RC4 in Clojure

gistfile1.clj

(ns rc4.core)

(defn init-sbox [enc-key]
  (let [k (mapv int enc-key)
        L (count enc-key)]
    (loop [s (into [] (range 0 256)) i 0 j 0]
      (if (= i 256) s
          (let [j (mod (+ j (s i) (k (mod i L))) 256)]
            (recur (assoc s i (s j) j (s i)) (inc i) j))))))

(defn crypt [clear s-box]
  (loop [enc (vector (repeat (count clear) 0))
         s s-box n 0 i 0 j 0]
    (if (= n (count clear)) enc
        (let [i (mod (inc i) 256)
              j (mod (+ j (s i)) 256)
              s (assoc s i (s j) j (s i))
              random (s (mod (+ (s i) (s j)) 256))]
          (recur (assoc enc n (bit-xor random (int (nth clear n))))
                 s (inc n) i j)))))

(def cyph (crypt "Hello World!" (init-sbox "abc")))
; checked against http://www.joonis.de/de/code/rc4-algorithm

(apply str (map char (crypt cyph (init-sbox "abc"))))
; => "Hello World!"