Contents

module Affine
  class Cipher
    def initialize(modulus, a_key, b_key)
      raise CoprimeError.new(modulus, a_key) if modulus.gcd(a_key) != 1
      @modulus = modulus
      @a_key = a_key
      @b_key = b_key
      @a_inv = extended_gcd(a_key, modulus)
    end
    def encipher(plaintext)
      raise RangeError.new(plaintext, @modulus) if plaintext > @modulus
      ((@a_key * plaintext) + @b_key) % @modulus
    end
    def decipher(ciphertext)
      raise RangeError.new(ciphertext, @modulus) if ciphertext > @modulus
      (@a_inv * (ciphertext - @b_key)) % @modulus
    end

    private
    # from http://snippets.dzone.com/posts/show/6074
    def extended_gcd(b,m,_recursion_depth=0)
      if b % m == 0
        temp = [0,1]
        return temp
      else
        temp = extended_gcd(m, b % m, _recursion_depth+1)
        temp2 = [temp[1], temp[0]-temp[1] * ((b/m).to_i)]
        if _recursion_depth == 0
          return temp2[0] % m
        else
          return temp2
        end
      end
    end
  end
end

Version data entries

1 entries across 1 versions & 1 rubygems

Version	Path
bkerley-affine-0.1.0	lib/affine/cipher.rb