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. 
So I want to pause and let you see a 
YouTube video, here from a, beloved movie 
called The Christmas Story, where little 
Ralphie gets his Little Orphan Annie 
secret decoder ring. 
And Little Orphan Annie sends a decoded 
message, an encoded ciphertext, through 
the radio. 
Everyone can hear it, but only those 
people who have the secret decoder ring 
can decrypt the message. 
And you can see that it is a, a Caesar 
cipher it has a shift. 
You'll note that the first thing they say 
before they say the encrypted message in 
the radio is that you're supposed to 
connect B and 13, or something like that, 
and then you rotate the two wheels of the 
secret decoder ring to the b13. 
And then you can read across the secret 
decoder ring and decrypt the message as 
it's decrypted, and then he slowly 
decrypts it. 
And then, of course, there is the 
delightful moment where he realizes the 
crash, crass commercialism that that, 
that are the complete lack of interesting 
meaning in, in all of this. 
So, without further ado, let's take a 
look at Ralphie and A Christmas Story. 
We'll be right back. 
So, I hope you liked that, hope you liked 
that. 
and so off we go. 
we're going to have a secret decoder ring 
for this class. 
I would love to be able to send you all, 
oops, come down. 
I would love to be able to send you all a 
little mechanical wheel to move the stuff 
back and forth. 
But instead I used the internet, and I'm 
going to send you a PDF, and at this 
point you might want to pause the video, 
and grab this PDF. 
Okay? 
grab it. 
Secretdecoder.pdf. 
Dr-chuck.com/SecretDecoder.pdf. 
And download it, and you might even want 
to print it out. 
Because we're going to, at this moment do 
a code breaking exercise. 
Okay? 
And so let me tell you how to use this 
secret decoder ring. 
So the top line here is the plain text. 
And if you recall Caesar shift has a, a, 
a shift number. 
And so to encode, you go from plain-text, 
let's say I want to do Chuck. 
Alright, I want to encode Chuck. 
And I want to encode it with a shift of 
two. 
So a shift of two means we select this. 
And we basically go C is our plain-text, 
and then down we go E. 
So then we go E is our first letter. 
And then H is our second letter, so we go 
down and that means H becomes J. 
Yeah, and so U becomes W. 
So now I'm doing my encryptions. 
So E H W is the encrypted ciphertext. 
So, let me clear that, and write back 
down here. 
Oop, maybe, a different, different color. 
E H W. 
Now to decrypt, remember, you need to 
know the shift. 
So, we somehow communicated separately 
and securely, what the shift was. 
And so, now we want, we have received our 
ciphertext. 
We received our ciphertext, and we need 
to decrypt it. 
OK? 
And so we know that the shift is two, so 
we go to E, we go in the shift row, and 
then we go up to the plain text row, and 
that says the first one is C. 
Then we go to H, we go to the the encoded 
text row. 
And we go back up to the plain text row, 
and so that's an H. 
Oh, wait, wait, wait. 
It's not H W. 
What am I thinking? 
This should have been a J. 
I got this wrong. 
Sorry about that. 
So, that's wrong. 
Here's a J, moves up to the H. 
Then the W, let's see if I got W right, 
yeah. 
So W is my last ciphertext, and it goes 
up to the U. 
OK? 
Dot, dot, dot, dot, dot. 
So you see the pattern that our in 
encoding is plain text down to shift 
position, and our decoding is shift 
position back up to plain text. 
OK? 
And, so this is our secret decoder ring. 
So go grab it and download it, so that 
you can participate in the next exercise. 
So now you are going to be cast in the 
role of Bletchley Park. 
OK, ready? 
So here is your first code breaking 
exercise. 
OK. 
So here we go. 
So you're Bletchley Park, right? 
You just intercepted this ciphertext. 
U B U P B T U. 
Whoa, it's encrypted. 
It's clearly meaningless. 
So, how are you going to be decrypt it? 
Well, the technique is, take a look, and 
decrypt it with all the shift numbers. 
Right? 
You're going to do this by hand, you're 
going to be a computer yourself. 
You're going to do all the shift 
encrypting, and just like in Bletchley 
Park, you know you've succeeded when the 
plain text makes sense. 
Right when the plain text makes no sense, 
then you haven't succeeded. 
But in some point the plain text makes 
sense. 
So what you need to do is take your 
secret decoder ring, and you need to 
decrypt it with a shift of one, a shift 
of two, a shift of three, a shift of 
four, and if you have your family members 
around, you can put out multiple copies 
of the secret decoder ring, and you can 
assign different shifts to different 
family members. 
So you have to decrypt this 26 times. 
Have to decrypt it 26 times. 
And then you'd look at all the 26 
decryptions, and then you decide which 
one makes the most sense. 
OK? 
So, don't peek, decrypt this one. 
I made it easy on you. 
OK? 
So, we'll stop now, and give you a little 
bit of time to decrypt this one. 
Don't start pause until you actually have 
decrypted it. 
OK? 
OK, this is your last chance for spoiler 
alert. 
So here we are. 
We're about to decrypt it. 
Here we go. 
I did make it easy on you. 
It was a shift of one. 
If was a shift of one. 
If you started at 12, you're kind of 
foolish. 
Right? 
So you started at one, and you go, like, 
oh great, so now, I'm going to decrypt 
it. 
I'll start at one. 
here's the plain text, this should be PP. 
That's the plain text. 
So I'll start with U. 
If it's one, then I go up, and it's T. 
And the second one is P, so I go up, and 
it's O, T O. 
Keep going, says toast. 
So you say to yourself, well that's a 
word, so it must be it. 
Well hello. 
What are you doing here? 
Do you want to say hi to my students? 
 >> Meow. 
 >> This is the cat. 
This is Eddie cat. 
He likes to come up into my office, and 
look. 
So do you know anything about encryption? 
Hm? 
Do you know anything about encryption? 
So you use a shift of one, and then you 
go from the encryption text up to the 
plain text. 
Meow. 
OK. 
You are clearly not interested in my 
lecture. 
So that was my cat. 
Hello, sorry, I can't open a window for 
you, because I'm doing a lecture. 
OK. 
so,[LAUGH] ,[CROSSTALK], you're going 
like just keep bugging that window, until 
I kick you out of the room aren't you? 
So, you're going to have to get kicked 
out. 
Out you go. 
[SOUND] He's was going to keep hitting, 
keep hitting that, until I opened it for 
him. 
OK. 
So, so now you've broken this code. 
And again, just like in Bletchley Park, 
you only knew that you broke it, if it 
made sense. 
And and so, luckily the, in Bletchley 
Park the messages were longer, and they 
were often looking for canonical things 
that they would say everyday. 
So here we go. 
And and so that's the breaking of that 
one. 
And it turns out the shift of one was the 
thing that we did. 
So here is your second task. 
This one's longer, and its not a shift of 
one. 
And so, so this is a situation where you 
would really have to get your whole 
family going on this. 
Right? 
Where you gotta do 26 decryptions of 
this, and it will, you know, make sense 
to you. 
You decrypt it 26 times. 
Right? 
And so, this one's going to be harder. 
I guess you could just, just decrypt one 
word, but it's just not a shift of one. 
But now we're going to do another trick. 
OK? 
So, I don't want you to try all 26. 
Because there's, there's a mistake in 
this. 
There is a leakage of information, that 
makes it so that you can, figure out what 
the right decryption to try it might be. 
So this is English. 
This is an English sentence. 
So stare at it for a while, and find a 
more optimal way to decrypt it, than 
trying all 26 shift patterns. 
OK? 
So there’s a way to optimize this. 
A way to cleverly figure out what might 
be the best shift, or how not to have to 
decrypt the entire message 26 times, to 
reduce the complexity. 
And that's because we've leaked some 
information here, that should be pretty 
obvious to you. 
OK? 
So, let me give you a moment to break 
this one. 
It shouldn't take you too long, and you 
shouldn't have to force your whole family 
to decrypt this stuff. 
OK? 
So here we go. 
Give you a chance to decrypt it. 
OK, this is your last chance before the 
reveal. 
You ready? 
Here we go. 
So here is the decrypted text. 
The shift turns out to be 13, it's a 
shift of 13. 
And the, the weakness of this whole thing 
is this, right here. 
In the English language, what is a single 
character, we're not encrypting the 
spaces you'll notice, because the spaces 
come across. 
So what is the one single character, 
word, that we have in the English 
language, that's capitalized? 
Well, that's usually I. 
I need a jet, money and a jet. 
What is the one thing that we do in the 
English language that is a single 
character word that is all lowercase, 
that's lower case typically, unless it's 
at the beginning of a sentence? 
And that is the letter A. 
So basically, you didn't have to decrypt 
the whole message. 
You see some weird pattern, and you go 
like this. 
I just have to figure out, and then you 
go look. 
And you look in the row 13, the plain 
text. 
And you go, like oh. 
Where's, you go look at the I, because 
you guessed at plain text. 
And then you just look down until you see 
the V. 
And within seconds, literally, within 
seconds, if you do it right, within 
seconds, you know it's a shift of 13. 
And then it's a trivial matter to convert 
it. 
So, you could figure out the shift code 
within seconds. 
And these, this was how Bletchley Park 
figured it out. 
And this is why the known plain text was 
so important. 
Because you'd only have to figure out, 
like, one letter, if you knew what the 
plain text was. 
And often they would know by length, and 
certain other things. 
Oh, this is, I, I think we can guess what 
this plain text was that this particular 
operator would send. 
And this has to do with the leakage of 
information. 
It's not the, it's not the mathematical 
perfection, or lack of perfection in the 
security key. 
It's some other leak, it's some other 
thing. 
When they're going like, oh, wait a sec, 
I can take advantage of something. 
It was just equally encrypted as any 
other message, but because I gave you 
this clue, of an uppercase single 
character word and a lower case single 
character word. 
Upper, lower case single character word 
and upper case single character word, I 
greatly reduced the amount of effort that 
you had to put in. 
OK? 
Now, what's cool about this, you can go 
to this website, www.rot13.com, is that, 
long ago, before Facebook and before 
Twitter and before all these things we 
had these things called newsgroups. 
And they were kind of this weird kind of, 
collective email list that we had. 
And this was, like, in the 80's. 
And it was even used in storing forward 
networks. 
Where, it was kind of like Facebook in 
storing forward networks. 
Meaning that it might take four hours for 
you to see the status update. 
But we kind of of would subscribe to 
these collective things. 
And there was one that was basically the 
dirty jokes. 
And the thing about dirty jokes was, part 
of the, part of what we were trying to do 
in this thing, was, you weren't supposed 
to swear. 
And there was software that would filter 
out swear words. 
from, if, if you put a swear word in to a 
dirty joke it would not forward the 
message. 
And so we had to have a way to encrypt 
messages that included swear words. 
So that we could tell dirty jokes to each 
other, for those who wanted to subscribe 
to the dirty joke list. 
And so they came up with this rote 13. 
So we came up with a simple Caesar 
cipher, with a shift of 13. 
13 beautifully, of course, is 26 divided 
by 2. 
So it's a symmetric shift. 
The shifting in by 13, is the same as 
shifting out. 
So, of all the Caesar's ciphers, a shift 
of 13, the encryption and the decryption 
are exactly the same calculation. 
And so we would we would type our dirty 
joke into, rot13, and convert it to 
rot13, and we would send it in rot13. 
And then we would, if we wanted to 
decrypt it. 
But what became funny after a while, was, 
we were so used to reading rot13, that it 
almost became like a second language. 
Right? 
We could, we could read second, we could 
start reading the dirty jokes in rot13, 
and we would laugh before we translated 
them up. 
So rot13, has an interesting sort of 
historical thing. 
And you can go to rot13 and sort of like 
encrypt, whatever you want to say. 
And I'll probably have some questions to 
ask you, where you will have to do some 
rot13 encryption. 
And so that's the end of this lecture, 
where we talk about Caesar ciphers and 
the various techniques, and how Caesar 
ciphers work. 
And, so we'll be back and talk about 
cryptographic hashes.