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CRYPTOGRAPHY AND NETWORK SECURITY PRINCIPLES AND PRACTICE SEVENTH EDITION GLOBAL EDITION

William Stallings

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Authorized adaptation from the United States edition, entitled Cryptography and Network Security: Principles and Practice, 7th Edition, ISBN 978-0-13-444428-4, by William Stallings published by Pearson Education © 2017.

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3

CONTENTS Notation 10

Preface 12

About the Author 18

PART ONE: BACKGROUND 19

Chapter 1 Computer and Network Security Concepts 19

1.1 Computer Security Concepts 21 1.2 The OSI Security Architecture 26 1.3 Security Attacks 27 1.4 Security Services 29 1.5 Security Mechanisms 32 1.6 Fundamental Security Design Principles 34 1.7 Attack Surfaces and Attack Trees 37 1.8 A Model for Network Security 41 1.9 Standards 43 1.10 Key Terms, Review Questions, and Problems 44

Chapter 2 Introduction to Number Theory 46

2.1 Divisibility and the Division Algorithm 47 2.2 The Euclidean Algorithm 49 2.3 Modular Arithmetic 53 2.4 Prime Numbers 61 2.5 Fermat’s and Euler’s Theorems 64 2.6 Testing for Primality 68 2.7 The Chinese Remainder Theorem 71 2.8 Discrete Logarithms 73 2.9 Key Terms, Review Questions, and Problems 78 Appendix 2A The Meaning of Mod 82

PART TWO: SYMMETRIC CIPHERS 85

Chapter 3 Classical Encryption Techniques 85

3.1 Symmetric Cipher Model 86 3.2 Substitution Techniques 92 3.3 Transposition Techniques 107 3.4 Rotor Machines 108 3.5 Steganography 110 3.6 Key Terms, Review Questions, and Problems 112

Chapter 4 Block Ciphers and the Data Encryption Standard 118

4.1 Traditional Block Cipher Structure 119 4.2 The Data Encryption Standard 129 4.3 A DES Example 131 4.4 The Strength of DES 134

4 CONTENTS

4.5 Block Cipher Design Principles 135 4.6 Key Terms, Review Questions, and Problems 137

Chapter 5 Finite Fields 141

5.1 Groups 143 5.2 Rings 145 5.3 Fields 146 5.4 Finite Fields of the Form GF(p) 147 5.5 Polynomial Arithmetic 151 5.6 Finite Fields of the Form GF(2n) 157 5.7 Key Terms, Review Questions, and Problems 169

Chapter 6 Advanced Encryption Standard 171

6.1 Finite Field Arithmetic 172 6.2 AES Structure 174 6.3 AES Transformation Functions 179 6.4 AES Key Expansion 190 6.5 An AES Example 193 6.6 AES Implementation 197 6.7 Key Terms, Review Questions, and Problems 202 Appendix 6A Polynomials with Coefficients in GF(28) 203

Chapter 7 Block Cipher Operation 207

7.1 Multiple Encryption and Triple DES 208 7.2 Electronic Codebook 213 7.3 Cipher Block Chaining Mode 216 7.4 Cipher Feedback Mode 218 7.5 Output Feedback Mode 220 7.6 Counter Mode 222 7.7 XTS-AES Mode for Block-Oriented Storage Devices 224 7.8 Format-Preserving Encryption 231 7.9 Key Terms, Review Questions, and Problems 245

Chapter 8 Random Bit Generation and Stream Ciphers 250

8.1 Principles of Pseudorandom Number Generation 252 8.2 Pseudorandom Number Generators 258 8.3 Pseudorandom Number Generation Using a Block Cipher 261 8.4 Stream Ciphers 267 8.5 RC4 269 8.6 True Random Number Generators 271 8.7 Key Terms, Review Questions, and Problems 280

PART THREE: ASYMMETRIC CIPHERS 283

Chapter 9 Public-Key Cryptography and RSA 283

9.1 Principles of Public-Key Cryptosystems 285 9.2 The RSA Algorithm 294 9.3 Key Terms, Review Questions, and Problems 308

CONTENTS 5

Chapter 10 Other Public-Key Cryptosystems 313

10.1 Diffie-Hellman Key Exchange 314 10.2 Elgamal Cryptographic System 318 10.3 Elliptic Curve Arithmetic 321 10.4 Elliptic Curve Cryptography 330 10.5 Pseudorandom Number Generation Based on an Asymmetric Cipher 334 10.6 Key Terms, Review Questions, and Problems 336

PART FOUR: CRYPTOGRAPHIC DATA INTEGRITY ALGORITHMS 339

Chapter 11 Cryptographic Hash Functions 339

11.1 Applications of Cryptographic Hash Functions 341 11.2 Two Simple Hash Functions 346 11.3 Requirements and Security 348 11.4 Hash Functions Based on Cipher Block Chaining 354 11.5 Secure Hash Algorithm (SHA) 355 11.6 SHA-3 365 11.7 Key Terms, Review Questions, and Problems 377

Chapter 12 Message Authentication Codes 381

12.1 Message Authentication Requirements 382 12.2 Message Authentication Functions 383 12.3 Requirements for Message Authentication Codes 391 12.4 Security of MACs 393 12.5 MACs Based on Hash Functions: HMAC 394 12.6 MACs Based on Block Ciphers: DAA and CMAC 399 12.7 Authenticated Encryption: CCM and GCM 402 12.8 Key Wrapping 408 12.9 Pseudorandom Number Generation Using Hash Functions and MACs 413 12.10 Key Terms, Review Questions, and Problems 416

Chapter 13 Digital Signatures 419

13.1 Digital Signatures 421 13.2 Elgamal Digital Signature Scheme 424 13.3 Schnorr Digital Signature Scheme 425 13.4 NIST Digital Signature Algorithm 426 13.5 Elliptic Curve Digital Signature Algorithm 430 13.6 RSA-PSS Digital Signature Algorithm 433 13.7 Key Terms, Review Questions, and Problems 438

PART FIVE: MUTUAL TRUST 441

Chapter 14 Key Management and Distribution 441

14.1 Symmetric Key Distribution Using Symmetric Encryption 442 14.2 Symmetric Key Distribution Using Asymmetric Encryption 451 14.3 Distribution of Public Keys 454 14.4 X.509 Certificates 459

6 CONTENTS

14.5 Public-Key Infrastructure 467 14.6 Key Terms, Review Questions, and Problems 469

Chapter 15 User Authentication 473

15.1 Remote User-Authentication Principles 474 15.2 Remote User-Authentication Using Symmetric Encryption 478 15.3 Kerberos 482 15.4 Remote User-Authentication Using Asymmetric Encryption 500 15.5 Federated Identity Management 502 15.6 Personal Identity Verification 508 15.7 Key Terms, Review Questions, and Problems 515

PART SIX: NETWORK AND INTERNET SECURITY 519

Chapter 16 Network Access Control and Cloud Security 519

16.1 Network Access Control 520 16.2 Extensible Authentication Protocol 523 16.3 IEEE 802.1X Port-Based Network Access Control 527 16.4 Cloud Computing 529 16.5 Cloud Security Risks and Countermeasures 535 16.6 Data Protection in the Cloud 537 16.7 Cloud Security as a Service 541 16.8 Addressing Cloud Computing Security Concerns 544 16.9 Key Terms, Review Questions, and Problems 545

Chapter 17 Transport-Level Security 546

17.1 Web Security Considerations 547 17.2 Transport Layer Security 549 17.3 HTTPS 566 17.4 Secure Shell (SSH) 567 17.5 Key Terms, Review Questions, and Problems 579

Chapter 18 Wireless Network Security 581

18.1 Wireless Security 582 18.2 Mobile Device Security 585 18.3 IEEE 802.11 Wireless LAN Overview 589 18.4 IEEE 802.11i Wireless LAN Security 595 18.5 Key Terms, Review Questions, and Problems 610

Chapter 19 Electronic Mail Security 612

19.1 Internet Mail Architecture 613 19.2 Email Formats 617 19.3 Email Threats and Comprehensive Email Security 625 19.4 S/MIME 627 19.5 Pretty Good Privacy 638 19.6 DNSSEC 639 19.7 DNS-Based Authentication of Named Entities 643 19.8 Sender Policy Framework 645 19.9 DomainKeys Identified Mail 648

CONTENTS 7

19.10 Domain-Based Message Authentication, Reporting, and Conformance 654 19.11 Key Terms, Review Questions, and Problems 659

Chapter 20 IP Security 661

20.1 IP Security Overview 662 20.2 IP Security Policy 668 20.3 Encapsulating Security Payload 673 20.4 Combining Security Associations 681 20.5 Internet Key Exchange 684 20.6 Cryptographic Suites 692 20.7 Key Terms, Review Questions, and Problems 694

APPENDICES 696

Appendix A Projects for Teaching Cryptography and Network Security 696

A.1 Sage Computer Algebra Projects 697 A.2 Hacking Project 698 A.3 Block Cipher Projects 699 A.4 Laboratory Exercises 699 A.5 Research Projects 699 A.6 Programming Projects 700 A.7 Practical Security Assessments 700 A.8 Firewall Projects 701 A.9 Case Studies 701 A.10 Writing Assignments 701 A.11 Reading/Report Assignments 702 A.12 Discussion Topics 702

Appendix B Sage Examples 703

B.1 Linear Algebra and Matrix Functionality 704 B.2 Chapter 2: Number Theory 705 B.3 Chapter 3: Classical Encryption 710 B.4 Chapter 4: Block Ciphers and the Data Encryption Standard 713 B.5 Chapter 5: Basic Concepts in Number Theory and Finite Fields 717 B.6 Chapter 6: Advanced Encryption Standard 724 B.7 Chapter 8: Pseudorandom Number Generation and Stream Ciphers 729 B.8 Chapter 9: Public-Key Cryptography and RSA 731 B.9 Chapter 10: Other Public-Key Cryptosystems 734 B.10 Chapter 11: Cryptographic Hash Functions 739 B.11 Chapter 13: Digital Signatures 741

References 744

Credits 753

Index 754

8 CONTENTS

ONLINE CHAPTERS AND APPENDICES1

PART SEVEN: SYSTEM SECURITY

Chapter 21 Malicious Software

21.1 Types of Malicious Software (Malware) 21.2 Advanced Persistent Threat 21.3 Propagation—Infected Content—Viruses 21.4 Propagation—Vulnerability Exploit—Worms 21.5 Propagation—Social Engineering—Spam E-mail, Trojans 21.6 Payload—System Corruption 21.7 Payload—Attack Agent—Zombie, Bots 21.8 Payload—Information Theft—Keyloggers, Phishing, Spyware 21.9 Payload—Stealthing—Backdoors, Rootkits 21.10 Countermeasures 21.11 Distributed Denial of Service Attacks 21.12 References 21.13 Key Terms, Review Questions, and Problems

Chapter 22 Intruders

22.1 Intruders 22.2 Intrusion Detection 22.3 Password Management 22.4 References 22.5 Key Terms, Review Questions, and Problems

Chapter 23 Firewalls

23.1 The Need for Firewalls 23.2 Firewall Characteristics and Access Policy 23.3 Types of Firewalls 23.4 Firewall Basing 23.5 Firewall Location and Configurations 23.6 References 23.7 Key Terms, Review Questions, and Problems

PART EIGHT: LEGAL AND ETHICAL ISSUES

Chapter 24 Legal and Ethical Aspects

24.1 Cybercrime and Computer Crime 24.2 Intellectual Property 24.3 Privacy 24.4 Ethical Issues 24.5 Recommended Reading 24.6 References 24.7 Key Terms, Review Questions, and Problems 24.A Information Privacy

1Online chapters, appendices, and other documents are at the Companion Website, available via the access card at the front of this book.

CONTENTS 9

Appendix C Sage Exercises

Appendix D Standards and Standard-Setting Organizations

Appendix E Basic Concepts from Linear Algebra

Appendix F Measures of Secrecy and Security

Appendix G Simplified DES

Appendix H Evaluation Criteria for AES

Appendix I Simplified AES

Appendix J The Knapsack Algorithm

Appendix K Proof of the Digital Signature Algorithm

Appendix L TCP/IP and OSI

Appendix M Java Cryptographic APIs

Appendix N MD5 Hash Function

Appendix O Data Compression Using ZIP

Appendix P PGP

Appendix Q The International Reference Alphabet

Appendix R Proof of the RSA Algorithm

Appendix S Data Encryption Standard

Appendix T Kerberos Encryption Techniques

Appendix U Mathematical Basis of the Birthday Attack

Appendix V Evaluation Criteria for SHA-3

Appendix W The Complexity of Algorithms

Appendix X Radix-64 Conversion

Appendix Y The Base Rate Fallacy

Glossary

NOTATION

Symbol Expression Meaning

D, K D(K, Y) Symmetric decryption of ciphertext Y using secret key K

D, PRa D(PRa, Y) Asymmetric decryption of ciphertext Y using A’s private key PRa

D, PUa D(PUa, Y) Asymmetric decryption of ciphertext Y using A’s public key PUa

E, K E(K, X) Symmetric encryption of plaintext X using secret key K

E, PRa E(PRa, X) Asymmetric encryption of plaintext X using A’s private key PRa

E, PUa E(PUa, X) Asymmetric encryption of plaintext X using A’s public key PUa

K Secret key

PRa Private key of user A

PUa Public key of user A

MAC, K MAC(K, X) Message authentication code of message X using secret key K

GF(p) The finite field of order p, where p is prime.The field is defined as the set Zp together with the arithmetic operations modulo p.

GF(2n) The finite field of order 2n

Zn Set of nonnegative integers less than n

gcd gcd(i, j) Greatest common divisor; the largest positive integer that divides both i and j with no remainder on division.

mod a mod m Remainder after division of a by m

mod, K a K b (mod m) a mod m = b mod m

mod, [ a [ b (mod m) a mod m ≠ b mod m

dlog dloga,p(b) Discrete logarithm of the number b for the base a (mod p)

w f(n) The number of positive integers less than n and relatively prime to n. This is Euler’s totient function.

Σ a n

i=1 ai a1 + a2 + g + an

Π q n

i=1 ai a1 * a2 * g * an

� i � j i divides j, which means that there is no remainder when j is divided by i

� , � �a � Absolute value of a

10

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NOTATION 11

Symbol Expression Meaning

} x } y x concatenated with y

≈ x ≈ y x is approximately equal to y

⊕ x⊕ y Exclusive-OR of x and y for single-bit variables; Bitwise exclusive-OR of x and y for multiple-bit variables

:, ; :x; The largest integer less than or equal to x ∈ x∈ S The element x is contained in the set S.

· A · (a1, a2, c ak)

The integer A corresponds to the sequence of integers (a1, a2, c ak)

PREFACE

WHAT’S NEW IN THE SEVENTH EDITION

In the four years since the sixth edition of this book was published, the field has seen contin- ued innovations and improvements. In this new edition, I try to capture these changes while maintaining a broad and comprehensive coverage of the entire field. To begin this process of revision, the sixth edition of this book was extensively reviewed by a number of professors who teach the subject and by professionals working in the field. The result is that, in many places, the narrative has been clarified and tightened, and illustrations have been improved.

Beyond these refinements to improve pedagogy and user-friendliness, there have been substantive changes throughout the book. Roughly the same chapter organization has been retained, but much of the material has been revised and new material has been added. The most noteworthy changes are as follows:

■ Fundamental security design principles: Chapter 1 includes a new section discussing the security design principles listed as fundamental by the National Centers of Academic Excellence in Information Assurance/Cyber Defense, which is jointly sponsored by the U.S. National Security Agency and the U.S. Department of Homeland Security.

■ Attack surfaces and attack trees: Chapter 1 includes a new section describing these two concepts, which are useful in evaluating and classifying security threats.

■ Number theory coverage: The material on number theory has been consolidated into a single chapter, Chapter 2. This makes for a convenient reference. The relevant portions of Chapter 2 can be assigned as needed.

■ Finite fields: The chapter on finite fields has been revised and expanded with addi- tional text and new figures to enhance understanding.

■ Format-preserving encryption: This relatively new mode of encryption is enjoying increasing commercial success. A new section in Chapter 7 covers this method.

■ Conditioning and health testing for true random number generators: Chapter 8 now provides coverage of these important topics.

■ User authentication model: Chapter 15 includes a new description of a general model for user authentication, which helps to unify the discussion of the various approaches to user authentication.

■ Cloud security: The material on cloud security in Chapter 16 has been updated and expanded to reflect its importance and recent developments.

■ Transport Layer Security (TLS): The treatment of TLS in Chapter 17 has been updated, reorganized to improve clarity, and now includes a discussion of the new TLS version 1.3.

■ Email Security: Chapter 19 has been completely rewritten to provide a comprehensive and up-to-date discussion of email security. It includes:

— New: discussion of email threats and a comprehensive approach to email security.

— New: discussion of STARTTLS, which provides confidentiality and authentication for SMTP.

12

PREFACE 13

— Revised: treatment of S/MIME has been updated to reflect the latest version 3.2.

— New: discussion of DNSSEC and its role in supporting email security.

— New: discussion of DNS-based Authentication of Named Entities (DANE) and the use of this approach to enhance security for certificate use in SMTP and S/MIME.

— New: discussion of Sender Policy Framework (SPF), which is the standardized way for a sending domain to identify and assert the mail senders for a given domain.

— Revised: discussion of DomainKeys Identified Mail (DKIM) has been revised.

— New: discussion of Domain-based Message Authentication, Reporting, and Confor- mance (DMARC) allows email senders to specify policy on how their mail should be handled, the types of reports that receivers can send back, and the frequency those reports should be sent.

OBJECTIVES

The subject, and therefore this book, draws on a variety of disciplines. In particular, it is impossible to appreciate the significance of some of the techniques discussed in this book without a basic understanding of number theory and some results from probability theory. Nevertheless, an attempt has been made to make the book self-contained. The book not only presents the basic mathematical results that are needed but provides the reader with an intuitive understanding of those results. Such background material is introduced as needed. This approach helps to motivate the material that is introduced, and the author considers this preferable to simply presenting all of the mathematical material in a lump at the beginning of the book.

SUPPORT OF ACM/IEEE COMPUTER SCIENCE CURRICULA 2013

The book is intended for both academic and professional audiences. As a textbook, it is intended as a one-semester undergraduate course in cryptography and network security for computer science, computer engineering, and electrical engineering majors. The changes to this edition are intended to provide support of the ACM/IEEE Computer Science Curricula 2013 (CS2013). CS2013 adds Information Assurance and Security (IAS) to the curriculum rec- ommendation as one of the Knowledge Areas in the Computer Science Body of Knowledge. The document states that IAS is now part of the curriculum recommendation because of the critical role of IAS in computer science education. CS2013 divides all course work into three categories: Core-Tier 1 (all topics should be included in the curriculum), Core-Tier-2 (all or almost all topics should be included), and elective (desirable to provide breadth and depth). In the IAS area, CS2013 recommends topics in Fundamental Concepts and Network Security

It is the purpose of this book to provide a practical survey of both the principles and practice of cryptography and network security. In the first part of the book, the basic issues to be addressed by a network security capability are explored by providing a tutorial and survey of cryptography and network security technology. The latter part of the book deals with the practice of network security: practical applications that have been implemented and are in use to provide network security.

14 PREFACE

in Tier 1 and Tier 2, and Cryptography topics as elective. This text covers virtually all of the topics listed by CS2013 in these three categories.

The book also serves as a basic reference volume and is suitable for self-study.

PLAN OF THE TEXT

The book is divided into eight parts.

■ Background

■ Symmetric Ciphers

■ Asymmetric Ciphers

■ Cryptographic Data Integrity Algorithms

■ Mutual Trust

■ Network and Internet Security

■ System Security

■ Legal and Ethical Issues

The book includes a number of pedagogic features, including the use of the computer algebra system Sage and numerous figures and tables to clarify the discussions. Each chap- ter includes a list of key words, review questions, homework problems, and suggestions for further reading. The book also includes an extensive glossary, a list of frequently used acronyms, and a bibliography. In addition, a test bank is available to instructors.

INSTRUCTOR SUPPORT MATERIALS

The major goal of this text is to make it as effective a teaching tool for this exciting and fast-moving subject as possible. This goal is reflected both in the structure of the book and in the supporting material. The text is accompanied by the following supplementary material that will aid the instructor:

■ Solutions manual: Solutions to all end-of-chapter Review Questions and Problems.

■ Projects manual: Suggested project assignments for all of the project categories listed below.

■ PowerPoint slides: A set of slides covering all chapters, suitable for use in lecturing.

■ PDF files: Reproductions of all figures and tables from the book.

■ Test bank: A chapter-by-chapter set of questions with a separate file of answers.

■ Sample syllabuses: The text contains more material than can be conveniently covered in one semester. Accordingly, instructors are provided with several sample syllabuses that guide the use of the text within limited time.

All of these support materials are available at the Instructor Resource Center (IRC) for this textbook, which can be reached through the publisher’s Web site www.pearsonglobaleditions.com/stallings. To gain access to the IRC, please contact your local Pearson sales representative.

PREFACE 15

PROJECTS AND OTHER STUDENT EXERCISES

For many instructors, an important component of a cryptography or network security course is a project or set of projects by which the student gets hands-on experience to reinforce concepts from the text. This book provides an unparalleled degree of support, including a projects component in the course. The IRC not only includes guidance on how to assign and structure the projects, but also includes a set of project assignments that covers a broad range of topics from the text:

■ Sage projects: Described in the next section.

■ Hacking project: Exercise designed to illuminate the key issues in intrusion detection and prevention.

■ Block cipher projects: A lab that explores the operation of the AES encryption algo- rithm by tracing its execution, computing one round by hand, and then exploring the various block cipher modes of use. The lab also covers DES. In both cases, an online Java applet is used (or can be downloaded) to execute AES or DES.

■ Lab exercises: A series of projects that involve programming and experimenting with concepts from the book.

■ Research projects: A series of research assignments that instruct the student to research a particular topic on the Internet and write a report.

■ Programming projects: A series of programming projects that cover a broad range of topics and that can be implemented in any suitable language on any platform.

■ Practical security assessments: A set of exercises to examine current infrastructure and practices of an existing organization.

■ Firewall projects: A portable network firewall visualization simulator, together with exercises for teaching the fundamentals of firewalls.

■ Case studies: A set of real-world case studies, including learning objectives, case description, and a series of case discussion questions.

■ Writing assignments: A set of suggested writing assignments, organized by chapter.

■ Reading/report assignments: A list of papers in the literature—one for each chapter— that can be assigned for the student to read and then write a short report.

This diverse set of projects and other student exercises enables the instructor to use the book as one component in a rich and varied learning experience and to tailor a course plan to meet the specific needs of the instructor and students. See Appendix A in this book for details.

THE SAGE COMPUTER ALGEBRA SYSTEM

One of the most important features of this book is the use of Sage for cryptographic examples and homework assignments. Sage is an open-source, multiplatform, freeware package that implements a very powerful, flexible, and easily learned mathematics and computer algebra system. Unlike competing systems (such as Mathematica, Maple, and MATLAB), there are

16 PREFACE

no licensing agreements or fees involved. Thus, Sage can be made available on computers and networks at school, and students can individually download the software to their own personal computers for use at home. Another advantage of using Sage is that students learn a powerful, flexible tool that can be used for virtually any mathematical application, not just cryptography.

The use of Sage can make a significant difference to the teaching of the mathematics of cryptographic algorithms. This book provides a large number of examples of the use of Sage covering many cryptographic concepts in Appendix B, which is included in this book.

Appendix C lists exercises in each of these topic areas to enable the student to gain hands-on experience with cryptographic algorithms. This appendix is available to instruc- tors at the IRC for this book. Appendix C includes a section on how to download and get started with Sage, a section on programming with Sage, and exercises that can be assigned to students in the following categories:

■ Chapter 2—Number Theory and Finite Fields: Euclidean and extended Euclidean algorithms, polynomial arithmetic, GF(24), Euler’s Totient function, Miller–Rabin, fac- toring, modular exponentiation, discrete logarithm, and Chinese remainder theorem.

■ Chapter 3—Classical Encryption: Affine ciphers and the Hill cipher.

■ Chapter 4—Block Ciphers and the Data Encryption Standard: Exercises based on SDES.

■ Chapter 6—Advanced Encryption Standard: Exercises based on SAES.

■ Chapter 8—Pseudorandom Number Generation and Stream Ciphers: Blum Blum Shub, linear congruential generator, and ANSI X9.17 PRNG.

■ Chapter 9—Public-Key Cryptography and RSA: RSA encrypt/decrypt and signing.

■ Chapter 10—Other Public-Key Cryptosystems: Diffie–Hellman, elliptic curve.

■ Chapter 11—Cryptographic Hash Functions: Number-theoretic hash function.

■ Chapter 13—Digital Signatures: DSA.

ONLINE DOCUMENTS FOR STUDENTS

For this new edition, a tremendous amount of original supporting material for students has been made available online.

Purchasing this textbook new also grants the reader six months of access to the Companion Website, which includes the following materials:

■ Online chapters: To limit the size and cost of the book, four …