function Rotate13(t)
  local byte_a, byte_A = string.byte('a'), string.byte('A')
  return (string.gsub(t, "[%a]",
      function (char)
        local offset = (char < 'a') and byte_A or byte_a
        local b = string.byte(char) - offset -- 0 to 25
        b = math.mod(b  + 13, 26) + offset -- Rotate
        return string.char(b)
      end
    ))
end