Show HN: Otter, a one time pad tool in lisp, my Saturday side project

Way too imperative. Tons of undeclared variables. why is 'modo' a macro? Local 'defvar'.

Try to write the code shorter. You should be able to write it in about 50%.

Example, from 6 to 3 lines:

    ; generate a random list of *max-int* integers
    (defun randlist (size)
      (let ((lst ()))
        (dotimes (i size)
          (setf lst (cons (random *max-int*) lst)))
      lst))

vs.

    (defun random-vector (size)
      "generate a random vector of *max-int* integers"
      (map-into (make-array size) (lambda () (random *max-int*))))

and

    ; generate random data
    (defun gen (size-string)
      (when (null size-string)
        (setf size-string "4096"))
      (setf size (parse-integer size-string :junk-allowed t))
      (unless (numberp size)
        (error "size must be a number"))
      (defvar buffer (randlist size))
      (princ (%octets-to-string (coerce buffer 'vector) *encoding*)))

vs.

    (defun generate-random-data (&optional (size 4096))
      (when (stringp size)
        (setf size (parse-integer size :junk-allowed t))
        (check-type size integer))
      (%octets-to-string (random-vector size) *encoding*))

Doesn't this only encrypt the first line of input? I thought the idea was to generate a pad-file of equal length to the whole file.

    (with-open-file (input input-path)
      (with-open-file (pad pad-path)
        (setq input-buffer (%string-to-octets (read-line input) *encoding*))
        (setq pad-buffer (%string-to-octets (read-line pad) *encoding*))

Also, I'm not sure what (specifically) read-line looks for to stop reading, but if it can occur randomly you could receive errors if this first random new-line occurs before the length of the file's first line. Or at all when using the same pad for many files as you describe. This may be handled by your %octets-to-string handler though.