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00 XOR Cypher.md

XOR Cypher

The XOR Cypher is simple: To encrypt, you do a bitwise XOR of the message with a key. To decrypt it, you XOR it again with the same key.

Write a function encrypt that does an XOR encryption of a message string given a key string. If the message is longer than the key, repeat the key.

Note, I see this as an exercise in JVM byte manipulation.

Thanks to this site for the problem idea, where it is rated Expert in JavaScript. The problem has been modified.

Please submit your solutions as comments on this gist.

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(defn encrypt [message key]
  (map bit-xor (map int message) (map int (cycle key))))

(defn decrypt [message key]
  (->> (map bit-xor (map int (cycle key)) (map int message))
       (map char)
       (apply str)))

(def ^java.nio.charset.Charset utf8 (java.nio.charset.Charset/forName "UTF-8"))

(defn enc8 [key message]
  (let [utf-bytes (fn [^String s] (.getBytes s utf8))]
    (String. (byte-array (map bit-xor (cycle (utf-bytes key)) (utf-bytes message))))))

(defn encrypt [key message]
  (->>
    (map vector (seq message) (cycle key))
    (map (fn [[m k]] (bit-xor (int m) (int k))))
    (map (fn [i] (format "%02x" i)))
    (apply str)))

(defn decrypt [key message]
  (let [int-seq (->> message
                     (partition 2)
                     (map (fn [[a b]] (str "0x" a b)))
                     (map read-string))]
    (->> (map vector int-seq (cycle key))
         (map (fn [[m k]] (bit-xor m (int k))))
         (map char)
         (apply str))))

(encrypt "abc" "hello world")
=> "09070f0d0d43160d110d06"
(decrypt "abc" (encrypt "abc" "hello world"))
=> "hello world"

(defn key-bytes [msg key]
  (->> key cycle (take (count msg)) (apply str) 
      (map int)))

(defn encrypt [msg key]
  (let [msg-bytes (->> msg (map int)) 
        key-bytes (key-bytes msg key)]
    (->>  
     (map bit-xor msg-bytes key-bytes)
     (map char)
     (apply str))))

(defn decrypt [encrypted key]
  (encrypt encrypted key))