- [Instructor] Modern symmetric algorithms
are much more complex than the simple substitution
and transposition ciphers of years past.
One of the most well-known symmetric algorithms
is the Data Encryption Standard, or DES.
DES is an historic encryption algorithm
designed and implemented by IBM in the 1970s
with the purpose of serving
as a standard encryption algorithm
for unclassified communication
throughout the federal government.
Up until that point,
different agencies used different encryption algorithms.
This caused issues with both security
because all of those algorithms weren't thoroughly tested
and interoperability because different agencies couldn't
easily communicate with each other in a secure manner.
Let's dig into how DES works
because it serves as a great example
of the complexity of modern cryptographic algorithms.
This picture shows the basic functioning of DES.
It takes 64 bits of plain text as input in the top
and then runs it through an encryption operation
known as the Feistel function.
Those are those yellow boxes with the Fs inside of them.
It uses the Feistel function 16 different times
in order to produce the cipher text.
And here's a view inside one of those F boxes.
Each of these F boxes that implements the Feistel function
takes half a block of input, or 32 bits,
and combines it with a piece of the 56-bit encryption key.
That's happening at the red icon
in the middle of this image.
Then the output of that function
is broken up into eight segments
and fed into eight different functions called S boxes,
these yellow boxes labeled S1 through S8
that just appeared on the screen.
S stands for substitution,
and each one of these boxes contains
a different substitution cipher.
The results of all of those substitutions
are then combined back together again
and fed into a P box,
the green box that just appeared.
P stands for permutation,
which is just another term for transposition.
So the output of all of those S boxes
is scrambled up to produce the cipher text.
That's a pretty complex encryption operation.
And don't forget, that complexity inside the F box
is repeated 16 times on any block of input.
DES was used widely throughout the government
and the private sector for decades,
but today it is no longer considered secure.
Mathematicians and cryptologists have published papers
containing several effective attacks
against the DES algorithm,
and it is no longer recommended for use.
You'll need to learn about many different
encryption algorithms for the exam,
so let me sum up here with some key facts
that you should know about DES.
DES is a symmetric encryption algorithm.
It is a block cipher that works on 64-bit blocks
using a 56-bit key.
And DES is now considered insecure.