code: function _mask(source, mask, output, offset, length) {
    for (let i = 0; i < length; i++) {
      output[offset + i] = source[i] ^ mask[i & 3];
    }
  }
docs: Masks a buffer using the given mask.
  
  @param {Buffer} source The buffer to mask
  @param {Buffer} mask The mask to use
  @param {Buffer} output The buffer where to store the result
  @param {Number} offset The offset at which to start writing
  @param {Number} length The number of bytes to mask.
  @public