>> Hello. Let's talk a little bit more in-depth on
how symmetric algorithms work and talk about
the two types, stream and block.
Now, stream ciphers encrypt data one bit at a time.
Sometimes it's one byte
but just think of it as being bit by bit,
and it's very fast.
As a matter of fact, a lot of times you'll
use a hardware encrypter for stream ciphers.
Just because you've got to have
something that you can keep up with
the process, it's very fast.
The way stream algorithms work is they
either use a process called transposition,
which is basically just shuffling
>> the characters around,
>> or substitution,
which is replacing one character for another,
or they use a process called XOR,
which we'll talk about in a minute.
These are some very fast algorithms,
but they're not as secure.
Like we said, sometimes we
trade security for performance.
The only algorithm that I want you to know for the test
that is a stream cipher is RC-4.
If I ask if AES is a stream or block, you say block.
Why? Because it's not RC-4.
What about RC-5?
It's a block because it's not RC-4.
That is the only stream cipher they
will ask you about is RC-4.
Now, remember that we already talked about
some of the things that made WEP work.
One of those reasons was that it actually use RC-4,
is not as secure as when they needed it to be.
RC-4 uses either a 64-bit or a 128-bit key.
But that's not very secure bytes, today's standards.
RC-4 was one of the problems with WEP.
Even when we improved it with WPA,
we were still using IC-4,
so it could be backward compatible.
But with WPA2,
we got away from RC-4 and use the block cipher.
Block ciphers chunked data into blocks.
Each block goes through a series of
math functions called S-box.
Now, going through all these functions takes more time,
but it is more secure.
The block algorithm that came with WPA2 is called AES,
and that stands for Advanced Encryption Standard.
AES is an algorithm that can
provide variable length encryption.
You can use AES in a 128-bit mode,
192 or 256, with 256 giving you the best performance.
I'll also mention that sometimes we hear about
these algorithms like the Advanced Encryption Standard,
but that's actually not an algorithm, it's a standard.
The government will release
these standards and then vendors
will produce algorithms that satisfy those standards.
Currently, the algorithm that satisfies
the AES is called Region del.
I'm not sure if they will ask
>> you about that on the test,
>> but it's something to be aware of.
This is an illustration of block ciphers.
You can see the S-blocks.
Within each conceptual S-block,
a math function is performed.
The key is going to dictate
what order those math functions are performed in.
Those algorithms and keys work together.
Now, I mentioned earlier that
stream ciphers either do transposition,
substitution, or a process called XOR,
and that stands for Exclusive OR.
For XOR, remember we are
encrypting one bit of data at a time.
If you look at the bottom of this slide,
we have plain text on the top line and the key is next,
and what we produce is ciphertext.
Let's look at the first bit of plain text,
which is one, and the first bit of the key is a zero.
If these two values are alike,
the ciphertext becomes zero.
If the values are different, it becomes one.
Since the first bit for each
>> one and zero are different,
>> the ciphertexts bit becomes one,
and that pattern follows through for each bit.
Only where there are both the same
does the ciphertexts bit come through as a zero.
That's how the XOR process works.
It's very quick and very easy to reverse,
particularly when you have a key.
We have to keep that in mind.
Anything quick to encrypt is
likely going to be quick to decrypt.
These are just some common symmetric algorithms.
Now, I already mentioned RC-4 and AES.
I think I already mentioned DES,
Data Encryption Standard.
That happens a lot where there is
a standard and the algorithm gets created for it.
When you see that S on the end,
it likely means it's a standard and
an algorithm gets created to satisfy it.
An AES is by far the most commonly used one,
it's the de-facto standard.
The government set apart this standard to encrypt
sensitive but unclassified information.
We know we're going to see AES pop up on the exam.
IDEA is a proprietary algorithm that is used for PGP,
which is pretty good privacy.
That's an email encryption application
created by Phil Zimmerman.
He noted that for national security reasons,
the government would like to decrypt
anything that we encrypt.
Zimmerman wondered why are
we using algorithms that the government
standardized if their goal is
to decrypt anything that's been encrypted.
Zimmerman came up with an algorithm called
IDEA, which was proprietary.
That's what he used to secure
email and his application, PGP.
That one will most likely come up on the test.
PGP is an alternative that's
proprietary to standards-based email.
Now, I do think that on the test,
you'll need to look at a list and pick out
which algorithms are symmetric versus asymmetric.
I'll show you how to do that later on.