[Cryptonic]

To be honest. I would run off. This is the kind of hell where nobody wants to work. Where only "experts" know how things work and how to expand or fix things. It will secure their job but will make yours hell.

You personally will gain no knowledge there, just that your codebase is hell.

You cat try to convince the management of creating a new gen implementation. Not a rewrite. New software, that can fulfill customer needs better. Compete better, is safer and better to extend to do all this in the future.

One thing you can do though is to immediately set up modern practices. SCM, Code Review, CI, Tests (most of the code might not be unit testable in this state, but some tests at least) - This way you can see what others do when they add if fix something and learn better (SCM, Reviews), make changes and know that you did not break the whole thing (Tests) and have CI to at least ensure the tests run and everything works and it will glue all together.

Good luck