<- Virtual Exhibitions in Informatics

Data Encryption Standard (DES)

The Data Encryption Standard (DES) was published in 1977 by the National Bureau of Standards and was based on the concept later known as Feistel’s Network, developed at IBM.

NBS and IBM agreed on a cooperation involving also the NSA (National Security Agency), who insisted on several modifications of the basic algorithm among which the decrease of the key length from 128 to 56 Bits became the most crucial change.
Moreover the NSA influenced the design of the so called 'S-boxes'.




DES- Scheme(Caption in German)


The NSA's bank of supercomputers

It is suspected, that the NSA successfully tried to enforce modifications of the DES-scheme to get the possibility to break the algorithm given an unique set of special equipment and computing power at that time. It turned out later that the reduced key length should become the end of the DES, when huge computing power became available to a bigger number of institutions.

DES uses a 56 bit key to encrypt a 64 bit input block of plaintext. The fact that the key is shorter than the incoming message block leads to the introduction of 8 additional parity bits.


DES works in general like the following:


Each 64 bit input is first permuted and then taken as an input for a process that carries out 16 rounds of Feistels Scheme, each of which takes the 64 bit output of the previous round and a 48 bit per-round key. The result of each round is again a 64 bit output. The per-round keys differ for each round and have a length of 48 bit. Each per-round key is a derivate of the initial 56 bit key. After the last round, the 64 bit output needs to go through the inverse initial permutation. The decryption is basically just the inverse of this process.

Links:

For a more detailed description of DES please visit:

http://home.ecn.ab.ca/~jsavard/crypto/co040201.htm
http://www.maths.uwa.edu.au/~praeger/teaching/3CC/WWW/chapter5.html