A couple of years ago I had a student do an independent study with me on Cryptography. It may be that you have students who are also interested in the subject. A lot of students are. I think that part of it is a normal desire to know how to keep private things private. Teens seem to be especially interested in keeping things from their parent but not as interested in keeping things from their peers.
In any case, Phil Zimmermann who created PGP, a powerful open source encryption suite of software wrote a wonderful Introduction to Cryptography paper. If your student(s) find that interesting than the classic textbook on the subject is Bruce Schneier's Applied Cryptography. I highly recommend that book. There is a lot of math in it and of course students need to know that math is involved with cryptography. There is also a lot of sample code that your students can try out.
The is a FAQ on using the Cryptography classes in the .NET Framework at the GotDotNet web site BTW.