I am trying to find an encryption algorithm for a game client, but I need to find the actual register/stack in the client before I can decrypt/encrypt the packets correctly (on my emulated server side). The problem is that I have no idea how to find the function.

In OllyDbg, is there a way I can trace during my debugging in order to find the function right after user input? The user input is a string converted to a char array, which is then encrypted in the executable. In this case, it would mean sending a packet to the game client, and then the game client decrypts the packet.