Tips to make reverse engineering a C shared library more difficult

Question

I am interested in precautionary measures you could take to make reverse engineering a C shared library more difficult

It seems that it is impossible to prevent entirely but you can take steps that might make it take longer and therefore be less attractive/more costly to do

Things like code obfuscation, compiler optimisation options compiler debug flags etc. (I would be specifically interested in the gcc compiler)

Also interested in any thoughts on the relative difficulty of reverse engineering a ~5KLOC C shared library (.so size of ~200kb) in terms of $ or man hours etc.

Answer 1

Those are some tricks that I know of:

• Use strip with the --strip-unneeded option to remove all symbols except the ones that are needed for relocation (since we're talking about a shared library):

  strip --strip-unneeded libmylib.so

• Link with the standard libc statically, this results in a bigger library/binary which means more code to go through and also obfuscation since it will be more difficult to separate your functions from the library functions:

  gcc -static ...

• Force the compiler to inline small functions, this embeds small functions in your code instead of calling them, so it makes it almost impossible to distinguish between your code and the standard library code.

Having said that, I must add that I've had a binary that used the above measures and I was able to reverse engineer it in a few hours, I started by rebuilding the symtab if you're curious, and I'm no reverse engineering guru.

Answer 2

- No need of code obfuscation (as in C# or Java).

The name of your internal variables and functions do not exist in the object code.

However the name of your exported functions will still appear in clear in the shared object file. This sounds reasonable.

- Better to use gcc optimization flags -O2 or -O3.

Using compiler optimization, object code can largely differ from source code. It is actually very difficult to go back to the original C source code.

However it is always possible to revert engineer at the assembly level. Usually, not all your program is valuable. It will be easier to reverse engineer the interesting portion in assembly rather than the whole program in C.