Decompress Xamarin .NET compressed binaries so they can be decompiled.
This pull request on the xamarin-android project implemented compression for .NET assemblies inside Android .apk files: dotnet/android#4686. This prevents those assemblies from being decompiled until they have been decompressed, which is where this script comes in.
The script checks for the XALZ header which indicates a compressed assembly and decompresses it using LZ4 block decompression so that the resulting file can be decompiled using your tool of choice.
To decompress a single file run the script as follows:
python3 xamarin-decompress.py target-assembly.dll
This will generate target-assembly.decompressed.dll which can be decompiled. To overwrite the original assembly file with the decompressed one, pass the -o flag e.g.
python3 xamarin-decompress.py -o target-assembly.dll
To scan a directory for all Xamarin .NET compressed .dll and .exe files, run the script as follows:
python3 xamarin-decompress.py dir-where-you-extracted-the-apk
Likewise, the -o flag can be used to replace the original files.