If you ask a group of programmers the most difficult topics they think there is in the area, there will be a lot of different responses, depending on which language they use and how skilled they are. Dynamic memory allocation, concurrency, coroutines, compiler optimizations are just some examples of those answers. I do hope I’m wrong here, but I don’t think there will be anyone saying “Cryptography”. Sadly, this is why a lot of developers get it wrong on their first hundred times trying to develop a minimal secure system.