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

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

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

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

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

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

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

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ciphertext. 
And the ciphertext is what we assume is 

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

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

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

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

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

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

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impossible. 
Encryption is the act of going from plain 

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text to ciphertext. 
And returning the ciphertext back to the 

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plain text is decryption. 
And there is a key. 

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

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

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we'll talk about in the upcoming 
lectures. 

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

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

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

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

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

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

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

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information. 
You basically use the same key material 

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

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

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

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

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

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

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

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

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

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why nobody thought of it until you know 
very recently. 

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So, here is the path. 
Right? 

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

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

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

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

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

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

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

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somebody in Eve. 
Now there not intercepting the 

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

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

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

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

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

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

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

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

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

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

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

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encryption. 
It uses the notion of a shift. 

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

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

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

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period of time. 
And there are some pretty good YouTube 

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

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

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

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

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pretty soon, ourselves. 
So the Caesar Cipher. 

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So I want to pause and let you see a 
YouTube video, here from a, beloved movie 

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called The Christmas Story, where little 
Ralphie gets his Little Orphan Annie 

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secret decoder ring. 
And Little Orphan Annie sends a decoded 

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message, an encoded ciphertext, through 
the radio. 

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Everyone can hear it, but only those 
people who have the secret decoder ring 

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can decrypt the message. 
And you can see that it is a, a Caesar 

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cipher it has a shift. 
You'll note that the first thing they say 

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before they say the encrypted message in 
the radio is that you're supposed to 

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connect B and 13, or something like that, 
and then you rotate the two wheels of the 

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secret decoder ring to the b13. 
And then you can read across the secret 

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decoder ring and decrypt the message as 
it's decrypted, and then he slowly 

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decrypts it. 
And then, of course, there is the 

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delightful moment where he realizes the 
crash, crass commercialism that that, 

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that are the complete lack of interesting 
meaning in, in all of this. 

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So, without further ado, let's take a 
look at Ralphie and A Christmas Story. 

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We'll be right back. 
So, I hope you liked that, hope you liked 

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that. 
and so off we go. 

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we're going to have a secret decoder ring 
for this class. 

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I would love to be able to send you all, 
oops, come down. 

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I would love to be able to send you all a 
little mechanical wheel to move the stuff 

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back and forth. 
But instead I used the internet, and I'm 

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going to send you a PDF, and at this 
point you might want to pause the video, 

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and grab this PDF. 
Okay? 

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grab it. 
Secretdecoder.pdf. 

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Dr-chuck.com/SecretDecoder.pdf. 
And download it, and you might even want 

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to print it out. 
Because we're going to, at this moment do 

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a code breaking exercise. 
Okay? 

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And so let me tell you how to use this 
secret decoder ring. 

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So the top line here is the plain text. 
And if you recall Caesar shift has a, a, 

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a shift number. 
And so to encode, you go from plain-text, 

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let's say I want to do Chuck. 
Alright, I want to encode Chuck. 

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And I want to encode it with a shift of 
two. 

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So a shift of two means we select this. 
And we basically go C is our plain-text, 

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and then down we go E. 
So then we go E is our first letter. 

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And then H is our second letter, so we go 
down and that means H becomes J. 

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Yeah, and so U becomes W. 
So now I'm doing my encryptions. 

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So E H W is the encrypted ciphertext. 
So, let me clear that, and write back 

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down here. 
Oop, maybe, a different, different color. 

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E H W. 
Now to decrypt, remember, you need to 

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know the shift. 
So, we somehow communicated separately 

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and securely, what the shift was. 
And so, now we want, we have received our 

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ciphertext. 
We received our ciphertext, and we need 

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to decrypt it. 
OK? 

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And so we know that the shift is two, so 
we go to E, we go in the shift row, and 

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then we go up to the plain text row, and 
that says the first one is C. 

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Then we go to H, we go to the the encoded 
text row. 

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And we go back up to the plain text row, 
and so that's an H. 

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Oh, wait, wait, wait. 
It's not H W. 

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What am I thinking? 
This should have been a J. 

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I got this wrong. 
Sorry about that. 

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So, that's wrong. 
Here's a J, moves up to the H. 

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Then the W, let's see if I got W right, 
yeah. 

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So W is my last ciphertext, and it goes 
up to the U. 

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OK? 
Dot, dot, dot, dot, dot. 

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So you see the pattern that our in 
encoding is plain text down to shift 

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position, and our decoding is shift 
position back up to plain text. 

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OK? 
And, so this is our secret decoder ring. 

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So go grab it and download it, so that 
you can participate in the next exercise. 

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So now you are going to be cast in the 
role of Bletchley Park. 

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OK, ready? 
So here is your first code breaking 

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exercise. 
OK. 

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So here we go. 
So you're Bletchley Park, right? 

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You just intercepted this ciphertext. 
U B U P B T U. 

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Whoa, it's encrypted. 
It's clearly meaningless. 

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So, how are you going to be decrypt it? 
Well, the technique is, take a look, and 

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decrypt it with all the shift numbers. 
Right? 

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You're going to do this by hand, you're 
going to be a computer yourself. 

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You're going to do all the shift 
encrypting, and just like in Bletchley 

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Park, you know you've succeeded when the 
plain text makes sense. 

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Right when the plain text makes no sense, 
then you haven't succeeded. 

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But in some point the plain text makes 
sense. 

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So what you need to do is take your 
secret decoder ring, and you need to 

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decrypt it with a shift of one, a shift 
of two, a shift of three, a shift of 

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four, and if you have your family members 
around, you can put out multiple copies 

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of the secret decoder ring, and you can 
assign different shifts to different 

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family members. 
So you have to decrypt this 26 times. 

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Have to decrypt it 26 times. 
And then you'd look at all the 26 

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decryptions, and then you decide which 
one makes the most sense. 

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OK? 
So, don't peek, decrypt this one. 

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I made it easy on you. 
OK? 

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00:11:23,359 --> 00:11:29,550
So, we'll stop now, and give you a little 
bit of time to decrypt this one. 

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Don't start pause until you actually have 
decrypted it. 

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OK? 
OK, this is your last chance for spoiler 

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alert. 
So here we are. 

153
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We're about to decrypt it. 
Here we go. 

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00:11:48,680 --> 00:11:51,495
I did make it easy on you. 
It was a shift of one. 

155
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If was a shift of one. 
If you started at 12, you're kind of 

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foolish. 
Right? 

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00:11:59,490 --> 00:12:03,638
So you started at one, and you go, like, 
oh great, so now, I'm going to decrypt 

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it. 
I'll start at one. 

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here's the plain text, this should be PP. 
That's the plain text. 

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00:12:09,290 --> 00:12:11,742
So I'll start with U. 
If it's one, then I go up, and it's T. 

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And the second one is P, so I go up, and 
it's O, T O. 

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Keep going, says toast. 
So you say to yourself, well that's a 

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word, so it must be it. 
Well hello. 

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What are you doing here? 
Do you want to say hi to my students? 

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00:12:33,652 --> 00:12:37,124
 >> Meow. 
 >> This is the cat. 

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00:12:37,124 --> 00:12:43,008
This is Eddie cat. 
He likes to come up into my office, and 

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look. 
So do you know anything about encryption? 

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00:12:47,114 --> 00:12:50,450
Hm? 
Do you know anything about encryption? 

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00:12:50,450 --> 00:12:55,130
So you use a shift of one, and then you 
go from the encryption text up to the 

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plain text. 
Meow. 

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00:12:59,125 --> 00:13:02,657
OK. 
You are clearly not interested in my 

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00:13:02,657 --> 00:13:05,485
lecture. 
So that was my cat. 

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00:13:05,485 --> 00:13:10,028
Hello, sorry, I can't open a window for 
you, because I'm doing a lecture. 

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00:13:10,028 --> 00:13:16,330
OK. 
so,[LAUGH] ,[CROSSTALK], you're going 

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00:13:16,330 --> 00:13:24,568
like just keep bugging that window, until 
I kick you out of the room aren't you? 

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00:13:24,568 --> 00:13:28,217
So, you're going to have to get kicked 
out. 

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00:13:28,217 --> 00:13:31,198
Out you go. 
[SOUND] He's was going to keep hitting, 

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keep hitting that, until I opened it for 
him. 

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00:13:34,960 --> 00:13:39,170
OK. 
So, so now you've broken this code. 

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00:13:39,170 --> 00:13:42,068
And again, just like in Bletchley Park, 
you only knew that you broke it, if it 

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00:13:42,068 --> 00:13:45,588
made sense. 
And and so, luckily the, in Bletchley 

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Park the messages were longer, and they 
were often looking for canonical things 

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that they would say everyday. 
So here we go. 

184
00:13:55,529 --> 00:13:59,421
And and so that's the breaking of that 
one. 

185
00:13:59,421 --> 00:14:04,390
And it turns out the shift of one was the 
thing that we did. 

186
00:14:05,780 --> 00:14:10,429
So here is your second task. 
This one's longer, and its not a shift of 

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one. 
And so, so this is a situation where you 

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would really have to get your whole 
family going on this. 

189
00:14:19,011 --> 00:14:21,820
Right? 
Where you gotta do 26 decryptions of 

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00:14:21,820 --> 00:14:26,400
this, and it will, you know, make sense 
to you. 

191
00:14:26,400 --> 00:14:28,514
You decrypt it 26 times. 
Right? 

192
00:14:28,514 --> 00:14:33,426
And so, this one's going to be harder. 
I guess you could just, just decrypt one 

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word, but it's just not a shift of one. 
But now we're going to do another trick. 

194
00:14:38,512 --> 00:14:39,144
OK? 
So, I don't want you to try all 26. 

195
00:14:39,144 --> 00:14:39,773
Because there's, there's a mistake in 
this. 

196
00:14:39,773 --> 00:14:47,891
There is a leakage of information, that 
makes it so that you can, figure out what 

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00:14:47,891 --> 00:14:57,460
the right decryption to try it might be. 
So this is English. 

198
00:14:57,460 --> 00:15:02,147
This is an English sentence. 
So stare at it for a while, and find a 

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00:15:02,147 --> 00:15:07,703
more optimal way to decrypt it, than 
trying all 26 shift patterns. 

200
00:15:07,703 --> 00:15:10,423
OK? 
So there’s a way to optimize this. 

201
00:15:10,423 --> 00:15:15,022
A way to cleverly figure out what might 
be the best shift, or how not to have to 

202
00:15:15,022 --> 00:15:20,970
decrypt the entire message 26 times, to 
reduce the complexity. 

203
00:15:20,970 --> 00:15:25,520
And that's because we've leaked some 
information here, that should be pretty 

204
00:15:25,520 --> 00:15:27,653
obvious to you. 
OK? 

205
00:15:27,653 --> 00:15:30,910
So, let me give you a moment to break 
this one. 

206
00:15:30,910 --> 00:15:34,260
It shouldn't take you too long, and you 
shouldn't have to force your whole family 

207
00:15:34,260 --> 00:15:36,590
to decrypt this stuff. 
OK? 

208
00:15:36,590 --> 00:15:43,256
So here we go. 
Give you a chance to decrypt it. 

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00:15:43,256 --> 00:15:49,410
OK, this is your last chance before the 
reveal. 

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You ready? 
Here we go. 

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So here is the decrypted text. 
The shift turns out to be 13, it's a 

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shift of 13. 
And the, the weakness of this whole thing 

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is this, right here. 
In the English language, what is a single 

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character, we're not encrypting the 
spaces you'll notice, because the spaces 

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come across. 
So what is the one single character, 

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word, that we have in the English 
language, that's capitalized? 

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Well, that's usually I. 
I need a jet, money and a jet. 

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What is the one thing that we do in the 
English language that is a single 

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character word that is all lowercase, 
that's lower case typically, unless it's 

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at the beginning of a sentence? 
And that is the letter A. 

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So basically, you didn't have to decrypt 
the whole message. 

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You see some weird pattern, and you go 
like this. 

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I just have to figure out, and then you 
go look. 

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And you look in the row 13, the plain 
text. 

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And you go, like oh. 
Where's, you go look at the I, because 

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you guessed at plain text. 
And then you just look down until you see 

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the V. 
And within seconds, literally, within 

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seconds, if you do it right, within 
seconds, you know it's a shift of 13. 

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And then it's a trivial matter to convert 
it. 

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So, you could figure out the shift code 
within seconds. 

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And these, this was how Bletchley Park 
figured it out. 

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And this is why the known plain text was 
so important. 

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Because you'd only have to figure out, 
like, one letter, if you knew what the 

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plain text was. 
And often they would know by length, and 

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certain other things. 
Oh, this is, I, I think we can guess what 

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this plain text was that this particular 
operator would send. 

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And this has to do with the leakage of 
information. 

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It's not the, it's not the mathematical 
perfection, or lack of perfection in the 

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security key. 
It's some other leak, it's some other 

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thing. 
When they're going like, oh, wait a sec, 

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I can take advantage of something. 
It was just equally encrypted as any 

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other message, but because I gave you 
this clue, of an uppercase single 

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character word and a lower case single 
character word. 

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Upper, lower case single character word 
and upper case single character word, I 

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greatly reduced the amount of effort that 
you had to put in. 

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OK? 
Now, what's cool about this, you can go 

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to this website, www.rot13.com, is that, 
long ago, before Facebook and before 

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Twitter and before all these things we 
had these things called newsgroups. 

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And they were kind of this weird kind of, 
collective email list that we had. 

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And this was, like, in the 80's. 
And it was even used in storing forward 

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networks. 
Where, it was kind of like Facebook in 

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storing forward networks. 
Meaning that it might take four hours for 

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00:18:49,624 --> 00:18:53,302
you to see the status update. 
But we kind of of would subscribe to 

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these collective things. 
And there was one that was basically the 

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dirty jokes. 
And the thing about dirty jokes was, part 

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00:19:01,261 --> 00:19:04,759
of the, part of what we were trying to do 
in this thing, was, you weren't supposed 

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to swear. 
And there was software that would filter 

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out swear words. 
from, if, if you put a swear word in to a 

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00:19:14,768 --> 00:19:20,480
dirty joke it would not forward the 
message. 

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And so we had to have a way to encrypt 
messages that included swear words. 

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So that we could tell dirty jokes to each 
other, for those who wanted to subscribe 

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to the dirty joke list. 
And so they came up with this rote 13. 

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So we came up with a simple Caesar 
cipher, with a shift of 13. 

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00:19:37,416 --> 00:19:40,599
13 beautifully, of course, is 26 divided 
by 2. 

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00:19:41,600 --> 00:19:45,173
So it's a symmetric shift. 
The shifting in by 13, is the same as 

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shifting out. 
So, of all the Caesar's ciphers, a shift 

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of 13, the encryption and the decryption 
are exactly the same calculation. 

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And so we would we would type our dirty 
joke into, rot13, and convert it to 

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00:19:59,312 --> 00:20:05,157
rot13, and we would send it in rot13. 
And then we would, if we wanted to 

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00:20:05,157 --> 00:20:08,204
decrypt it. 
But what became funny after a while, was, 

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we were so used to reading rot13, that it 
almost became like a second language. 

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00:20:13,978 --> 00:20:16,514
Right? 
We could, we could read second, we could 

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00:20:16,514 --> 00:20:20,804
start reading the dirty jokes in rot13, 
and we would laugh before we translated 

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00:20:20,804 --> 00:20:25,500
them up. 
So rot13, has an interesting sort of 

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00:20:25,500 --> 00:20:28,789
historical thing. 
And you can go to rot13 and sort of like 

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encrypt, whatever you want to say. 
And I'll probably have some questions to 

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ask you, where you will have to do some 
rot13 encryption. 

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And so that's the end of this lecture, 
where we talk about Caesar ciphers and 

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the various techniques, and how Caesar 
ciphers work. 

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00:20:45,570 --> 00:20:49,090
And, so we'll be back and talk about 
cryptographic hashes.