decrypt.rb

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