affine.rb

MULTIPLICATION_VALUES = [3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25]
ALPHABET = (?A..?Z).to_a
dictionary = {}
File.open("enable1.txt", "r") do |file| # loads a dictionary to check words against, feel free to replace this with the path to any dictionary
  file.each_line do |line|
    dictionary[line.chomp] = 1
  end
end

print "Please give CIPHERTEXT: "
ciphertext = gets.chomp.upcase
results = []

def affine(character, multiply, add)
  if ALPHABET.include? character # only shift if the character is a letter
    # This next line is pretty complicated so I'm going to break it down
    # It takes the character and gets the human-readable index
    # (e.g. a => 1, b => 2, etc)
    # Then it multiplies by multiply
    # Then it adds by add
    # Then it gives the least positive residue in mod 26
    # Then it converts to the array index
    # Then it returns the letter at that array index
    ALPHABET[(((ALPHABET.index(character) + 1) * multiply + add) % 26) - 1]
  else
    character
  end
end

def score (result, dictionary)
  score = 0.0
  words = result.split.map { |word| word.gsub(/[^a-z]/i, '') } # only letters
  words.each do |word|
    score += 1 if dictionary[word]
  end
  score /= words.length
  score
end

(1..25).each do |add|
  MULTIPLICATION_VALUES.each do |multiply|
    results << ciphertext.split("").map { |letter| affine(letter, multiply, add) }.join.downcase
  end
end

results = results.sort_by { |result| score(result, dictionary) }.reverse
results[0..4].each do |result|
  puts result
end