 2008
License: MIT

I plan on cleaning this code up quite a bit, but it works in the current state, just a bit messy

This is a simple library that takes any 4 byte integer and encrypts it with a modified version of AES. I designed it for a project where URL brevity mattered, thereby ruling out UUID's. I simply wanted to prevent two things:

• Keep the total number of records somewhat secret
• Prevent people from easily guessing the next record

mySQL integers are 4 bytes long, so I wanted to account for that, and keep the encrypted number the same length.

I certainly wouldn't base my security on this system - there are only 4.3 billion possible combinations of a 4 byte number, so with 1 million records in the system it would be quite trivial to guess a valid record number. This library isn't designed to prevent that. It's simple designed to keep your total record count secret, and prevent the average joe/jane from easily guessing the next number, all in the shortest possible manner.

Details about the code

I followed the AES standard as much as possible, except I used 8 rounds, and a 256 bit unexpanded key. The ShiftRows method became a rotation of the 4 bytes, and the MixColumns became one single column mix. Other than that, it's essentially the same algorithm.

Example code

    require 'four_bites_aes'
    CRYPT = FourBitesAES.new("key goes here")

    100.times do |i|
     p enc = FourBitesAES.enc(i)
     p dec = FourBitesAES.decrypt(enc)
    end