 Cryptography is part of communication, so let's talk about talking. In communication, a concept known to one person is delivered to a second person. So let's introduce Alice and Bob, who are the main characters in our story of cryptography. And maybe Alice has an idea in her head, a number, like 23. Now, if she wants to communicate this to Bob, she has several options. She might just simply say 23. And when Bob hears 23, he now has this concept of 23 in his head. Well, now let's introduce secrecy. With cryptography, we add a new requirement. The information should be known only to the intended recipient. And so maybe Alice and Bob need to communicate, but there's somebody else, who we usually call Eve, short for Eve's dropper, who's interested in the information that Alice and Bob are communicating. To keep Eve from learning the information that Alice and Bob are communicating, there are some simple methods. So, for example, don't speak around Eve, go someplace else. We could also use communication method Eve doesn't understand, or we could just speak in a language Eve doesn't understand. And so we can look at information security in the following way. If Alice wants to communicate information to Bob, but not to Eve, she encrypts a plain text message to produce the cipher text, which is then sent to Bob. Now, in order for this to be a communication system, Bob has to be able to recover the original message, so Bob decrypts the cipher text message to recover the plain text. Now, Eve's task is to attack or break the encryption method. And until Eve can break the encryption method, the system is secure. Otherwise, it's said to be insecure. Now, let's make some realistic assumptions. A good strategy when dealing with any adversarial confrontations is to engage in wishful thinking. Wait, wait, no, that's wrong. A good strategy is to give your adversary every possible advantage. And in the case of cryptography, we'll assume the following. First, Eve can always intercept messages between Alice and Bob, and Eve can always read messages between Alice and Bob. We'll make some other assumptions later, but we'll start off with these. And what this means is we should assume that sooner or later Eve can break the system. What do we do then? Well, one possibility is we can scrap the system and start all over. Or we can change part of the system and make Eve start all over. And what this means is that a good cryptosystem consists of two parts. First, a method of encryption which doesn't change. And second, a key which can be changed periodically. And so one of the other assumptions we'll make is that we'll assume that Eve always knows the method of encryption but doesn't know the key. And this brings us to our first truly important definition. In order for Alice and Bob to change the key, they have to have multiple keys available. And so this leads to the following idea. The key space consists of all possible keys. So this gives us a framework for looking at cryptographic systems. Now let's take a look at a few actual systems.