; rc40
 
(define (string-index c str)
  (let loop ((ss (string->list str)) (k 0))
    (cond ((null? ss) #f)
          ((char=? (car ss) c) k)
          (else (loop (cdr ss) (+ k 1))))))
 
; tilde represents shift
 
(define alpha "abcdefghijklmnopqrstuvwxyz0123456789.? ~")
(define shift "ABCDEFGHIJKLMNOPQRSTUVWXYZ)!@#$%^&*(,/|~")
 
(define (c->i c) (string-index c alpha))
(define (i->c i) (string-ref alpha i))
 
; convert list [~burrito ~~sale~~ 2 ~2 ~47.99] to "Burrito SALE 2 @ $7.99"
(define (rc40->string cs)
  (let loop ((cs cs) (locked? #f) (ss (list)))
    (cond ((null? cs) (list->string (reverse ss)))
          ((and locked? (pair? (cdr cs)) (char=? (car cs) #\~) (char=? (cadr cs) #\~))
            (loop (cddr cs) #f ss))
          (locked? (loop (cdr cs) #t (cons (string-ref shift (c->i (car cs))) ss)))
          ((and (pair? (cdr cs)) (char=? (car cs) #\~) (char=? (cadr cs) #\~))
            (loop (cddr cs) #t ss))
          ((and (pair? cs) (char=? (car cs) #\~))
            (loop (cddr cs) #f (cons (string-ref shift (c->i (cadr cs))) ss)))
          (else (loop (cdr cs) #f (cons (car cs) ss))))))
 
; convert "Burrito SALE 2 @ $7.99" to list [~burrito ~~sale~~ 2 ~2 ~47.99]
(define (string->rc40 str)
  (let loop ((cs (string->list str)) (locked? #f) (ss (list)))
    (cond ((null? cs)
            (if locked?
                (reverse (cons #\~ (cons #\~ ss)))
                (reverse ss)))
          (locked?
            (if (string-index (car cs) shift)
                (loop (cdr cs) #t
                      (cons (string-ref alpha (string-index (car cs) shift)) ss))
                (loop cs #f (cons #\~ (cons #\~ ss)))))
          ((string-index (car cs) shift)
            (if (and (pair? (cdr cs)) (string-index (cadr cs) shift))
                (loop cs #t (cons #\~ (cons #\~ ss)))
                (loop (cdr cs) #f
                      (cons (string-ref alpha (string-index (car cs) shift))
                            (cons #\~ ss)))))
          (else (loop (cdr cs) #f (cons (car cs) ss))))))
 
(define (rc40-init key)
  (let ((kvec (make-vector 40)) (klen (string-length key)) (j 0)
        (key (list->string (string->rc40 key))))
    (do ((i 0 (+ i 1))) ((= i 40)) (vector-set! kvec i i))
    (do ((i 0 (+ i 1))) ((= i 40) kvec)
      (set! j (modulo (+ j (vector-ref kvec i)
                (c->i (string-ref key (modulo i klen)))) 40))
      (let ((t (vector-ref kvec i)))
        (vector-set! kvec i (vector-ref kvec j))
        (vector-set! kvec j t)))))
 
(define (rc40-stream key)
  (let ((i 0) (j 0) (kvec (rc40-init key)))
    (lambda ()
      (set! i (modulo (+ i 1) 40))
      (set! j (modulo (+ j (vector-ref kvec i)) 40))
      (let ((t (vector-ref kvec j)))
        (vector-set! kvec j (vector-ref kvec i))
        (vector-set! kvec i t))
      (vector-ref kvec (modulo (+ (vector-ref kvec i) (vector-ref kvec j)) 40)))))
 
(define (rc40-plus a b) (modulo (+ a b) 40))
(define (rc40-minus a b) (modulo (- b a) 40))
 
(define (rc40-encipher key text)
  (let ((rc40 (rc40-stream key)))
    (let loop ((ts (map c->i (string->rc40 text))) (zs '()))
      (if (null? ts) (rc40->string (map i->c (reverse zs)))
        (loop (cdr ts) (cons (rc40-plus (rc40) (car ts)) zs))))))
 
(define (rc40-decipher key text)
  (let ((rc40 (rc40-stream key)))
    (let loop ((ts (map c->i (string->rc40 text))) (zs '()))
      (if (null? ts) (rc40->string (map i->c (reverse zs)))
        (loop (cdr ts) (cons (rc40-minus (rc40) (car ts)) zs))))))
 
(display (rc40-encipher "tedunangst" "Programming Praxis sharpens your saw.")) (newline)
(display (rc40-decipher "tedunangst" "5cxaxlfrfhy6kh38fbplm0mDko58xs.l9Hkz8")) (newline)

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