Simplified DES

CS-480b

Network Security

Dick Steflik

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

S-DES Scheme

IP

fk

SW

fk

IP - 1

P10

P8

IP

fk

SW

fk

IP - 1

SHIFT

SHIFT

P8

EncryptionDecryption

8-bit plaintext 8-bit plaintext

8-bit ciphertext 8-bit ciphertext

K1 K1

K2 K2

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

Encryption DetailI P

E/P

S0 S1

P4

E/P

S0 S1

P4

SW

I P -1

K 1

K 2

4

4

8

4 4

2 2

4

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

Expansion/Permutation (E/P)

k4 k1 k2 k3 k2 k3 k4 k1

k1 k2 k3 k4

Expand 4 bits into 8 and permutate them…

Key Generation

P10

P8

P8

LS-1 LS-1

LS-2 LS-2

8

8

5 5

5 5

5 5

K1

K2

10

P10 Permutation

k1 k2 k3 k4 k5 k6 k7 k8 k9 k10

k3 k5 k2 k7 k4 k10 k1 k9 k8 k6

P8 Permutation

k1 k2 k3 k4 k5 k6 k7 k8 k9 k10

k6 k3 k7 k4 k8 k5 k10 k9

Permutate 10 into 8

LS-1

k3 k 5 k 2 k 7 k 4

k5 k 2 k 7 k 4 k 3

k10 k1 k9 k8 k6

k1 k9 k8 k6 k10

Left circular shift 1 each 5 bit group

LS-2

k3 k 5 k 2 k 7 k 4

k2 k7 k4 k3 k5

k10 k1 k9 k8 k6

k9 k8 k6 k10 k1

Left circular shift 2 each 5 bit group

Substitution Boxes

1 0 3 2

3 2 1 0

0 2 1 3

3 1 3 2

0 1 2 3

2 0 1 3

3 0 1 0

2 1 0 3

S0 S1