"hello".rot(13)

rot13.rb

class String
  def rot num = 13
    self.upcase.split("").collect { |char|
      ((char.ord - 'A'.ord + num) % 26 + 'A'.ord).chr()
    }.join()
  end

  def tryRot
    (0..26).each do |i|
      p "#{i}: '#{self.rot(i)}'"
    end
  end
end