As we kick off the Applied Cryptography course, that is a good question to ask.
The answer of course, is that it is an an elephant! The goal of the course is to try and get a clearer picture of this animal (though we probably won’t be able to see the whole herd). We’ll start off from the theory, with definitions of some basic notions of information and secrecy.
At this point, it doesn’t hurt to reiterate a point we briefly touched up on in the introductory lecture, especially if it lets me recycle an old blog post. The very notion of information (from which stems the notion of secrecy) is based on a probabilistic model of the world. Pretty much all the cryptographic schemes we will encounter will require flipping coins in order to create uncertainty for an adversary. We will see this in action right away, with our first topic — namely, secret-sharing. Typically, cryptography courses would start off with encryption, but I think secret-sharing is perhaps a simpler context in which (perfect) secrecy can be understood. More on that later.
Well, or rather, teach a few fundamental concepts which manifest in different ways (and unfortunately under different names) in various applications. 