Semi-automatic Vigenère cipher decrypter.

cipher.txt

xsjztliyjvphmamicnwlriemsotjpsgcdtensyyllyydjwlxicnidtjonjqjvpsxnfidfvnntsjvdgedjhzsxsjppyxpwwzkevjchtvonxtxednqaqiqtvxtjatpjfpamemjxthwfgwenxfymzsxsjztliyjvphmamicmedgipsvpnrgjrejhxfrjymxjwemixjxsthhfwzwmrnrlqpjiidhvtgiogcrnsgfrmfxtxxlgiwfwznrsnwqnjejiykmqycemvpjfztowfgtkvliiwxmrlmzaeygeenweffpqedtlzbigjvemidhlprihfwwfxpwqtxeeyvtgyejhetfwfmdjhpamrjrpwitsxsjrtsiejiyylnjrezvjfronwytahnhpqcvsshsedylpamrjrpwinntsjvemmdhmamicnwhjpwprzbrmjglzwpbltqitymdjeddxzzrojvdyeyieyimxuppriyymetjejrlutpfvdysmjktsrpwwetfpzrmwilpemqiemmdjecsionxemiojwnwmaymzspphltkjcjmyiinmmqkvlgppkvpsgskscylpzrmwilpemqinntsjvntrdjufjreqcxfrjuizuppmegjxcnioystrtwjqpsxzgjfxglymzsscjrnwcaymzswnmixjwemeefvpjwdjrenewqcgnkpsicjgtulpwwzspjyssfzpylprfctopsxsjwewiylxsgisnroylpamrjrpwinntsjvwnopfpwuswdewullgiengnntsjvdnwtywlgmwnxjysdycxniqwibziyhclsewdwtxjcjufjrndeyfpjxmdnwemiawenymnjsqiinwcaymylexjwdfkpgcntyyymylxsjjcjufjrndsqhmamicyiiyppyxpwwlshpvylymylmeysemiwjxejvqwibziyhczkrzwqlqxpcxqtvtswefrnjmqusnhycwiorsdymyfgtulpwxpcxhmsdjtwfmyyiiymdnrpskwnwstrphsfqhdzwajgeyllytntvcjwatrojhetimjglzwpjmdylprsdyjcjufjreqcfxioqieyicnrpskwnwszwtskemignkpsicjgtulpwinfrmjiyhmamicjhlxeydsqxigjvlqppyxpwwtsxsjewullgiefxonjqjvpsxatmyywtsxsjqpxwlliemydiiqjeenrrxmxuppkvpvypsgjfrlqcdnweminwmenglqapfoyjwdnremignkpsicjgtulpwmdylpwiwfxtaiwdwstvefrowiajeejhyfxfwizkmexopdmqfgcdtefrlqcdyhtxgzaicxxsjopdwwjrrylemiyylphmamicyiiyglsfpyvpfxpiedfwpwmpxsqimqkicjrehepxechmamicxasngsnronztiylqpjfvpyvtamlqpjgvzpiyylppednwvneyijcnioreyyidywsjpaiiejvxnrpfgtulpwxpcxdpijqiylxsyltxxpcxtxjctqhnotuione

vigenere_decrypter.rb

module Enumerable
  def frequencies
    self.to_a.inject(Hash.new(0)) do |frequencies, element|
      frequencies.tap { |fqs| fqs[element] += 1 }
    end
  end
end

class CaesarCipher
  attr_reader :shift

  def initialize(shift, alphabet = Array('a'..'z').join)
    @shift = shift % alphabet.length
    @alphabet = alphabet
  end

  def shifted_alphabet
    @alphabet[@shift..-1] + @alphabet[0...@shift]
  end

  def encrypt(string)
    string.tr(@alphabet, shifted_alphabet)
  end

  def decrypt(string)
    string.tr(shifted_alphabet, @alphabet)
  end
end

class VigenereCipher
  attr_reader :key
  attr_writer :caeser_cipher_factory

  def initialize(key, alphabet = Array('a'..'z').join)
    @key, @alphabet = key, alphabet
  end

  def encrypt(string)
    process(string) { |cipher, char| cipher.encrypt(char) }.flatten.join
  end

  def decrypt(string)
    process(string) { |cipher, char| cipher.decrypt(char) }.flatten.join
  end


  def caesar_cipher_factory
    @caesar_cipher_factory ||= CaesarCipher.public_method(:new)
  end

  private

  def process(string)
    string.chars.each_slice(key.length).map do |characters|
      characters.zip(caesar_ciphers).map do |character, cipher|
        yield(cipher, character)
      end
    end
  end

  def caesar_ciphers
    key.each_char.map do |key_letter|
      caesar_cipher_factory.call(key_letter.ord - 'a'.ord, @alphabet)
    end
  end
end

encrypted_text = IO.read('./cipher.txt').strip

# Guessing...
sequence_length = n = 3

# Analyse letter frequencies
letter_frequencies = n.times.map do |offset|
  (offset...encrypted_text.length).step(n).map { |i| encrypted_text[i] }.frequencies
end

# Find the most frequent letters
most_frequent_letters = letter_frequencies.map do |lf|
  lf.max_by { |_, frequency| frequency }[0]
end

# Calculate key
key = most_frequent_letters.inject('') do |key, most_frequent_letter|
  key << CaesarCipher.new(most_frequent_letter.ord - 'e'.ord).encrypt('a')
end

# Decrypt text
puts VigenereCipher.new(key).decrypt(encrypted_text)