Generating XKCD-style passwords in Racket

password.rkt

#lang racket

(define dict "/etc/dictionaries-common/words")

(define (choose l) (list-ref l (random (length l))))
(define words (for/list ([i (file->lines dict)]
                         #:when (regexp-match? "^[a-z]*$" i)
                         #:when (3 . < . (string-length i))
                         #:when (10 . > . (string-length i)))
                i))
(define (password)
  (apply string-append
         (add-between (for/list ([j 4])
                        (choose words))
                      " ")))

(module+ test
   (require rackunit)
   (for ([i 50])
     (define e (password))     
     (check member (choose words) words)
     (check regexp-match? "^[a-z]* [a-z]* [a-z]* [a-z]*$" e)
     (check-true (< 19 (string-length e) 39))))

You would need to provide a different implementation of the random function. However, since you're only generating one result, I doubt there's much point (esp. if you go through a few passwords until you find one you like).