Block Ciphers

Contents in Brief

  • 7.1 Introduction and overview.....................223
  • 7.2 Background and general concepts.................224
  • 7.3 Classical ciphers and historical development............237
  • 7.4 DES.................................250
  • 7.5 FEAL................................259
  • 7.6 IDEA................................263
  • 7.7 SAFER, RC5, and other block ciphers...............266
  • 7.8 Notes and further references....................271

Introduction and overview

Symmetric-key block ciphers are the most prominent and important elements in many cryptographic systems. Individually, they provide confidentiality. As a fundamental building block, their versatility allows construction of pseudorandom number generators, stream ciphers, MACs, and hash functions. They may furthermore serve as a central component in message authentication techniques, data integrity mechanisms, entity authentication protocols, and (symmetric--key) digital signature schemes. This chapter examines symmetric-key block ciphers, including both general concepts and details of specific algorithms. Public- key block ciphers are discussed in Chapter 8.

No block cipher is ideally suited for all applications, even one offering a high level of security. This is a result of inevitable tradeoffs required in practical applications, including those arising from, for example, speed requirements and memory limitations (e.g., code size, data size, cache memory), constraints imposed by implementation platforms (e.g., hardware, software, chipcards), and differing tolerances of applications to properties of various modes of operation. In addition, efficiency must typically be traded off against security. Thus it is beneficial to have a number of candidate ciphers from which to draw.

Of the many block ciphers currently available, focus in this chapter is given to a subset of high profile and/or well-studied algorithms. While not guaranteed to be more secure than other published candidate ciphers (indeed, this status changes as new attacks become known), emphasis is given to those of greatest practical interest. Among these, DES is paramount; FEAL has received both serious commercial backing and a large amount of independent cryptographic analysis; and IDEA (originally proposed as a DES replacement) is widely known and highly regarded. Other recently proposed ciphers of both high promise and high profile (in part due to the reputation of their designers) are SAFER and RC5. Additional ciphers are presented in less detail.

Chapter outline

Basic background on block ciphers and algorithm-independent concepts are presented in §7.2, including inodes of operation, multiple encryption, and exhaustive search techniques. Classical ciphers and cryptanalysis thereof are addressed in §7.3, including historical details on cipher machines. Modern block ciphers covered in chronological order are DES (§7.4), FEAL (§7.5), and IDEA (§7.6), followed by SAFER, RC5, and other ciphers in §7.7, collectively illustrating a wide range of modem block cipher design approaches. Further notes, including details on additional ciphers (e.g., Lucifer) and references for the chapter, may be found in §7.8.

 
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