iOS obfuscation: mono-xcompile produces broken .dll.s

[Thomas_GF]

I’ve been experimenting with obfuscation for Assembly-CSharp.dll on Android and iOS.
For Android it was quite simple.

• extract the dll from the apk
• obfuscate it
• put it back in the apk
• re-sign it

For iOS it proved to be much more difficult. After Unity generates the XCode project, there is a corresponding .dll.s file for every managed .dll. After lots of googling I found that these .dll.s files are generated from the .dll files by a tool named mono-xcompiler. This tool is invoked by Unity during the build process.

So my strategy was:

• let Unity build an XCode project
• remove the old Assembly-CSharp.dll.s from Libraries/
• obfuscate Data/Managed/Assembly-CSharp.dll
• generate a new Assembly-CSharp.dll.s from the obfuscated Assembly-CSharp.dll using mono-xcompiler which comes with Unity
• let XCode build the app

However, building the app fails while XCode tries to compile Assembly-CSharp.dll.s into Assembly-CSharp.dll.o.
I don’t have access to the build environment atm, so I can’t give you the exact CompileC command, but the error was something like

/path/to/Assembly-CSharp.dll.s:[numbers]:[number]: error: invalid symbol redefinition
plt__class_init_____:
^

/path/to/Assembly-CSharp.dll.s:[numbers]:[number]: error: invalid symbol redefinition
plt__class_init_____:
^

/path/to/Assembly-CSharp.dll.s:[numbers]:[number]: error: invalid symbol redefinition
plt__class_init_____:
^

I suspect one of the following:

• I used Eazfuscator.NET to obfuscate the Assembly-CSharp.dll on Windows, however the file was surrounded only by iOS-versions of dependency DLLs so it should be fine. Still, it might be a problem.

• I might have used mono-xcompiler in a wrong way:

export MONO_PATH=[xcode-project]/Data/Managed
export GAC_PATH=[xcode-project]/Data/Managed
export GC_DONT_GC="yes please"
mono-xcompiler --aot=full,static,nodebug,asmonly,outfile="../Libraries/Assembly-CSharp.dll.s" "Managed/Assembly-CSharp.dll"

• mono-xcompiler or .dll.s-files might be unable to handle an obfuscated .dll. This would totally suck.

Any ideas? Any information about mono-xcompiler or .dll.s that might be helpful?

[Thomas_GF]

And I answer myself: plt__class_init_____ (the board filters out the underscores, sorry for that) is actually the static class initializer of a .NET class. And there are exactly as many errors as there are static class initializers in my dll So I removed name obfuscation for those classes and voila: everything works as intended, I have an obfuscated dll in my iOS app.

I am still planning to figure out how I can have class names of classes obfuscated, that use static initializers, but that shouldn’t be much of a problem. In the worst case I’ll have to replace the static initializers or encapsulate them into another class or something.

Still: obfuscation for Android and iOS works, q.e.d.