Data encryption standard DES (DES) in cryptography

DES encryption scheme

•         The plain-text (64 bits) passes through an initial permutation IP(on 64 bits)

•         Then follow 16 identical rounds –in each round a different sub key is used; each sub key is generated from the key

•        After round 16, swap the left half with the right half

•             Apply the inverse of the initial permutation IP-1(on 64 bits)

Sub key generation

Before round 1 of DES, they key is permuted according to a table labeled Permuted Choice One –the resulting 56-bit key is split into its two 28-bit halves labeled C0and D0􀂄

•         In each round, Ci-1 and Di-1 are separately subjected to a circular left shift of one or two bits according to the table on the next slide –the shifted values will be input to next round􀂄

•         The shifted values serve as input to Permuted Choice Two which produces a 48-bit output: the sub key of the current round.

      Strength of DES

Two main concerns with DES: the length of the key and the nature of the algorithm

•         The key is rather short: 56 bits –

–        In average, only half of the keys have to be tried to break the system

–        In principle it should take long time to break the system

–       Things are quicker with dedicated hardware: 1998 –a special machine was built for less than 250 000 $ breaking DES in less than 3 days, 2006 –estimates are that a hardware costing around 20.000$ may break DES within a day.

•         Nature of the algorithm

•         There has always been a concern about the design of DES, especially about the design of S-boxes –perhaps they have been designed in such a way as to ensure a trapdoor to the algorithm –break it without having to search for the key

•         The design criteria for the S-boxes (and for the rest of the algorithm) have been classified information and NSA was involved in the design

•         Many regularities and unexpected behavior of the S-boxes have been reported

•         On the other hand, changing the S-boxes slightly seems to weaken the algorithm

•         No fatal weaknesses in the S-boxes have been (publicly) reported so far.

DES Algorithm examples | How DES Algorithm works with examples