Bruteforces keys for audibles aax-files.
Adjusted for single-threaded (https://github.com/MaxMatti/aax-bruteforce/releases) or build it yourself, then run it.
Just run the executable with the checksum as a parameter:
./aax-bruteforce 23456789abcde23456789abcde23456789abcdef
You can find the checksum out so:
ffprobe audiobook.aax 2>&1 | grep checksum