A Naive (Somewhat Readable) Vigenére Cipher

vigenere.rb

module Vigenere
  def self.encrypt(key, phrase)
    exec key, phrase
  end
  
  def self.decrypt(key, phrase)
    exec key, phrase, -1
  end
  
  def self.exec(key, phrase, direction=1)
    kl      = key.length
    pl      = phrase.length
    offset  = 10
    base    = 36
    
    chs = pl.times.map do |i|
      ch = ((phrase[i].to_i(base) - offset) + (key[i%kl].to_i(base) - offset) * direction) % 26
      (ch + offset).to_s(base)
    end
    
    chs.join
  end
end