IOC.rb

#!/usr/bin/env ruby
# Open up the String class and give it an alpha? check
class String
  def alpha?
    self =~ /^[A-Za-z]$/
  end
end

# Run across each alpha character in string and tally letter occurances
# This returns something like [ 1, 2, <24 0's>] for "ABB"
def frequencies(string)
  prob = string.upcase.each_char.inject(Array.new(26,0)) do |array,char|
    if char.alpha?
      array[char[0] - ?A] += 1
    end
    array
  end
end

# This method defines a new enumerator. One can simply all every_x_letters
# on a piece of text and in the block expect to be passed sets of every X
# letters.
#
# Eg. "ABCABCABC", 3 would yield
#     "AAA"
#     "BBB"
#     "CCC"
#
# Then method returns an array of results
def every_x_letters(string,m)
  string.
    split(/\s|\n/).
    map {|word| word.split('')}.
    flatten.
    each_slice(m).to_a.
    map {|slice| slice << nil until slice.length == m; slice }.
    transpose.map(&:compact).
    map(&:join).map do |every_x|
      yield every_x
    end
end

# Calculate the index of coincidence for a string given key length m
def index_of_coincidence(string,m=1)
  every_x_letters(string,m) do |substring|
    frequency = frequencies(substring)
    total = frequency.inject(&:+).to_f

    i_o_c = 0.upto(25).inject(0) do |ioc, i| 
      ioc  + frequency[i] * (frequency[i] - 1)
    end
    i_o_c /= total * (total - 1)
  end
end

# Drive the program
if __FILE__ == $0
  begin
    string = File.read(ARGV.shift)
    m = ARGV.length != 0 ? ARGV.shift.to_i : 1
    i_o_c = index_of_coincidence(string,m)
    puts "I_c(x) = #{i_o_c.inspect}"
  rescue
    puts "ruby IOC.rb <text file> <key length (m)>"
  end
end

M_g.rb

#!/usr/bin/env ruby
require 'IOC.rb'

# English Language Frequencies - Page 26 of book
ELF = [82,15,28,43,127,22,20,61,70,2,8,40,24,67,75,19,1,60,63,91,28,10,23,1,20,1].map {|p| p / 1000.0}

# Method to calculate the M_g table for a given string and key length
# Present version prints the actual table instead of returning a hash
def m_g_table(string,m)
  s = 0
  every_x_letters(string,m) do |subtext|
    printf "Subtext #{s+1}:  "
    f = frequencies(subtext)
    n = f.inject(&:+).to_f

    # Formula Summation
    (0..25).to_a.each do |g|
      # For each value of G
      m_g = (0..25).inject(0) do |sum,i|
        sum + (ELF[i]*f[(i+g)%26])/n
      end
      printf("%0.4f ",m_g+0.00005)
    end
    printf "\n\n"
    s += 1
  end
end

# Vroom, vroom
if __FILE__ == $0
  begin
    cipher = File.read(ARGV.shift)
    m = ARGV.shift.to_i
    m_g_table(cipher,m)
  rescue
    puts "ruby M_g.rb <text file> <key length (m)>"
  end
end