; blum blum shub

(define (digits n . args)
  (let ((b (if (null? args) 10 (car args))))
    (let loop ((n n) (d '()))
      (if (zero? n) d
          (loop (quotient n b)
                (cons (modulo n b) d))))))

(define (undigits ds . args)
  (let ((b (if (null? args) 10 (car args))))
    (let loop ((ds ds) (n 0))
      (if (null? ds) n
          (loop (cdr ds) (+ (* n b) (car ds)))))))

(define (logxor x y)
  (let loop ((x (reverse (digits x 2))) (y (reverse (digits y 2))) (z '()))
    (cond ((and (null? x) (null? y)) (undigits z 2))
          ((and (null? x) (zero? (car y))) (loop x (cdr y) (cons 0 z)))
          ((null? x) (loop x (cdr y) (cons 1 z)))
          ((and (null? y) (zero? (car x))) (loop (cdr x) y (cons 0 z)))
          ((null? y) (loop (cdr x) y (cons 1 z)))
          ((= (car x) (car y)) (loop (cdr x) (cdr y) (cons 0 z)))
          (else (loop (cdr x) (cdr y) (cons 1 z))))))

(define (make-blum-blum-shub n s)
  (let ((s s))
    (lambda ()
      (set! s (modulo (* s s) n))
      (modulo s 256))))

(define (encipher n s plain-text)
  (let ((bbs (make-blum-blum-shub n s)))
    (let loop ((ps (string->list plain-text)) (cs '()))
      (if (null? ps)
          (list->string (reverse cs))
          (loop (cdr ps) (cons (integer->char
            (logxor (bbs) (char->integer (car ps)))) cs))))))

(define decipher encipher)

(display (decipher 974153 17 (encipher 974153 17 "PROGRAMMING PRAXIS")))
(newline)
(display (map char->integer (string->list (encipher 974153 17 "PROGRAMMING PRAXIS"))))

PROGRAMMING PRAXIS
(113 19 134 103 76 42 105 181 191 14 93 17 69 245 63 201 240 71)