Arcfour encryption

arcfour_str.js

goog.provide('ArcfourString');

goog.scope(function() {

/**
 * Arcfour implementation.
 * @param {string} key secret key.
 * @param {string} plainString plain text.
 * @param {number=} opt_dummyLoop dummy encryption loop count.
 * @return {string}
 */
function ArcfourString(key, plainString, opt_dummyLoop) {
  /** @type {!(Array.<number>|Uint8Array)} */
  var s  = new (goog.global.Uint8Array !== void 0 ? Uint8Array : Array)(256);
  /** @type {number} */
  var i;
  /** @type {number} */
  var j;
  /** @type {number} */
  var tmp;
  /** @type {number} */
  var k;
  /** @type {number} */
  var kl;
  /** @type {number} */
  var x;
  /** @type {Array.<string>} */
  var charArray;

  // initialize S-box
  for (i = 0; i < 256; ++i) {
    s[i] = i;
  }

  // shuffle S-box
  for (i = j = 0, kl = key.length; i < 256; ++i) {
    j = (j + s[i] + key.charCodeAt(i % kl) & 0xff) & 0xff;
    tmp = s[i];
    s[i] = s[j];
    s[j] = tmp;
  }

  // initialize
  i = j = 0;
  charArray = [];

  // dummy loop
  if (typeof opt_dummyLoop === 'number') {
    for (k = 0, kl = opt_dummyLoop; k < kl; ++k) {
      i = (i + 1)    & 0xff;
      j = (j + s[i]) & 0xff;

      tmp = s[i];
      s[i] = s[j];
      s[j] = tmp;
    }
  }

  // encryption
  for (x = -1, k = 0, kl = plainString.length; k < kl;) {
    i = (i + 1)    & 0xff;
    j = (j + s[i]) & 0xff;

    tmp = s[i];
    s[i] = s[j];
    s[j] = tmp;

    if (x < 0) {
      x = s[(s[i] + s[j]) % 0xff];
    } else {
      tmp = plainString.charCodeAt(k);
      charArray[k] = String.fromCharCode(
        (((tmp >>   8) ^ x                      ) << 8) |
        (( tmp & 0xff) ^ s[(s[i] + s[j]) % 0xff])
      );
      x = -1;
      ++k;
    }
  }

  // clear
  i = j = k = x = tmp = 0;
  for (i = 0; i < 256; ++i) {
    s[i] = 0;
  }
  s = null;

  return charArray.join('');
}

});