How does one obfuscate code in C? [closed]

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I want to obfuscate code just for fun. I'm looking at code from the international obfuscated c contest: http://www.ioccc.org/ And I seriously just have no idea how to even start reverse engineering some of this code to make anything of sense.

What are some common obfuscation techniques and how do you make sense of obfuscated code?

Answer 1

There is a lot of different techniques to obfuscate code, here is a small, very incomplete list:

1. Identifier mangling. Either you will find people using names like a, b, c exclusively, or you find identifiers that have absolutely nothing to do with the actual purpose of the variable/function. Deobfuscation would be to assign sensible names.

2. Heavy use of the conditional evaluation operator ? :, replacing all occurences of if() else. In most cases that's a lot harder to read, deobfuscation would reinsert if().

3. Heavy use of the comma operator instead of ;. In combination with 2. and 4., this basically allows the entire program to be one single statement in main().

4. Recursive calls of main(). You can fold any function into main by having an argument that main can use to decide what to do. Combine this with replacing loops by recursion, and you end up with the entire program being the main function.

5. You can go the exact opposite direction to 3. and 4., and hack everything into pieces by creating an insane amount of functions that all do virtually nothing.

6. You can obfuscate the storage of an array by storing the values on the stack. Should you need to walk the data twice, there's always the fork() call handy to make a convenient copy of your stack.

As I said, this is a very incomplete list, but generally, obfuscation is usually the heavy, systematic abuse of any valid programming technique. If the IOCCC were allowing C++ entries, I would bet on a lot of template code entering, making heavy use of throwing exceptions as an if replacement, hiding structure behind polymorphism, etc.