The best way to let other user make a kind of plugin

Hello,

I'm making a new chat program wich has some standard encryption. But now I want to give the user teh ability to create it's own encryption. This could be done with a dll or something. But now I'm thinking. I want to a much flexibility as possible, but what can I use best then? Cause an ActiveX isn't flexible cause they can't be made with for example Delphi or C++ (as far as I know). Any suggestions how this can be best done?

In other words someone else must be able to make a "standalone" program wich I will put in the data wich the user wants to send and then that program returns me the data to send, and they other way around, If I give the program some encrypted data it can decrypt?

Any suggestions?

You better implement this with interfaces ...search the forum for previous results .;)

Well I have searched a little on the interfaces thing, but that's not something I want, I want the user to give me (for example with a textfile) information about the functions and subs wich my program must use out of the dll, that's much more flexible, and that's something I really want.