Ceasar's cipher in ruby

ceasars_cipher_ruby.rb

# https://github.com/alexavlonitis
# Ceasar's cipher in ruby 

def cipher(word, index_shift = 13)
  alphabet = ('a'..'z').to_a
  alphabet_upcase = ('A'..'Z').to_a
  cipher_store = {}

  alphabet.each.with_index do |letter, i|
    cipher_store[letter] = alphabet[(index_shift + i) - alphabet.size]
  end

  alphabet_upcase.each.with_index do |letter, i|
    cipher_store[letter] = alphabet_upcase[(index_shift + i) - alphabet_upcase.size]
  end

  word.split('').map { |letter| cipher_store[letter] || ' ' }.join
end