(defun rot-13 (string)
  (let* ((len (length string))
         (output (make-string len ?\s)))
    (dotimes (i len)
      (let ((char (aref string i)))
        (cond
         ((or (and (>= char ?n) (<= char ?z))
              (and (>= char ?N) (<= char ?Z)))
          (aset output i (- char 13)))
         ((or (and (>= char ?a) (<= char ?m))
              (and (>= char ?A) (<= char ?M)))
          (aset output i (+ char 13)))
         (t
          (aset output i char)))))
    output))