lecture 6 DES part1.pdf

Computer Security
Prepared by: Assistant Prof. Eman Sanad
Faculty of computers and Artificial Intelligence
Cairo University 1

Lecture 6
• Feistel cipher
• DES
• DES encryption overview
• DES Round structure
Dr. Eman Sanad, faculty of computers and
Artificial Intelligence , Cairo University
2

Feistel cipher cont…
Example:
The fifteenth round of encryption, corresponding to the second round of
decryption. “prove that”
• Suppose that the blocks at each stage are 32 bits (two 16-bit halves)
• the key size is 24 bits.
• at the end of encryption round fourteen, the value of the intermediate
block (in hexadecimal) is DE7F03A6.
LE 14 = DE7F and RE 14 = 03A6.
• Also assume that the value of K 15 is 12DE52.
• After round 15, we have LE 15 = 03A6
RE 15 = F(03A6, 12DE52) ⊕ DE7F.
3

Feisel cipher cont…
4

• assume that :
LD 1 = RE 15 , RD 1 = LE 15
• prove that:
LD 2 = RE 14 , RD 2 = LE 14
• start with:
LD 1 = F(03A6, 12DE52) ⊕ DE7F
RD 1 = 03A6
LD 2 = 03A6 = RE 14 ,
RD 2 = F(03A6, 12DE52) ⊕ [F(03A6, 12DE52) ⊕ DE7F]
= DE7F = LE14
5

6

Data Encryption Standard (DES)
• Most widely used block cipher in world
• Adopted in 1977 by NBS (now NIST)
• Encrypts 64-bit data using 56-bit key
• Has widespread use
• has been considerable controversy over
its security
7

DES Encryption Overview
8

Initial Permutation IP
• first step of the data computation
• IP reorders the input data bits
• quite regular in structure (easy in h/w)
Example:
IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb)
9

10

Initial Permutation (IP)
11

DES Round Structure
• uses two 32-bit L & R halves
• As for any Feistel cipher can describe as:
L i = R i–1
R i = L i–1 ⊕ F(R i–1 , K i )
• 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
12

13

Single round of DES algorithm
14

8 S-Boxes
15

Substitution Boxes S
• have eight S-boxes which map 6 to 4 bits
o each S-box is actually 4 little 4 bit boxes
o outer bits 1 & 6 (row bits) select one row of 4
o inner bits 2-5 (col bits) are substituted
o 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
16

Example:
17

• The S-boxes are the core of DES in
terms of cryptographic strength. They
are the only nonlinear element in the
algorithm and provide confusion.
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Thank you

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