So the first topic that we're going to talk about is confidentiality, encryption

and decryption. And of course, this was what was going on

at Bletchley Park in World War II. So the terminology that we'll use in this

is plain text and ciphertext. And the idea is whether it's text or

other information, there is the information that we actually want to

transmit whether it's a credit card number or something else.

And then there is the encrypted version of that, and we'll call that the

ciphertext. And the ciphertext is what we assume is

revealed to intermediate parties. Whether they are stopping it, and

changing it, or they're just watching it. It's still, the ciphertext is the stuff

that we are just by the nature of the communication we are forced to reveal it,

or there is a probability that we'll reveal it.

So it is hopefully unintelligible, and hopefully it is difficult to go from the

ciphertext to the plain text, except if you are the actual intended recipient, or

impossible. Encryption is the act of going from plain

text to ciphertext. And returning the ciphertext back to the

plain text is decryption. And there is a key.

Some kind of a key, which is really sort of a, some data plus a technique plus an

algorithm that goes back and forth. So there are two kinds of systems that

we'll talk about in the upcoming lectures.

One is called a secret key, and the other is called a public key.

the secret key is the one we talked about at the very beginning.

The secret key was really used, from the Romans and Caesar, on up to World War II.

the public key encryption really is much more recent, in the 60's and the 70's and

we'll talk about that later on. So the first thing we'll talk about is

the shared secret, or secret key. the secret key is also called symmetric

key, which means that both parties have to be in possession of the same

information. You basically use the same key material

to encrypt as you do to decrypt. The the public key is asymmetric, which

means you use one key to encrypt, and a different key to decrypt.

We'll get to that later. and so the problem that secret key has,

that led to the need to invent a public key, is the fact that you need to at some

point have a secure communication. Whether you're sitting in a room together

and you hand each other code books, whatever it is, you have to have a way to

distribute the key in a secure manner. The public key, which we'll get to later,

has a way of distributing the key in a in a using insecure medium.

And you'll see when we get there it's like, so obvious and clever, you wonder

why nobody thought of it until you know very recently.

So, here is the path. Right?

You had some plain-text, you you have say the word candy that you want to send.

You're going to encrypt with a shift, where you, you just go to the next later

letters, so C becomes D, A becomes B, N becomes O, and so now we have the D, B,

O, E, Z. That is the plain text coming from Alice.

Alice sends it in the dangerous, dangerous, nasty wide world of you know

routers or radio with with Morse code, or whatever it is we're going to do,

whatever it is were going to do, where the message might be intercepted by

somebody in Eve. Now there not intercepting the

plain-text, we assume that this part here is secure, and this part here is secure.

It's only dangerous while it's in flight, somehow, in the middle.

And we only worry about Eve getting it. And then, then at some point, because Bob

has the key which is subtract one, Bob goes from each of the ciphertext letters

back to the plain text letters. And voila, out comes the plain-text

again. And so, Eve's problem is I'm, Eve is only

handed, well no, sorry, sorry. Eve's not, Eve's not given the key, Eve

is given the ciphertext and nothing else, and she must, like Bletchley Park, must

derive whatever it is. Derive the key, derive the plain-text,

whatever it is. That's Eve's goal.

The Caesar cipher, is the kind of the oldest, most widely used forms of

encryption. It uses the notion of a shift.

The shift number is just as I've shown. A shift of 1, means A becomes B and X

becomes Y, and L becomes L becomes M. So, you just take and move a fixed

position down the al. This was used for a surprisingly long

period of time. And there are some pretty good YouTube

videos that kind of you can, if you want to see more about sort of the, the how

this works and the math behind it, and how you break it.

It's, it's pretty fascinating. I mean, finally it's just, it's

completely breakable. and we'll actually going to break it here

pretty soon, ourselves. So the Caesar Cipher.