Block ciphers like DES encrypt data in fixed-size blocks and use symmetric encryption keys. DES is a 64-bit block cipher that uses a 56-bit key. It employs a Feistel network structure with 16 rounds to provide diffusion and confusion of the plaintext block. Each round uses subkey-dependent substitution boxes and permutation functions. While DES was widely adopted, cryptanalysis techniques showed it could be broken with less than 256 tries, making the key size too short by modern standards.

1. Block Cipher and the
data encryption
standard (DES)
Lecture slides
By
Rasha

2. Content
 Block Cipher Principles
 The Data Encryption Standard
 DES Details
 DES Design Issues and Attacks

3. The objectives
 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

4. Cryptography Classification
 The old encryption and decryption techniques before the
implementation of computer system are called classical techniques ,
with implementation computer are called modern techniques.
 However , cryptography system (classical or modern ) are generally
classified along three independent dimensions:
1- type of operations : used for transforming plaintext to ciphertext .
all encryption algorithm are based on general principle:
a) substitution
b) transposition
c) bit manipulation

5. Cryptography Classification
2- Number of keys used
a) symmetric: if the same key is used by both the sender and receiver
for encryption and decryption it might be called single key ,secret key,
conventional encryption.
b) asymmetric: each sender and receiver using different key.
3- The way in which plaintext processed
a) block cipher: the input massage is divided in the blocks of elements
and each block is processes at a time ,producing an output block for
input.
b)stream cipher :the input elements are processed individually ,
producing an output as one element at a time too.

6. Block Ciphers
 Encrypt data one block at a time
 „Used in broader range of applications
 „Typical block size 64 – 128 bits 128 bits
 „Most algorithms based on a structure referred to as
Feistel block cipher

7. Block vs Stream Ciphers

8. Block cipher principles
 n-bit block cipher takes n bit plaintext and produces n bit ciphertext
 2n possible different plaintext blocks
 Encryption must be reversible (decryption possible)
 Each plaintext block must produce unique ciphertext block
 Total transformations is 2n!

9. Ideal Block Cipher
key is mapping ; Key length 16 × 4 bits = 64 bits . i.e. concatenate all bits of ciphertext table

10. Encryption/decryption table

11. Ideal Block Cipher
 n-bit input maps to 2n possible input states
 Substitution used to produce 2n output states
 Output states map to n-bit output
 Ideal block cipher allows maximum number of possible encryption mappings from
plaintext block
 Problems with ideal block cipher:
– Small block size: equivalent to classical substitution cipher; cryptanalysis based
on statistical characteristics feasible
– Large block size: key must be very large; performance/implementation problems
 Key length :
– In general, key length is 2n × n
– „Actual block size is at least 64 bit ( „Key length will be 264× 64 ≈ 1021 „bits)

12. Feistel Structure for Block Ciphers
 Feistel proposed applying two or more simple ciphers in sequence so final result
cryptographically stronger than component ciphers
 n-bit block length; k-bit key length; 2k transformations (rather than 2n !)
 Feistel cipher alternates: substitutions, transpositions (permutations)
 Applies concepts of diffusion and confusion
 Applied in many ciphers today
 Approach:
– Plaintext split into halves
– Subkeys (or round keys) generated from key
– Round function, F, applied to right half
– Apply substitution on left half using XOR
– Apply permutation: interchange to halves
 implements Shannon’s S-P net concept

13. Feistel Cipher Structure

14. Confusion and Diffusion
 Diffusion
– Statistical nature of plaintext is reduced in ciphertext
– E.g. A plaintext letter affects the value of many ciphertext letters
– How: repeatedly apply permutation (transposition) to data, and
then apply function
 Confusion
– Make relationship between ciphertext and key as complex as
possible
– Even if attacker can find some statistical characteristics of
ciphertext, still hard to find key

15. Using the Feistel Structure
 Exact implementation depends on various design features
 Block size, e.g. 64, 128 bits: larger values leads to more diffusion
 Key size, e.g. 128 bits: larger values leads to more confusion, resistance
against brute force
 Number of rounds, e.g. 16 rounds
 Subkey generation algorithm: should be complex
 Round function F: should be complex
 Other factors include fast encryption in software and ease of
analysis

16. Feistel Example

17. Data Encryption Standard (DES)
 Symmetric block cipher
– 56-bit key, 64-bit input block, 64-bit output block
 One of most used encryption systems in world
– Developed in 1977 by NBS/NIST
– Designed by IBM (Lucifer) with input from NSA
– Principles used in other ciphers, e.g. 3DES, IDEA.

18. DES
Encryption
Algorithm

19. Permutation Tables for DES

20. 3: Expansion permutation (E )
4 : Permutation Function (P)
Permutation Tables for DES

21. Single Round of DES Algorithm
21

22. DES Round Structure

23. Definition of DES S-Boxes

24. Definition of DES S-Boxes

25. DES Key Schedule Calculation
25

26. Table
3.2
DES
Example
Note: DES subkeys are shown as eight 6-bit values in hex format
(Table can be found on
page 75 in textbook)

27. DES Example

28. Avalanche Effect
 Aim: small change in key (or plaintext) produces large change in ciphertext
 Avalanche effect is present in DES (good for security)
 Following examples show the number of bits that change in output when two
different inputs are used, differing by 1 bit
– Plaintext 1: 02468aceeca86420
– Plaintext 2: 12468aceeca86420
– Ciphertext difference: 32 bits
– Key 1: 0f1571c947d9e859
– Key 2: 1f1571c947d9e859
– Ciphertext difference: 30

29. Table 3.3 Avalanche Effect in DES: Change in Plaintext

30. Table 3.4 Avalanche Effect in DES: Change in Key

31. Table 3.5
Average Time Required for Exhaustive Key Search

32. Key size
 Although 64 bit initial key, only 56 bits used in
encryption (other 8 for parity check)
 256 = 7.2 x 1016
– Today: 56 bits considered too short to
withstand brute force attack
 3DES uses 128-bit keys

33. Attacks on DES
 Timing Attacks
– Information gained about key/plaintext by observing how
long implementation takes to decrypt
– No known useful attacks on DES
 Differential Cryptanalysis
– Observe how pairs of plaintext blocks evolve
– Break DES in 247 encryptions (compared to 255); but
require 247 chosen plaintexts
 Linear Cryptanalysis
– Find linear approximations of the transformations
– Break DES using 243 known plaintexts

34. DES Algorithm Design
 DES was designed in private; questions about the
motivation of the design
– S-Boxes provide non-linearity: important part
of DES, generally considered to be secure
– S-Boxes provide increased confusion
– Permutation P chosen to increase diffusion

35. Summary
 have considered:
– block vs stream ciphers
– Feistel cipher design & structure
– DES
» details
» strength