; hardware random number generator
 
(define (take n xs)
  (let loop ((n n) (xs xs) (ys '()))
    (if (or (zero? n) (null? xs))
        (reverse ys)
        (loop (- n 1) (cdr xs)
              (cons (car xs) ys)))))
 
(define (rand-letter)
  (with-input-from-file "/dev/random" (lambda ()
    (do ((i 4 (- i 1)) (r 0 (+ (* r 256) (char->integer (read-char)))))
        ((zero? i) (integer->char (+ (modulo r 26) 65)))))))
 
(define (keypad n)
  (do ((n n (- n 1)) (cs (list) (cons (rand-letter) cs)))
      ((zero? n) (list->string cs))))
 
(define (trigraph msg key)
  (define (i->a i) (integer->char (+ i 65)))
  (define (a->i a) (- (char->integer a) 65))
  (define (tri c k) (i->a (modulo (- 25 (a->i k) (a->i c)) 26)))
  (let* ((msg (string->list msg))
         (key (take (length msg) (string->list key))))
    (list->string (map tri msg key))))
 
(display (rand-letter)) (newline)
(define key "HDFHIJNULLPVJNHTNKNBAXPYR")
(display (trigraph "ATTACKATDAWN" key)) (newline)
(display (trigraph (trigraph "ATTACKATDAWN" key) key)) (newline)

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