cipher.rb

def is_lower(c)
  c.ord >= 'a'.ord && c.ord <= 'z'.ord
end

def is_upper(c)
  c.ord >= 'A'.ord && c.ord <= 'Z'.ord
end

def rotx(x, string, encrypt = true)
  x = encrypt ? x : -x

  string.each_char.map do |c|
    if is_lower(c)
      delta = 'a'.ord
    elsif is_upper(c)
      delta = 'A'.ord
    else
      next c
    end

    (((c.ord - delta + x) % 26) + delta).chr
  end.join
end

p rotx(10,(p rotx(10, 'Hello World')),false)