15 Simplified DES

Objectives

• To discuss about the basis of Simplified Data Encryption Standard (DES).
• To understand the working of S-DES
• To learn the Key generation process in S-DES

1.  Introduction

Data Encryption Standards became a federal standard in November 1976. S-DES is a simplified version of the well-known DES (Data Encryption Standard) algorithm. It closely resembles the real thing, with smaller parameters, to facilitate operation by hand for pedagogical purposes. It is not intended as a real encryption tool, rather as a teaching tool. All conventional encryption algorithms are based on two kinds of manipulation of the data: substitution and permutation. In substitution, each element of the plaintext is mapped into another element and in permutation, the elements in the plaintext are rearranged. Permutation is also called transposition.

8 bits block with a 10 bits key is used in S-DES. The encryption process consists of Initial Permutation, Function fk1, Switch of the key halves, Function fk2, Final Permutation (inverse of initial permutation).

Key generation process do the following steps like First Initial permutation P10, Divide in left and right parts then Left shift and merge 8 bits permutation, resulting in a 8 bits K1.Then divide in left and right parts, double left shift and Merge an 8 bits permutation, resulting in a 8 bits K2

• Example of key generation:
  • Key: 1010000010
  • P10: 1000001100
  • Split: 10000 01100
  • Lshift: 00001 11000
  • P8: 10100100  K1
  • 2 Lshift: 00100 00011
  • P8: 01000011  K2

2.Structure of S-P boxes

The following gives structure of S-P boxes in Simplified DES

S-Box

• Initial Permutation

S-DES involves several fixed operations involving particular permutations or other kinds of operators. It starts by using the following

8-bit permutation, its “initial permutation,” hence dubbed IP:

S-DES uses an “expansion” permutation whereby 2 separate permutations are applied to a 4-bit number, and the 2 results pasted together to form an 8-bit final output.

• And it uses two so-called s-boxes, S0 and S1.
• S0 is :

• And here is S1:

The function F is taken from S0 and S1, such as:R is expanded by E,The expansion is xored with the subkey

Finally, it uses a swap operation called SW, wherein the right and left 4 bits of a byte are swapped. This is a permutation, and can be represented as such The first 4 bits are the input for S0 the last are input to S1

If the input is I1I2I3I4, then I1I3 is the row to consider and I2I3 is the column. The

output                                goes                                 then                                 through                                 P4.

Summary

Outlined the Simplified DES algorithm and discussed the encryption process in S- DES.