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Filename : rc4.js
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import {
  StreamCipher,
} from './cipher-core.js';

function generateKeystreamWord() {
  // Shortcuts
  const S = this._S;
  let i = this._i;
  let j = this._j;

  // Generate keystream word
  let keystreamWord = 0;
  for (let n = 0; n < 4; n += 1) {
    i = (i + 1) % 256;
    j = (j + S[i]) % 256;

    // Swap
    const t = S[i];
    S[i] = S[j];
    S[j] = t;

    keystreamWord |= S[(S[i] + S[j]) % 256] << (24 - n * 8);
  }

  // Update counters
  this._i = i;
  this._j = j;

  return keystreamWord;
}

/**
 * RC4 stream cipher algorithm.
 */
export class RC4Algo extends StreamCipher {
  _doReset() {
    // Shortcuts
    const key = this._key;
    const keyWords = key.words;
    const keySigBytes = key.sigBytes;

    // Init sbox
    this._S = [];
    const S = this._S;
    for (let i = 0; i < 256; i += 1) {
      S[i] = i;
    }

    // Key setup
    for (let i = 0, j = 0; i < 256; i += 1) {
      const keyByteIndex = i % keySigBytes;
      const keyByte = (keyWords[keyByteIndex >>> 2] >>> (24 - (keyByteIndex % 4) * 8)) & 0xff;

      j = (j + S[i] + keyByte) % 256;

      // Swap
      const t = S[i];
      S[i] = S[j];
      S[j] = t;
    }

    // Counters
    this._j = 0;
    this._i = this._j;
  }

  _doProcessBlock(M, offset) {
    const _M = M;

    _M[offset] ^= generateKeystreamWord.call(this);
  }
}
RC4Algo.keySize = 256 / 32;
RC4Algo.ivSize = 0;

/**
 * Shortcut functions to the cipher's object interface.
 *
 * @example
 *
 *     var ciphertext = CryptoJS.RC4.encrypt(message, key, cfg);
 *     var plaintext  = CryptoJS.RC4.decrypt(ciphertext, key, cfg);
 */
export const RC4 = StreamCipher._createHelper(RC4Algo);

/**
 * Modified RC4 stream cipher algorithm.
 */
export class RC4DropAlgo extends RC4Algo {
  constructor(...args) {
    super(...args);

    /**
     * Configuration options.
     *
     * @property {number} drop The number of keystream words to drop. Default 192
     */
    Object.assign(this.cfg, { drop: 192 });
  }

  _doReset() {
    super._doReset.call(this);

    // Drop
    for (let i = this.cfg.drop; i > 0; i -= 1) {
      generateKeystreamWord.call(this);
    }
  }
}

/**
 * Shortcut functions to the cipher's object interface.
 *
 * @example
 *
 *     var ciphertext = CryptoJS.RC4Drop.encrypt(message, key, cfg);
 *     var plaintext  = CryptoJS.RC4Drop.decrypt(ciphertext, key, cfg);
 */
export const RC4Drop = StreamCipher._createHelper(RC4DropAlgo);