Let's start first with a brief history lesson with a data encryption standard D.S. is a symmetric block
cipher developed by IBM in the 70s.
At that time IBM was an an champion worldwide leader IBM Research would set the trends for all other
competitors in the field.
Did encryption standard encrypt 64 bit blocks of data with eight letters in the ASCII code and a parity
bit using a 56 bit key.
How is this possible after all you probably remember that it was claimed that D.S. used 64 bits.
That's actually true but the remaining 16 bits is an earlier checksum.
The true entropy is 56 bits in this case.
If the key is truly random it has at best fifty six bits of randomness.
56 bits doesn't have quite the same ring to it.
It's always better to claim that you use a longer key.
This is the source of the misconception that it's still popular today.
The data encryption standard never used 64 bit keys.
The encryption process is made of 26 Feistel rounds.
The rounds we mentioned before
and two permutations called the initial and final permutations
the initial and final permutations don't contribute in any way to overall ciphertext security or their
contribution is small and insignificant.
It's unclear why they were brought in in the first place.
It's usually assumed that they were introduced to make the encryption and decryption hardware implemented
the two permutations are cost intensive to an extent hardware is able to go through them at a much faster
rate than software their implementation can be explained simply as a consequence of hardware manufacturers
lobbying.
The permutations don't add to the security of a ciphertext in any way.
In each round a half of the round key 32 bits is expanded to 48 bits.
The result is two pieces 48 bits each one piece is encrypted using the function f and the result of
this operation is combined using the exclusive disjunction DXO our operation with the other half the
function f is above all a substitution permutation box.
This is the S-box you saw earlier one of the eight X-boxes.
Each time a different one is selected to use in a permutation Why are the S-box is structured in this
exact way.
What is the basis and source for the values located in the intersections of individual columns.
Well this is also one of the greatest enigmas of the 20th century cryptography the data encryption standard
was developed in the US where IBM has established some people believe that the specific structure of
the X-boxes has made it possible from the onset for U.S. government agencies to reverse the whole process
at a small cost.
This has been either proved or disproved and still no one knows on what principle the S-box were selected.
Our S-box is a 4 by 16 table the values it contains range from zero to 15 to be able to transpose some
data.
You only need to locate appropriate table cells.
The first and last bits of the encrypted parts of the data are used as row numbers.
The remaining bits are used as column numbers find the coordinates and look up the content of the S-box
and completing all Feistel rounds you have to perform a final permutation just like in the initial permutation.
This permutation does not enhance the security of the ciphertext.
In retrospect it seems that the block used by the D.S. algorithm is too short.
While block length does not always correspond to the ciphertext security blocks that are too short can
prove problematic.
64 bits are not really sufficient.
The key meanwhile is definitely too short we'll discuss later.
The reason why the practical security of a system amounts in fact to have the keys size D.S. was correct.
Using the brute force attack in 1999.
In less than 24 hours this attack was proved and deployed by Electronic Frontier Foundation to build
a machine capable of mounting a successful attack.
The organization made use of distributed member computers
a large Geo cluster was built each note was provided with a share of keyspace is to test together the
cluster was able to crack the D.S. cipher in 22 hours.
D.S. is vulnerable for purely cryptographic reasons.
The first point is that a significant dependence can be observed between round keys and cipher keys.
This relationship is evident especially if zero is the key round keys in that case are also zero.
Another problem the standard exhibit's is the complement to nation property which refers to some relationships
existing between the key the plaintext and the ciphertext itself.
If you encrypt all big complements using key complements the output is the ciphertext complement
today.
The DNS server text can be cracked in a matter of minutes.
In 2004 the nest Institute withdrew the recommendation for the algorithm the image depicts Copacabana.
One of the many internet available devices for cracking D.S. in real time.