Authorization code

I have a remote assistance package based on VNC. It is a compiled .exe application. It uses reverse connections, initiating from the client to me. I want to add a authorization code box that the client will have to put a code into to initiate the outbound connection to me inside of my application. Some sort of algorithm based application that works with my remote support application, and a random code generator for my side to give the codes to the clients. Something secure as well.I know there are free tools that do this, but i want this on my own support package. I am unsure of the categories, so please let me know if this can be done.