Simplified DES What is Simplified DES Developed 1996

Simplified DES

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 Encryption Decryption P 10 8 -bit plaintext SHIFT IP fk IP - 1 K 1 P 8 K 1 SHIFT SW K 2 P 8 fk SW K 2 fk fk IP - 1 IP 8 -bit ciphertext

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

IP 4 E/P 8 4 4 K 1 4 S 1 S 0 2 Encryption Detail P 4 2 4 SW E/P K 2 S 0 S 1 P 4 I P -1

Initial Permutation (IP) Move the bits of the original character around a little… k 1 k 2 k 6 k 3 k 4 k 5 k 6 k 7 k 8 k 3 k 1 k 4 k 8 k 5 k 7

Expansion/Permutation (E/P) Expand 4 bits into 8 and permutate them… k 1 k 2 k 4 k 1 k 3 k 4 k 2 k 3 k 4 k 1

Key Generation 10 P 10 5 5 LS-1 5 5 P 8 8 K 1 LS-2 5 5 P 8 K 2 8

P 10 Permutation k 1 k 2 k 3 k 4 k 5 k 6 k 7 k 8 k 9 k 10 k 3 k 5 k 2 k 7 k 4 k 10 k 1 k 9 k 8 k 6

P 8 Permutation Permutate 10 into 8 k 1 k 2 k 3 k 4 k 5 k 6 k 7 k 8 k 9 k 10 k 6 k 3 k 7 k 4 k 8 k 5 k 10 k 9

LS-1 Left circular shift 1 each 5 bit group k 3 k 5 k 2 k 7 k 4 k 10 k 1 k 9 k 8 k 6 k 5 k 2 k 7 k 4 k 3 k 1 k 9 k 8 k 6 k 10

LS-2 Left circular shift 2 each 5 bit group k 3 k 5 k 2 k 7 k 4 k 10 k 1 k 9 k 8 k 6 k 2 k 7 k 4 k 3 k 5 k 9 k 8 k 6 k 10 k 1