code: function fast_decrypt(data, k) {
    var b = new Uint8Array(data);
    var key = new Uint8Array(k);
    var previous = 0x36;
    for (var i = 0; i < b.length; ++i) {
      var t = (b[i] >> 4 | b[i] << 4) & 0xff;
      t = t ^ previous ^ i & 0xff ^ key[i % key.length];
      previous = b[i];
      b[i] = t;
    }
    return new _bl2.default(b);
  }
docs: fast_decrypt buffer
  @param {Buffer} data data buffer
  @param {Buffer} k key