Ch 3: Block Ciphers and Data Encryption Standard DES

1
Ch 3 Block Ciphers and Data Encryption Standard
(DES)

• Fourth Edition
• by William Stallings
• Lecture slides by Lawrie Brown
• (modified by Prof. M. Singhal, U of Kentucky)

2
Modern Block Ciphers

• 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

3
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
• broader range of applications

4
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

5
Ideal Block Cipher
6
Claude Shannon and Substitution-Permutation
Ciphers

• Claude Shannon introduced idea of
  substitution-permutation (S-P) networks in 1949
• form basis of modern block ciphers
• S-P nets are based on the two primitive
  cryptographic operations
• substitution (S-box)
• permutation (P-box)
• provide confusion diffusion of message key

7
Diffusion and Confusion

• Diffusion
• The statistical structure of the plaintext is
  spread (dissipated) into long-range statistics of
  the ciphertext.
• Achieved by having each plaintext digit affect
  the value of many ciphertext digits.
• Objective is to globalize the local affects.

8
Diffusion and Confusion

• Confusion
• Attempts to make the relationship between the
  ciphertext and the encryption key as complex as
  possible.
• Achieved by using a complex substitution
  algorithm.
• Even if an attacker can some handle on the
  statistics of the ciphertext, it is very
  difficult to deduce the key.

9
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 Shannons S-P net concept

10
Feistel Cipher Structure
11
Feistel Cipher Design Elements

• block size
• key size
• number of rounds
• subkey generation algorithm
• round function
• fast software en/decryption
• ease of analysis

12
Feistel Cipher Decryption
13
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

14
DES History

• IBM developed Lucifer cipher
• by team led by Feistel in late 60s
• 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

15
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

16
(No Transcript)
17
DES

• Initial Permutation (IP)
• The plaintext block undergoes an intial
  permutation.
• gt 64 bits of the block are permuted.
• A Complex Transformation
• 64 bit permuted block undergoes 16 rounds of
  complex transformation. (Using subkeys)

18
DES

• 32-bit swap
• 32 bit left and right halves of the output of
  the 16th round are swapped.
• Inverse Initial Permutation (IP-1)
• The 64 bit output undergoes a permutation that
  is inverse of the intial permutation.
• gtThe 64 bit output is the ciphertext.

19
(No Transcript)
20
DES

• The complex processing at each iteration/round
• Li Ri-1
• Ri Li-1 F(Ri-1, Ki)
• Details of function F
• It takes 32 bits input and produces a 32 bit
  output.

21
DES

• Details of function F
• gt32 bit input is expanded into 48 bits.
• -This is done by permuting and
• duplicating some bits of 32 bits.
• gtExclusive OR operation is performed between
  these 48 bits and 48 bit subkey.

22
DES

• Details of function F...
• gt 48 bit output of the Exclusive OR operation
  is grouped into 8 groups of 6 bits each.
• gt Each 6 bit group is fed into a 6-to-4
  substitution box that transforms 6 bits to 4 bits.

23
DES

• Details of function F...
• gt 32 bit output of 8 substitution boxes is fed
  into a permutation box.
• gt The 32 bit output of the permutation box is
  F(Ri-1, Ki).

24
DES

• Concerns about
• The key length (56-bits)
• gt 56 bit key was adequate in 70s.
• gt With faster processors, this encryption method
  is no longer safe.

25
Time to break a code (106 decryptions/µs)
26
Triple DEA

• Use three keys and three executions of the DES
  algorithm (encrypt-decrypt-encrypt)
• C ciphertext
• P Plaintext
• EKX encryption of X using key K
• DKY decryption of Y using key K
• Effective key length of 168 bits

C EK3DK2EK1P
27
Triple DEA
28
Cipher Block Modes of Operation

• Cipher Block Chaining Mode (CBC)
• - A method to increase the security of DES
  or
• any block cipher.
• The input to the encryption algorithm is the XOR
  of the current plaintext block and the preceding
  ciphertext block.
• - Processing of a sequence of plaintext
• blocks is chained together.

29
(No Transcript)
30
Basis of Cipher Block Chaining