Cryptography is obviously extremely important in what
we are doing today, with our networks and our computers.
The term cryptography comes from the Greek word
"cryptos," which means hidden or secret.
And that's exactly, obviously, what
we're doing when we're talking about cryptography.
There are number of features that cryptography
brings to the table.
One is confidentiality.
This is the one we normally think about.
If we're going to send our credit card information
across the internet we want to be sure that nobody in between
would be able to see that credit card information.
They'll have all of that information absolutely secret.
Nobody's able to see it except for me and the person
that I'm sending it to.
Everything else, completely confidential.
The authentication and access control
is also a capability that's provided
to us through cryptography.
This authentication method means that if I'm
putting in a username and then a password,
the cryptography around that password and the way that we
keep it secret validates that it is me.
I'm able to send that message in an encrypted form,
and the other side is able to validate that that is indeed
exactly the person I would be expecting there.
There's also a capability called non-repudiation,
which means if I receive a message from you
there's no way you can say that you did not send this.
I can really tell that this was you that sent it.
There's no way you can deny what you put into this message
because I have a way to determine that it really
did come from you, and that you really did write these things.
And that also brings us to integrity.
We know that when we receive this message,
and we check it with these cryptography and the methods
that we're going to talk about, that we can be assured
that nobody has tampered with anything
inside of this message.
If anybody was to change one word, or one letter,
we would be able to notice it.
And cryptography makes sure that when we receive a message
we can be sure that it was not tampered somewhere
along the way.
There are a number of common terms
you'll need to know about cryptography.
The first is plaintext.
Before you encrypt anything it is plaintext, or something
that we sometimes will call in the clear.
If we were to send this information out
over the network anyone would be able to read it.
We haven't done any type the hiding or encrypting
of this message.
Once we do encrypt the information
it becomes ciphertext.
We have taken our plaintext, we have applied a cipher to it,
and the resulting encrypted information is the ciphertext.
That cipher is what we're using to encrypt that message.
It's an algorithm.
It's a mathematical method that we
are using to take the plaintext and encrypt it or convert it
into something that people can't read.
And that is the cipher that allows us to do that.
The art of cracking this encryption
then is cryptanalysis.
Having your cryptographers in a government facility
trying to understand and make sure that the messages
that they are sending our secure,
and trying to crack the messages that
are coming from other places, is a big, big business.
And even though it's something that is relatively hush, hush
and top secret, it's something that is happening every day.
There are many different ways to scramble up a message,
and encrypt it in a way that no one would be able to read it.
One common way is something called a substitution cipher.
This is one of the oldest methods
that you'll find of encrypting data.
This is also called Caesar's cipher
because Caesar is one of the people that originated
this method of sending information back and forth,
and certainly made it very popular.
What he did was take a normal alphabet
and he changed all of the letters down a certain number.
In fact, a very common one is ROT13.
You've rotated this group 13 steps
to the right so that instead of writing a letter A,
you would write the letter N. Instead of writing the letter
B, you would write the letter O.
So your cipher is really this 13.
As long as you know the number 13 you can take a message
and encrypt it and decrypt it with that particular key.
This is obviously something that would turn this funny
looking message into, hello world.
It's a very simple one to do.
And it made it very easy to send this message.
It's encrypted, you can't read it.
You would have to know exactly what you would need to do,
how many methods, how many sections you needed
to move this down and rotate it, to be
able to read that message.
Another type of cipher is a transposition cipher,
where we keep exactly the same letters
but we just scramble them up and put them in a different format.
And then you would provide the person on the other end
with the key-- what letter should go in what position.
And they would transpose them back into the order
that it was designed so that you could make hello world out
of this scrambled bunch of letters that
has exactly the right letters in it,
they're just all in the wrong place.
We often see people trying to figure out
these particular sections of ciphertext.
And one of the methods it they'll use, especially
on these older substitution ciphers and transposition
ciphers, is to use frequency analysis.
They'll examine the entire ciphertext
and they'll try to determine how many times do I
see the letter R?
How many times do I see the letter Q?
How many times do I see the letter W?
And they'll start creating a frequency
table of how many times we're seeing different characters.
And if we think that this is the English language,
we know the letter E is certainly
one of the most popular letters.
T is the second most, A is the third most,
O is the fourth most, and so on.
And you can start substituting in some of these frequencies
that we're seeing in our ciphertext
to try to figure out exactly what the real message might
be inside of that ciphertext.
Well obviously, in modern times, a ROT13 or a transposition
cipher is not really going to keep things very safe.
So we started to see things like mechanical ciphers appear
around the World War II time frame.
This is one that Germany used.
This is an Enigma machine, which took a message
and encrypted it using a piece of machinery, which
means you would have to have this piece of machinery on one
end, and this piece of machinery on the other end,
to be able to decrypt the messages that
were being sent back and forth.
Being able to hack or crack that particular encryption code
was an extremely important part of what
happened during World War II.
A lot of interesting history there.
There are also mathematical ciphers.
And these are the ones that we will probably
see the most of, especially in modern times,
because if you can make it a very, very complex cipher then
obviously it will be very, very, very
difficult to be able to decrypt that message in some way.
And we're going to go through a number of mathematical ciphers,
not just in this video, but in many others were
we will talk about hashing.
We'll talk about doing symmetric encryption,
and asymmetric encryption.
If you're at all interested in getting into cryptography
you will need to have an extremely strong background
in mathematics.
We've talked a little bit so far about keys.
And we've talked about if you know what the key is then
when that encrypted message gets on the other side,
you can apply a key to it, with the correct cipher,
and come up with the plaintext, back to the information
we wanted to get our hands on.
These keys can be very simple, like the ROT13 key.
We needed that number 13.
That was the important part of it.
If it's something like a PGP or GPG,
and we'll talk more about those as well,
the keys are very, very complex.
This is my public key, for instance,
that I use when I start encrypting information
using some asymmetric encryption method.
So you have to make sure you have the right kind of key
for the message that you're getting
so that you can encrypt it properly.
Another cryptography technique is a one-time pad.
This is one where you would have a page of letters on one side.
You have exactly the same page of letters on the other side.
The person who is encrypting the message
will go through each letter of the message,
and they would combine the first letter of this pad
with the first letter the message to come up
with ciphertext.
Then the second letter of the pad with the second letter
of the plaintext message to come up
with the next letter of the ciphertext, and so on.
They would go all the way through your plaintext message
to create the ciphertext.
And obviously this would be very, very difficult
to be able to decrypt unless you had
this exact key on both sides.
You'd have to have that exact pad of paper, which
is where the one-time pad comes from.
You use that message one time, you get rid of it.
And you have a completely new key, a completely new
one-time pad, to be able to use.
You use it one time and you're done.
So the next message, even if the first message
was decrypted in some way, the second message someone
would have to start all the way from the beginning again
to try to decrypt that message.
It is all of these different cryptography methods
and cryptography features that really provide us
with a very, very powerful way to keep all of our information
private on our computers and across our network.