This video illustrates some of the key principles behind "internet security" (actually, dual-key cryptography) using tennis-balls and padlocks. It's does a good job, but it misses out on explaining the hardest -- and most interesting -- concept: private/public key-pairs that have the mathematically provable capability of unscrambling one-another's scrambled messages. I have a long section on this in Little Brother, my forthcoming kids' book about hacking for democracy, and I had to rewrite it more than once to get it right. I'll never forget the moment of satori my editor had when I succeeded in conveying to him the extreme coolth and elegant simplicity of key-pairs over lunch -- then went back to my computer and tried to recapture that in the text. Link, Link 2 (Thanks, Jason!)
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.