Could this be reverse engineered?

im familiar with Kerckhoffs principle and the importance of transparency of implementation when it comes to cryptography, but as a thought excersise, i want to investigate how far i can go with close source on the frontend. i notice there are big players in the field of secure messaging that are close-source and seem to get away with claims of being secure, private, e2ee, etc. i would like to get your thoughts about how easy it could be to reverse