A common technique for obfuscating data is to use exclusive-or (XOR) with some key; it is inexpensive and symmetric. A problem occurs when this is used on file formats like portable executable where there are many null bytes, since XORing nulls with the key ends up writing the key out. A slightly more complex algorithm uses a Linear Feedback Shift Register (LFSR) to produce a key stream, which will be XORed with the data. A generic LFSR is: