Vigenere cipher (source: http://www.problemotd.com/)

README

Vigenere cipher

The Vigenere cipher made its rounds in the mid-1550s up until the end of the American Civil War.
It was very easy for soldiers to encode messages and pass them around to all the allied camps. It
was very easy for soldiers to encode messages and pass them aroudn to all the allied camps.

The cipher requires a key and a message. It works like this:

Key:
REDDIT
Message:
TODAYISMYBIRTHDAY

REDDITREDDITREDDI
TODAYISMYBIRTHDAY
-----------------
KSGDGBJQBEQKKLGDG

Using a 0 based alphabet (A=0), R is the 17th letter of the alphabet and T is the 19th letter
of the alphabet. (17 + 19) mod 26 = 11 which is where K resides in the alphabet. Repeat for each
key/message letter combination until done.

Today's problem of the day is two part. The first part is to implement a Vigenere cipher in the
programming language of your choice. Feel free to post solutions or links to solutions in the
comments.

The second part is to try and implement something to crack the message below (the key is 5 or less
characters).

ZEJFOKHTWHCGAW

Good Luck!

cipher.rb

# Part 1

class Cipher
  attr_reader :key, :msg
  def initialize(key, msg)
    @key = key
    @msg = msg
  end

  def index_position(char)
    alphabet = ('A'..'Z').to_a
    alphabet.index(char)
  end

  def character_at(position)
    alphabet = ('A'..'Z').to_a
    alphabet[position]
  end

  def encrypt
    result = []
    zipped_msg = combine

    zipped_msg.each do |pair|
      code_position = (index_position(pair[0]) + index_position(pair[1])) % 26
      result << character_at(code_position)
    end

    result.join
  end

  def combine
    # expand key to match length of message
    expanded_key = key * msg.length.fdiv(key.length).ceil

    # turn key into array to pop extras
    expanded_key = expanded_key.split(//)

    # calc how many extra chars exist
    key_muffintop = expanded_key.length - msg.length

    # match message length
    expanded_key.pop(key_muffintop)

    # convert msg to array
    expanded_msg = msg.split(//)

    expanded_key.zip(expanded_msg)
  end
end

key = 'reddit'.upcase
message = 'TODAYISMYBIRTHDAY'.upcase

cipher = Cipher.new(key, message)
p cipher.encrypt