function _mask(source, mask, output, offset, length) {
  for (let i = 0; i < length; i++) {
    output[offset + i] = source[i] ^ mask[i & 3];
  }
}
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