L 13 Data Encryption Standard -part1

Chapter 4
Data Encryption Standard (DES)
General Notes:
 DES is by far the most popular private-key algorithm.
 It was published in 1975 and standardized in 1977.
 Expired in 1998.
4.1 Encryption
System Parameters:
! block cipher.
! 64 input/output bits.
! 56 bits of key.
Principle: 16 rounds of encryption.
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Initial
Permutation
Final
Permutation
Encryption
16
Encryption
1
K
1
K
16
K
X Y
Figure 4.1: General Model of DES
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4.1.1 Overview
f
32
32
32
L 0 R 0
Initial Permutation
IP(X)
Message X
64
64
f
32
32
32
L 1 R 1
L 15 R 15
K16
K1
Transform 1
Final Permutation
Key K
56
32
32
32
32
56
Cipher Y = DES (X) K
IP (R , L ) -1
16 16
L 16 R 16
48
48
Transform 16
round 1
round 16
Figure 4.2: The Feistel Network
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4.1.2 Permutations
a) Initial Permutation IP.
IP
58 50 42 34 26 18 10 2
60 52 44 36 28 20 12 4
62 54 46 38 30 22 14 6
64 56 48 40 32 24 16 8
57 49 41 33 25 17 9 1
59 51 43 35 27 19 11 3
61 53 45 37 29 21 13 5
63 55 47 39 31 23 15 7
X
1 50 58 64
1 2 40
IP(X)
Figure 4.3: Initial permutation
b) Inverse Initial Permutation IP??1 (nal permutation).
Note:
IP??1(IP(X)) = X.
4.1.3 Core Iteration / f-Function
General Description:
Li = Ri??1.