How would a python program check its own md5sum?

Question

I was playing a game about a year ago. Wanting to speed it up a bit, I edited the main .py file and added psyco to the very beginning. The game ran faster and better, but it would not let me send my scores. I went into the file and it was checking its own md5sum. The part that confused me is that they had its md5 in the file. My question is, how do I get the md5 of a file with the sum of the file including the md5 I don't have yet. Seems a catch 22 to me, but they did it.

Hope I explained that well enough. Example:

File: Example.example

check md5 of Example.example
If md5 I just obtained == "###################################"
allow send score.

Answer 1

You can't (easily) embed an MD5 of a program inside the program that it is computing the MD5, because when you change the MD5 of that you are embedding, the program's MD5 changes. Loop for a while and eventually you may reach a stable point. Then again, you may not. "a while" typically means 2^128 steps, so that's going to be a very long while --- like after the sun becomes a red giant.

So there's a trick. When the program computes its own MD5, it specifically recognizes where the embedded MD5 is located and uses a set value, like all NULLs.

To make it harder for hackers, don't use all NULLs --- use a SALT value. Obfuscate that value in your binary. Now it will be harder for people to edit the embedded MD5 and "fix" it after they have edited your program.