RC4 (e.g. ARCFOUR) in (Guile) Scheme. Yes, this is vulnerable in many ways, since it's straight up RC4.

rc4.scm

;;; RC4, just for the hell of it.
;;; Written for Guile 2.0+
;;; Use like so: 
;;;   Encypher: (rc4 "Key" (map char->integer (string->list "Plaintext")))
;;;    $1 = (187 243 22 232 217 64 175 10 211)
;;;   Decypher: (rc4 "Key" $1)
;;;    $2 = (80 108 97 105 110 116 101 120 116)

(define (rc4-make-schedule key)
  (let ((L (string-length key)))
    (let loop ((K (apply vector (map char->integer (string->list key))))
               (S (apply vector (iota 256)))
               (i 0)
               (j 0))
      (if (< i 256)
          (begin
            (set! j (remainder (+ j 
                                  (vector-ref S i)
                                  (vector-ref K (remainder i L)))
                               256))
            (let ((t (vector-ref S i)))
              (vector-set! S i (vector-ref S j))
              (vector-set! S j t)
              (loop K S (+ i 1) j)))
          S))))

(define (rc4-make-generator S)
  (let ((i 0)
        (j 0))
    (lambda ()
      (set! i (remainder (+ i 1) 256))
      (set! j (remainder (+ j (vector-ref S i)) 256))
      (let ((t (vector-ref S i)))
        (vector-set! S i (vector-ref S j))
        (vector-set! S j t))
      (vector-ref S (remainder
                     (+ (vector-ref S i) (vector-ref S j))
                     256)))))

(define (rc4 K P)
  (let* ((S (rc4-make-schedule K))
         (G (rc4-make-generator S)))
    (map (lambda (x)
           (logxor (G) x)) 
         P)))