private static void WriteRandomMask(byte[] buffer, int offset) { byte[] mask = t_headerMask ?? (t_headerMask = new byte[MaskLength]); Debug.Assert(mask.Length == MaskLength, $"Expected mask of length {MaskLength}, got {mask.Length}"); s_random.GetBytes(mask); Buffer.BlockCopy(mask, 0, buffer, offset, MaskLength); }