Module 1-Block Ciphers and the Data Encryption Standard.pptx

Block Ciphers and the
Data Encryption
Standard

Modern Block Ciphers
 now look at modern block ciphers
 one of the most widely used types of
cryptographic algorithms
 provide secrecy /authentication services
 focus on DES (Data Encryption Standard)
 to illustrate block cipher design principles

Block vs Stream Ciphers
 block ciphers process messages in blocks,
each of which is then en/decrypted
 like a substitution on very big characters
 64-bits or more
 stream ciphers process messages a bit or
byte at a time when en/decrypting
 many current ciphers are block ciphers
 better analysed
 broader range of applications

Block Cipher Principles
 most symmetric block ciphers are based on a
Feistel Cipher Structure
 needed since must be able to decrypt ciphertext
to recover messages efficiently
 block ciphers look like an extremely large
substitution
 would need table of 264 entries for a 64-bit block
 instead create from smaller building blocks
 using idea of a product cipher

Claude Shannon and Substitution-
Permutation Ciphers
 Claude Shannon introduced idea of substitution-
permutation (S-P) networks in 1949 paper
 form basis of modern block ciphers
 S-P nets are based on the two primitive
cryptographic operations seen before:
 substitution (S-box)
 permutation (P-box)
 provide confusion & diffusion of message & key

Confusion and Diffusion
 cipher needs to completely obscure
statistical properties of original message
 a one-time pad does this
 more practically Shannon suggested
combining S & P elements to obtain:
 diffusion – dissipates statistical structure
of plaintext over bulk of ciphertext
 confusion – makes relationship between
ciphertext and key as complex as possible

Feistel Cipher Structure
 Horst Feistel devised the feistel cipher
 based on concept of invertible product cipher
 partitions input block into two halves
 process through multiple rounds which
 perform a substitution on left data half
 based on round function of right half & subkey
 then have permutation swapping halves
 implements Shannon’s S-P net concept

Feistel Cipher Design Elements
 block size
 key size
 number of rounds
 subkey generation algorithm
 round function
 fast software en/decryption
 ease of analysis

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

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  F(Ri–1, Ki)
 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

Substitution Boxes S
 have eight S-boxes which map 6 to 4 bits
 each S-box is actually 4 little 4 bit boxes
 outer bits 1 & 6 (row bits) select one row of 4
 inner bits 2-5 (col bits) are substituted
 result is 8 lots of 4 bits, or 32 bits
 row selection depends on both data & key
 feature known as autoclaving (autokeying)
 example:
 S(18 09 12 3d 11 17 38 39) = 5fd25e03

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

Differential Cryptanalysis
 one of the most significant recent (public)
advances in cryptanalysis
 known by NSA in 70's cf DES design
 Murphy, Biham & Shamir published in 90’s
 powerful method to analyse block ciphers
 used to analyse most current block ciphers
with varying degrees of success
 DES reasonably resistant to it, cf Lucifer

 a statistical attack against Feistel ciphers
 uses cipher structure not previously used
 design of S-P networks has output of
function f influenced by both input & key
 hence cannot trace values back through
cipher without knowing value of the key
 differential cryptanalysis compares two
related pairs of encryptions

Compares Pairs of Encryptions
 with a known difference in the input
 searching for a known difference in output
 when same subkeys are used

 have some input difference giving some
output difference with probability p
 if find instances of some higher probability
input / output difference pairs occurring
 can infer subkey that was used in round
 then must iterate process over many
rounds (with decreasing probabilities)

 perform attack by repeatedly encrypting plaintext pairs
with known input XOR until obtain desired output XOR
 when found
 if intermediate rounds match required XOR have a right pair
 if not then have a wrong pair, relative ratio is S/N for attack
 can then deduce keys values for the rounds
 right pairs suggest same key bits
 wrong pairs give random values
 for large numbers of rounds, probability is so low that
more pairs are required than exist with 64-bit inputs
 Biham and Shamir have shown how a 13-round iterated
characteristic can break the full 16-round DES

Linear Cryptanalysis
 another recent development
 also a statistical method
 must be iterated over rounds, with
decreasing probabilities
 developed by Matsui et al in early 90's
 based on finding linear approximations
 can attack DES with 243 known plaintexts,
easier but still in practise infeasible

Linear Cryptanalysis
 find linear approximations with prob p != ½
P[i1,i2,...,ia]  C[j1,j2,...,jb] =
K[k1,k2,...,kc]
where ia,jb,kc are bit locations in P,C,K
 gives linear equation for key bits
 get one key bit using max likelihood alg
 using a large number of trial encryptions
 effectiveness given by: |p–1/2|

DES Design Criteria
 as reported by Coppersmith in [COPP94]
 7 criteria for S-boxes provide for
 non-linearity
 resistance to differential cryptanalysis
 good confusion
 3 criteria for permutation P provide for
 increased diffusion

Block Cipher Design
 basic principles still like Feistel’s in 1970’s
 number of rounds
 more is better, exhaustive search best attack
 function f:
 provides “confusion”, is nonlinear, avalanche
 have issues of how S-boxes are selected
 key schedule
 complex subkey creation, key avalanche

Summary
 have considered:
 block vs stream ciphers
 Feistel cipher design & structure
 DES
• details
• strength
 Differential & Linear Cryptanalysis
 block cipher design principles

Module 1-Block Ciphers and the Data Encryption Standard.pptx

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Module 1-Block Ciphers and the Data Encryption Standard.pptx