We started out talking about how encryption is used on DVDs, and what DRM is. That led into a discussion about DeCSS, DVD Jon, and the DMCA (these are all terms you can look up in a search engine for more informaiton).
Moving on to authentication, we discussed Mary, Queen of Scots, and how encryption is sometimes not enough if you don't also have authentication. It got her killed! The "Babington Plot" which brought her down was an example of a Man-In-The-Middle attack.
The opposite of authentication is anonymity, which might be useful if you don't want anyone to know you're the author of a message: like if you're a whistleblower, exposing misconduct of your boss. You can, however, encrypt your authentication, and then send it anonymously. This would be a way to have a secure authenticated communication, without anyone in the middle knowing who sent the message.
Steganography is all about hiding information. We used the examples of invisible ink, or the more modern technique of hiding information in the low-order bits of image pixel components. That's a fancy way to say that you change whether a red, green, or blue value is even or odd to encode your message. Steganography could be used in situations where you don't want anyone to know you're even sending messages, and any or all of the previous three techniques can be used here too.
We touched on the social implications of this. We spent a good deal of time throughout the class talking about freedom, politics, and even the US Constitution! I ended things by asking everyone to think about the downsides to having such strong techniques available for everyone to use: child pornographers, organized crime, or seditious groups trying to overthrow the government, can all use these powerful tools too. Some within law enforcement argue that these tools are too powerful for ordinary people and should be made illegal. What do you think?
Finally, I managed to get everybody excited about the book Little Brother by Cory Doctorow. You can buy it at the bookstore, check it out from the library, or download it for free in many formats, including for iPods, many mobile phones, and computers (PDF, HTML, plain text). One of the reasons that the author makes it available for free is because he wants teenagers to read it, so please check it out. It covers everything we're talking about in these classes.