Modern block Cipher : Key Distribution

Data Encryption Standard (DES)

• most widely used block cipher in world

• adopted in 1977 by NBS (now NIST)

– as FIPS PUB 46

• encrypts 64?bit data using 56?bit key

has widespread use

• has been considerable controversy over its

Security

DES History

• IBM developed Lucifer cipher

– by team led by Feistel in late 60’s

– used 64?bit data blocks with 128?bit key

• then redeveloped as a commercial cipher with

input from NSA and others

• in 1973 NBS issued request for proposals for a

national cipher standard

• IBM submitted their revised Lucifer which was

eventually accepted as the DES

DES Design Controversy

• although DES standard is public

• was considerable controversy over design

– in choice of 56?bit key (vs Lucifer 128?bit)

– and because design criteria were classified

• subsequent events and public analysis show in

fact design was appropriate

• use of DES has flourished

– especially in financial applications

– still standardised for legacy application use

DES Encryption Overview

DES Encryption Overview

Initial Permutation IP

?first step of the data computation

?IP reorders the input data bits

?even bits to LH half, odd bits to RH half

?quite regular in structure (easy in h/w)

?example:

IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb)

DES Round Structure

• uses two 32?bit L ; R halves

• as for any Feistel cipher can describe as:

Li = Ri–1

Ri = Li 1 Li–? F(Ri 1 Ki)

Ri–1, • F takes 32?bit R half and 48?bit subkey:

– expands R to 48?bits using perm E

– adds to subkey using XOR

– passes through 8 S?boxes to get 32?bit result

– finally permutes using 32?bit perm P

DES Key Schedule

?forms subkeys used in each round

?initial permutation of the key (PC1) which selects

56?bits in two 28?bit halves

?16 stages consisting of:

• rotating each half separately either 1 or 2 places

depending on the key rotation schedule K

• selecting 24?bits from each half ; permuting them by

PC2 for use in round function F

?note practical use issues in h/w vs s/w

DES Decryption

• decrypt must unwind steps of data computation

• with Feistel design, do encryption steps again using

subkeys in reverse order (SK16 … SK1)

– IP undoes final FP step of encryption

– 1st round with SK16 undoes 16th encrypt round

– ….

– 16th round with SK1 undoes 1st encrypt round

– then final FP undoes initial encryption IP

– thus recovering original data value

Avalanche Effect

• key desirable property of encryption alg

• where a change of one input or key bit results

in changing approx half output bits

• making attempts to “home?in” by guessing

keys impossible

• DES exhibits strong avalanche

Strength of DES – Key Size

• 56?bit keys have 256 = 7.2 x 1016 values

• brute force search looks hard

• recent advances have shown is possible

– in 1997 on Internet in a few months

– in 1998 on dedicated h/w (EFF) in a few days

– in 1999 above combined in 22hrs!

• still must be able to recognize plaintext

• must now consider alternatives to DES

Strength of DES – Analytic Attacks

? now have several analytic attacks on DES

? these utilise some deep structure of the cipher

? by gathering information about encryptions

? can eventually recover some/all of the sub?key bits

? if necessary then exhaustively search for the rest

? generally these are statistical attacks

? differential cryptanalysis

? linear cryptanalysis

? related key attacks

Strength of DES – Timing Attacks

?attacks actual implementation of cipher

?use knowledge of consequences of

implementation to derive information about

some/all subkey bits

?specifically use fact that calculations can take

varying times depending on the value of the

inputs to it

?particularly problematic on smartcards