cipher.rb

#
# This file demonstrates a Caesar cipher
#   There is an encrypt function for encoding plaintext into ciphertext
#   and a decrypt function for converting ciphertext into plaintext
#

# given plaintext and key, return encoded ciphertext
def encrypt(plaintext, key, letters)
  plaintext_as_numbers = []
  # Represent each plaintext letter as a corresponding number
  # for example a=>0, b=>1, ... y=>25, z=>26, '<space>'=>27
  plaintext.downcase.each_char do |l|
    plaintext_as_numbers << letters.index(l)
  end

  # Encryption - Caesar cipher shift
  # Add the key to each number and mod by the length of the letters
  # - or 27
  encrypted = []
  plaintext_as_numbers.each do |n|
    encrypted << ((n+key) % letters.length)
  end

  # convert the shifted/encrypted numbers back to letters
  ciphertext = ""
  encrypted.each do |n|
    ciphertext << letters[n]
  end

  ciphertext
end

# given ciphertext and key, return decoded plaintext
def decrypt(ciphertext, key, letters)
  # Represent each plaintext letter as a corresponding number
  #  for example a=>0, b=>1, ... y=>25, z=>26, '<space>'=>27
  ciphertext_as_numbers = []
  ciphertext.downcase.each_char do |l|
    ciphertext_as_numbers << letters.index(l)
  end

  # encryption... Caesar cipher shift
  #  add the key to each number and mod by 27
  decrypted = []
  ciphertext_as_numbers.each do |n|
    decrypted << ((n-key) % letters.length)
  end

  # convert the shifted/encrypted numbers back to letters
  plaintext = ""
  decrypted.each do |n|
    plaintext << letters[n]
  end

  plaintext
end

# This is the message to encode
plaintext = "hello world"

# This is the secret key used for encoding the Caesar cipher
key = 4

letters = "abcdefghijklmnopqrstuvwxyz "

ciphertext = encrypt(plaintext, key, letters)
puts "Ciphertext = #{ciphertext}"
puts "Decrypted  = #{decrypt(ciphertext, key, letters)}"