I have to give a presentation on CRYPTOGRAPHY. any ideas on what topics to cover ?

I really wouldn't even worry about explaing algorythms at all, since in reality you only need to explain different ways that cryptograghy is in use today, and what the government and other authorities are doing to try to break the codes to stop terrorism, etc. One of the newest and most common ways of sending data that is encrypted, is thru pictures. I would do a search for: "Cryptography methods in use today" and essay that for the presentation. People may ask a few questions, but this should give you enough information to give authoritive answers. Hope this helps