CE-408: Cryptography & Network Security SSUET/QR/114
LAB#04
Modern Encryption Techniques
Simplified DES (S-DES)
OBJECTIVE
The purpose of this lab is to implement SDES encryption and decryption in MATLAB.
THEORY
Encryption: It takes an 8-bit block of plain text and a 10-bit key as input and produces an
8-bit block of cipher text as output.
Decryption: It takes an 8-bit block of cipher text and the same 10-bit key used to produce
that ciphertext as input and produces the original 8-bit block of plaintext.
Algorithm involves 5 functions:
1. An initial permutation (IP).
2. A complex function, fK , that involves both permutation and substitution operations and
depends on the sub key input. In the first fK block, sub key 1 is used.
3. A simple permutation function that switches the two halves of the data (SW).
4. The function fK again with sub key 2 being used in this case.
5. A permutation function that is the inverse of the initial one (IP-1) as shown below
C =( IP
-1
∘f
K
2
∘SW ∘f
K
1
∘IP)
or
Ciphertext=IP
-1
(f
K
2
(SW (f
K
1
(IP(plaintext ))) ))
where
K
1
=P8(Left Shift-1 (P10 (key )))
K
2
=P8(Left Shift-2(Left Shift-1(P10(key ))))
and
Plaintext=IP
-1
(f
K
1
(SW (f
K
2
(IP (ciphertext )))))
Lab#04: Simplified DES (SDES) Page | 17
