Simplified DES

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Simplified DES

• CS-480b
• Network Security
• Dick Steflik

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What is Simplified DES

• Developed 1996 as a teaching tool
• Santa Clara University\
• Prof. Edward Schaefer
• Takes an 8-bit block plaintext, a 10 bit key and
  produces an 8-bit block of ciphertext
• Decryption takes the 8-bit block of ciphertext,
  the same 10-bit key and produces the original
  8-bit block of plaintext

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S-DES Scheme
Encryption
Decryption
P10
8-bit plaintext
8-bit plaintext
SHIFT
IP
IP - 1
P8
K1
K1
fk
fk
SHIFT
SW
SW
P8
K2
K2
fk
fk
IP - 1
IP
8-bit ciphertext
8-bit ciphertext
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Five Functions to Encrypt

• IP an initial permutation
• fk - a complex, 2-input function
• SW a simple permutation that swaps the two
  nybles
• fk - a complex, 2-input function again
• IP inverse permutation of the initial
  permutation

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Encryption Detail
I P
4
E/P
8
K 1
4
4
4
S1
S0
2
2
P4
4
SW
E/P
K 2
S0
S1
P4
I P -1
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Initial Permutation (IP)?
Move the bits of the original character around a
little
k1 k2 k3 k4 k5 k6 k7 k8
k2 k6 k3 k1 k4 k8 k5 k7
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Expansion/Permutation (E/P)?
Expand 4 bits into 8 and permutate them
k1 k2 k3 k4
k4 k1 k2 k3 k2 k3 k4 k1
8
Key Generation
10
P10
5
5
LS-1
LS-1
5
5
P8
8
K1
LS-2
LS-2
5
5
P8
8
K2
9
P10 Permutation
k1 k2 k3 k4 k5 k6 k7 k8 k9 k10
k3 k5 k2 k7 k4 k10 k1 k9 k8 k6
10
P8 Permutation
Permutate 10 into 8
k1 k2 k3 k4 k5 k6 k7 k8 k9 k10
k6 k3 k7 k4 k8 k5 k10 k9
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LS-1
Left circular shift 1 each 5 bit group
k3 k 5 k 2 k 7 k 4
k10 k1 k9 k8 k6
k5 k 2 k 7 k 4 k 3
k1 k9 k8 k6 k10
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LS-2
Left circular shift 2 each 5 bit group
k3 k 5 k 2 k 7 k 4
k10 k1 k9 k8 k6
k2 k7 k4 k3 k5
k9 k8 k6 k10 k1
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Substitution Boxes
S1
S0
0 1 2 3 2 0 1 3 3 0 1 0 2 1
0 3
1 0 3 2 3 2 1 0 0 2 1 3 3 1
3 2