solved it. The biggest issue I had was to find a decent debugger to work with in Linux. I am spoiled in Windows land with X64dbg and the likes. Combining GDB with Peda helped a bit but I still struggle with all-terminal debuggers. My hint on this one would be find the secret routine, find the info you need to reverse the secret code and most important, use a hexdump of the encrypted secret for reversing, the string representation gave me a false decrypted result.
All in all a nice challenge, thanks to the creator!