Data Encryption Standard

Data Encryption StandardIntroductionDES developed in 1970sBased on IBM Lucifier cipherFederal Information processing standard(FIPS)DES development was controversial:Design process was not openKey length was small(56 bits)

DESDES is a feistel cipher64 bit block length56 bit key length16 rounds48 bits of key used each round(subkey)Each round is simpleSecurity depends on primarily on S-BoxesEach S-Boxes maps6 bits to 4 bitsInitial PermutationsDES has an initial permutations and a final permutation after 16 rounds.These permutations are inverse of each other and operate on 64 bitsThe designers did not disclose their purpose.Single round

DES ExpansionOutput 48 bits32 12345456789891011121312131415161716171819202120212223242524252627282928293031321Input 32 bits0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

DES S-box (Substitution Box)8 Substitution boxes or S-boxesEach S-box maps 6 bits to 4 bits

There are 8 s-boxes in DES round.Each S box maps 6 bits into 4 bits out put.This input 6 bits serve as a look up or serve as a index to look up in this table.In that 6 bits (0,1,2,3,4,5)-First and last bits represents row index.Remaining middle 4 bits represents column index.ExampleInput bits are101000First and last bits are1 0=2(row). 0100=4(column).

Output is :13- (1101)

Properties of S-boxesThere are several propertiesHere highlight some properties:The rows are permutationsThe outputs are a non linear combination of the inputs.Change one bit of the input, and half of the output bits changed, which is called Avalanche effect.Each output bit is dependent on all the input bits

DES P-box(Permutation box)Input bits 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31Out bits15 6 19 20 28 11 27 16 0 14 22 25 4 17 30 91 7 23 13 31 26 2 8 18 12 29 5 21 10 3 24DES SubkeyInput Key size: 64 bits, of which 8 are parity bits.0,1,2,3.55 -56 bit key.Left half key bits, LK49 42 35 28 21 14 70 30 43 36 29 22 158 15 14 4 37 30 23 16 9 2 52 45 38 31

Right half key bits, RK55 48 41 34 27 20 136 54 47 40 3326 1912 5 53 46 39 32 2518 11 4 24 17 10 313DES Sub keyFor rounds i=1,2,..nLet LK=(LK circular shift left by ri)Let RK=(RK circular shift left by ri)Left half of sub key Ki is of 24 bits13 16 10 23 0 4 2 27 14 5 20 9 22 18 11 3 25 7 15 6 26 19 12 1Right half of sub key Ki is of 24 bits 12 23 2 8 18 26 1 11 22 16 4 19 15 20 10 27 5 24 17 13 21 7 0 3DES SubkeyFor rounds 1,2,9 and 16 the shift ri is 1,and in all other rounds ri is 2.Bits 8,17,21,24 of LK omitted each roundBits 6,9,14,25 of RK omitted each roundCompression permutation yields 48 bit subkey ki from 56 bits of LK and RKKey schedule generates subkey.

Weak KeysA weak key is the one which after parity drop operation, consists either of all 0s, all 1s or half 0s or half 1s.Four out of the 256 keys are weak keys.Example:00000000 0000000000000000 FFFFFFFFConsequence of weak keysThe round keys are created from any of these weak keys are the same.For example, for the first weak key, all the round keys are 0.The second key leads to half 0s, and half 1s.If we encrypt a block with a weak key and subsequently encrypt the result with the same weak key, we get the original block.Semi Weak KeysA semi weak keys creates only two different round keys and each of them is repeated eight times.There are six key pairs that are called semi weak keys.The round keys created from each pair arethe same in different order.Multiple DESThe major criticism against DES is the key length.So, we may try cascading several DES applications.The total key size is 56*2=112bits.However 2DES is vulnerable to a known plaintext.Man in middle attack is possible.

Security of 2 DESThen the attacker checks for a match in the table in the value of M.Here note the key pair of (k1,k2)If there are more than one keys, takes another p,c value.The attacker continues until there is only ley left.Attack complexity is around 257 .

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