affine
An affine cipher is a monoalphabetic substitution cipher, that uses math to generate the substitution alphabet based on three paramaters:
 modulus

specifies how many different plaintexts and ciphertexts are available.
 a_key

multiplied against the plaintext. Must be coprime with the modulus.
 b_key

added to the multiplied plaintext. No restrictions, but it's modulus math, so making it larger than
modulus
is useless.
These parameters are passed in order to the Cipher constructor:
a = Affine::Cipher.new(2176782371, 65182241782, 123235151)
r = rand(123235150) # (should be smaller then the modulus)
ciphertext = a.encipher r
# ciphertext = ((r * 65182241782) + 123235151) % 2176782371
r == a.decipher(ciphertext)
Giant security caveat
Don't use this cipher for security related tasks. I'm using it to make primary keys alphanumeric and lesspredictable: github.com/bkerley/havecode/
How to crack it
Know two plaintexts and their ciphertexts, put into linear system, solve.
Know the relation between two plaintexts, predict future ciphertexts.
Know lots of ciphertexts, guess it's English, solve like a cryptogram.
Copyright
Copyright © 2009 Bryce Kerley. See LICENSE for details.