I’ve never done this before so I had to do some research, and then analyze what the app is doing. This was an excellent learning exercise about this kind of crypto. I feel like I just did a homework assignment out of a cryptanalysis textbook.
No brute force required but it will require a bit of reading the code and analyzing aspects of the cipher algorithm to know what to manipulate without messing everything else up. You won’t need to get into intricate algorithmic details, just basic structure. It’s all right there though.