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.

Chapter **3 **

**BLOCK CIPHERS AND THE DATA ENCRYPTION STANDARD**

**3.1
****Block
Cipher Principles **

Stream Ciphers and Block Ciphers

Motivation for the Feistel Cipher
Structure

The Feistel Cipher

**3.2
****The
Data Encryption Standard **

DES Encryption

DES Decryption

**3.3
****A
Des Example **

Results

The Avalanche Effect

**3.4
****The
Strength of Des **

The Use of 56-Bit Keys

The Nature of the DES Algorithm

Timing Attacks

**3.5
****Differential
and Linear Cryptanalysis **

Differential Cryptanalysis

Linear Cryptanalysis

**3.6
****Block
Cipher Design Principles **

DES Design Criteria

Number of Rounds

Design of Function F

Key Schedule Algorithm

*All the
afternoon Mungo had been working on Stern’s code, principally with the aid of
the latest messages which he had copied down at the Nevin Square drop. Stern
was very confident. He must be well aware London Central knew about that drop.
It was obvious that they didn’t care how often Mungo read their messages, so
confident were they in the impenetrability of the code.*

—*Talking to Strange Men*, Ruth
Rendell

**KEY POINTS**

◆
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 permu-tation 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.

◆
Two important methods of
cryptanalysis are differential cryptanalysis and linear cryptanalysis. DES has
been shown to be highly resistant to these two types of attack.

The objective of this chapter is to
illustrate the principles of modern symmetric ciphers. For this purpose, we
focus on the most widely used symmetric cipher: the Data Encryption Standard
(DES). Although numerous symmetric ciphers have been developed since the
introduction of DES, and although it is destined to be replaced by the Advanced
Encryption Standard (AES), DES remains the most important such algorithm.
Furthermore, a detailed study of DES provides an understanding of the
principles used in other symmetric ciphers.

This chapter begins with a discussion
of the general principles of symmetric block ciphers, which are the type of
symmetric ciphers studied in this book (with the exception of the stream cipher
RC4 in Chapter 7). Next, we cover full DES. Following this look at a specific
algorithm, we return to a more general discussion of block cipher design.

Compared to public-key ciphers, such
as RSA, the structure of DES and most symmetric ciphers is very complex and
cannot be explained as easily as RSA and simi-lar algorithms. Accordingly, the
reader may wish to begin with a simplified version of DES, which is described
in Appendix G. This version allows the reader to perform encryption and
decryption by hand and gain a good understanding of the working of the
algorithm details. Classroom experience indicates that a study of this
simplified version enhances understanding of DES.

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Cryptography and Network Security Principles and Practice : One Symmetric Ciphers : Block Ciphers and the Data Encryption Standard : Block Ciphers and the Data Encryption Standard |

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