rc4.js

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function RC4(key) {
    this.privateKey = keySetup(key);
    this.byteStream = byteStreamGenerator(this.privateKey.slice(0));
}
 
/**
 * Converts the text into an array of the characters numeric Unicode values
 * @param  {String} text, the text to convert
 * @return {Array} the array of Unicode values
 */
function convert(text) {
    var codes = [];
    for (var i = 0, ii = text.length; i < ii; i++) { codes.push(text.charCodeAt(i)); }
    return codes;
}
 
/**
 * Sets up the key to use with the byte stream
 * @param  {String} key, The key that you want to use
 * @return {Array}, the key stream which with be used in the byteStreamGenerator
 */
function keySetup(key) {
    var K = [...Array(256).keys()], j = 0, key = convert(key);
    for (var i = 0, ii = K.length; i < ii; i++) {
        j = (j + K[i] + key[i % key.length]) % 256;
        [K[i], K[j]] = [K[j], K[i]];
    }
    return K;
}
 
/**
 * byteStreamGenerator uses ES6 generators which will be 'XOR-ed' to encrypt and decrypt
 * @param {Array} K, the array generated from the keySetup
 * @yield {Integer}, the current value which will be 'XOR-ed' to encrypt or decrypt
 */
var byteStreamGenerator = function* (K) {
    var i = 0, j = 0;
    while (true) {
        i = (i + 1) % 256;
        j = (j + K[i]) % 256;
        [K[i], K[j]] = [K[j], K[i]];
        yield (K[(K[i] + K[j]) % 256]);
    }
}
 
/**
 * Encrypts the input text
 * @param  {String} input, the text to encrypt
 * @return {String}, the encrypted text
 */
RC4.prototype.encrypt = function (input) {
    var outputText = '';
    for (var i = 0, ii = input.length; i < ii; i++) { outputText += ('00' + (input.charCodeAt(i) ^ this.byteStream.next().value).toString(16)).substr(-2); }
    return outputText;
}
 
/**
 * Decrypts the input text
 * @param  {String} input, the text to decrypt
 * @return {String}, the decrypted text (if the same key was used)
 */
RC4.prototype.decrypt = function (input) {
    var outputText = '';
    input = input.match(/[a-z0-9]{2}/gi);
    for (var i = 0, ii = input.length; i < ii; i++) { outputText += String.fromCharCode((parseInt(input[i], 16) ^ byteStream.next().value)); }
    return outputText;
}
 
module.exports = RC4;