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Block Ciphers and the Data Encryption Standard (DES) ◆ A block cipher is an encryption/decryption scheme in which a block of

plaintext is treated as a whole and used to produce a ciphertext block of equal length. ◆ Many block ciphers have a Feistel structure. Such a structure consists of a number of identical rounds of processing. In each round, a substitution is performed on one half of the data being processed, followed by a permutation that interchanges the two halves. The original key is expanded so that a different key is used for each round. ◆ The Data Encryption Standard (DES) has been the most widely used encryption algorithm until recently. It exhibits the classic Feistel structure. DES uses a 64-bit block and a 56-bit key.

Block Ciphers and the Data Encryption Standard 1. Block Cipher Principles  Stream Ciphers and Block Ciphers  Motivation for the Feistel Cipher Structure  The Feistel Cipher 2. The Data Encryption Standard  DES Encryption  DES Decryption

(i) Stream Ciphers and Block Ciphers

(ii) Motivation for the Feistel Cipher Structure A block cipher operates on a plaintext block of n bits to produce a ciphertext block of n bits. There are possible different plaintext blocks and, for the encryption to be reversible (i.e., for decryption to be possible), each must produce a unique ciphertext block. Such a transformation is called reversible, or nonsingular. The following examples illustrate nonsingular and singular transformations for n = 2.

Ideal Block Cipher

Confusion and Diffusion – makes the statistical relationship between plaintext and ciphertext as complex as possible.  confusion – makes the statistical relationship between ciphertext and the value of encryption key as complex as possible.  diffusion

Feistel Cipher Structure FEISTEL ENCRYPTION ALGORITHM

FEISTEL DECRYPTION ALGORITHM

Feistel Cipher Design Features  block

size  key size  number of rounds  subkey generation algorithm  round function  fast software encryption/decryption  ease of analysis

2. The Data Encryption Standard DES Encryption  The

Data Encryption Standard (DES) has been the most widely used encryption algorithm until recently. It exhibits the classic Feistel structure.  DES uses a 64-bit block and a 56-bit key.

DES Encryption Overview

DETAILS OF SINGLE ROUND

DES Round Structure (Calculation of F(R,K)

Avalanche Effect 

A desirable property of any encryption algorithm is that a small change in either the plaintext or the key should produce a significant change in the ciphertext.  In particular, a change in one bit of the plaintext or one bit of the key should produce a change in many bits of the ciphertext. This is referred to as the avalanche effect.  If the change were small, this might provide a way to reduce the size of the plaintext or key space to be searched.

The End