protect c++ output file(pe file) from editing using crc

Question

How to protect c++ output file(pe file) from editing using crc(Cyclic Redundancy Check)?

                            **Best Regards**

Answer 1

You can use CRC's to effectively check to see if a file was accidentally altered, but they are not effective for copy protection, or preventing cheats on a game.

Usually, when I program has some sort of CRC check, I find the code which does the check, and change the assembly instruction from a conditional branch to an unconditional branch. This is usually quite easy to find, because normally after a CRC fail, the program displays a message and exits. I place a break point when the message occurs, and examine all the frames in the stack. I then put break points on each point in the stack, run the program again, and see which one does the CRC check.

This isn't particularly difficult, and people often bundle little programs which will apply the same changes to the software of your choice.

Answer 2

1. You need a static variable in your code. The variable needs to be initialized to a value that can easily found with an hex editor (e.g. DEADBEEF)
2. you need a crc-algorithm (try searching google)
3. The tricky part. You need to get pointer in memory to the start and to the end of your exe. You can parse the pe file header for the code location and run the crc-algorithm from start of code to end of code. Then you have the value.
4. Of course you have to check the calculated value with the one in the static variable.
5. Inserting the value - depending on how often you build, you might want to programm a tool. You can always run your program and set a breakpoint on the comparison. Then you note down the value and hex-edit it into the executable. Or you create a standalone program that parses the pe-header as well, uses the same function (this time on the file) and patches it in. This could be complicated though, because I don't know what is changed by the OS during loading.