Cryptography Decrypted / Edition 1

by H. Mel, Doris Baker

Overview

Cryptography is at the heart of computer security: without it, secure e-commerce and Internet communications would be impossible. Decision-makers and sophisticated computer users need to understand cryptography — but most explanations are highly mathematical and technical. Cryptography Decrypted explains cryptography in "plain English" — and is authoritative and thorough enough to address the needs of professionals. It explains the processes step-by-step, with extensive visuals. The authors present the elements of cryptography systems; public key infrastructure (PKI); and the IPSec standard for virtual private network security; then review real-world systems and their applications. They show how real-world systems are attacked, and how to protect them; introduce essential cryptographic terms; and present the fascinating history of cryptography through sidebars highlighting its important events, people, and breakthroughs. For every decision-maker and computer user who needs to understand cryptography, this book is also ideal for security pros who need to educate management about cryptography.

Product Details

ISBN-13:	9780201616477
Publisher:	Pearson Education
Publication date:	12/21/2000
Edition description:	New Edition
Pages:	384
Product dimensions:	7.30(w) x 9.10(h) x 0.90(d)

About the Author

H.X. Mel consults and develops computer systems for organizations such as Lucent, Xerox, MIT, the U.S. Government, Motorola, Goldman Sachs, and PricewaterhouseCoopers. Doris Baker is a freelance technical writer and editor.

0201616475AB04062001

Read an Excerpt

Chapter 9: Pioneering Public Key: Public Exchange of Secret Keys

Let's recap and lay the groundwork to see how a new twist on secret key distribution empowers a new form of cryptography.

Alice and Bob have developed secure secret keys.Alice encrypts her computer files and feels secure that no one can decrypt the files without her individual secret key. Alice and Bob's digital conversations use their shared secret key to authenticate each other, confidentially exchange files, and validate the integrity of the files (ensure that the files have not been altered during transit).

But as you saw in Chapter 8, sharing secret keys is difficult and expensive. Alice must either personally deliver the shared secret key to Bob or unequivocally trust a courier. Trustworthy couriers are expensive. And if Bob forgets their shared secret key, Alice must repeat the same key delivery process.

The Search for an Innovative Key Delivery Solution

The secret key delivery problem has plagued cryptographers, governments,a nd kings for thousands of years. How do you securely deliver a secret key to a confidant using insecure public lines of communication? Although the key may pass through BlackHat's hands, BlackHat must not be able to ascertain the secret key. It's a tough problem. What advantage does Bob have over BlackHat that Bob can exploit?

Developing an Innovative Secret Key Delivery Solution

While a graduate student at Berkeley in the early 1970s, Ralph Merkle devised a system that enabled people like Alice and Bob to exchange secret keys over a public line, marking the beginning of public key cryptography. Even though BlackHat is assumed to be listening to Alice and Bob's communications, Merkle envisioned a way to create a difficult, time-consuming problem for BlackHat. At the same time, Merkle's approach makes it easier for Alice and Bob to establish their shared secret key.

The goal was to create a problem that would take BlackHat a long time to solve even with the aid of a computer. Here's what Merkle devised.

First Attempt: A Database of Key/Serial Number Pairs

Suppose that Alice makes 1,000,000 new secret keys and stamps a unique serial number on each one (see Figure 9-1). Note that there's no reason to order the serial numbers. Alice keeps a database of each secret key and serial number.

If Alice then sends Bob a plaintext electronic copy of that database, he can easily pick a serial number (say, serial number 500,121) and its paired secret key (1yt8a42x35); then he calls Alice to tell her to use the secret key associated with serial number 500,121. But as Figure 9-2 shows, what's easy for Alice is also easy for the eavesdropping BlackHat. BlackHat has copied the database Alice sent to Bob—remember that it was sent over public lines—and quickly figures out the secret key. So this secret key exchange doesn 't work for Alice and Bob....

Second Attempt: An Encrypted Database of Key/Serial Number Pairs

Now suppose that Alice encrypts her serial number/secret key database and then ships the encrypted database to Bob. Alice tells Bob the encryption method she used but not the encryption key. Because Bob doesn't know Alice's encryption key,he must try all possible keys. Say it takes Bob about one hour, using his desktop computer, to find Alice's encryption key.¹ After decrypting the entire database, Bob selects a secret key and tells Alice the matching serial number — say, serial number 500,121. As before, Alice knows to use secret key 1yt8a42x35 (see Figure 9-3). But, like Bob,BlackHat can also spend about an hour decrypting Alice's database, so BlackHat can also figure out that serial number 500,121 matches secret key 1yt8a42x35. We still need a method that will make BlackHat's job much tougher than Alice and Bob's job.

Merkle's Insight: Individually Encrypted Key/Serial Number Pairs

This brings us to Merkle's creative insight. As the result of this innovation, Bob’s work doesn’t change, but BlackHat's work increases dramatically. Let's see how. Previously, Alice encrypted the entire database with one secret key. But that didn't make BlackHat work longer than Bob. Now Alice sends 1,000,000 encrypted secret key/serial number pairs (see Figure 9-4).

Each secret key/serial number pair (second column, Table 9-1) is encrypted with a unique secret key (third column, Table 9-1) to make the encrypted pair (final column, Table 9-1). Alice uses a million different secret keys to encrypt the 1,000,000 individual secret key/serial number pairs. Table 9-1 shows each secret key/serial number pair encrypted with a separate key.

Bob gets 1,000,000 encrypted secret key/serial number pairs and picks one encrypted pair—say,Pair3. He spends an hour deciphering it and learns that Pair3 means secret key 1yt8a42x35 and serial number 500,121 (see Figure 9-5). As before,he tells Alice that he will encrypt with the secret key matching the serial number 500,121. Alice quickly matches the serial number to the corresponding secret key in her database.

As before, Alice and Bob assume that BlackHat is listening, has copied all 1,000,000 encrypted pairs Alice sent to Bob, and has heard Bob tell Alice to use the secret key associated with serial number 500,121.

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^1. Obviously, Alice does not choose a "strong" cryptographic method to encrypt her database. Recall from Chapter 4 that a strong encryption method is one in which the most practical attack is to try each possible key and there are so many possible keys that it's infeasible to try even half of them.

Table of Contents

Foreword.


Preface.


Introduction.

I. SECRET KEY CRYPTOGRAPHY.

1. Locks and Keys.

Locks and Combinations.

Defining Cryptographic Terms.

Making and Solving Puzzles.

Review.

2. Substitution and Caesar's Cipher.

Cryptanalysis of Caesar's Cipher.

Empowering the Masses.

The Importance of Separating the Method and the Key.

Adding Keys.

A Weakness of Caesar's Ciphers: The Failure to Hide Linguistic Patterns.

More Complex Substitution: Vigenere's Cipher.

Review.

3. Transposition Ciphers: Moving Around.

Patterns and Cryptanalysis.

Adding Complexity.

Computer Transposition.

Combining Substitution and Transposition.

Review.

4. Diffuse and Confuse: How Cryptographers Win the End Game.

Diffusion.

The Polybius Cipher.

The Principle of Confusion.

Cryptographic Locks and Keys.

Review.

5. DES Isn't Strong Anymore.

The Historical Need for an Encryption Standard.

Cycling Through Computer Keys.

Double and Triple DES.

DES (and Other Block Cipher) Modes.

The Avalanche Effect.

Supplement: Binary Numbers and Computer Letters.

Review.

6. Evolution of Cryptography: Going Global.

Early Cryptography.

Commercial and Military Needs.

Entering the Computer Age.

Review.

7. Secret Key Assurances.

Confidentiality.

Authentication.

An Authentication Attack.

Not Really Random Numbers.

Integrity.

Using the MAC for Message Integrity Assurance.

Why Bother Using a Message Authentication Code?

File and MAC Compression.

Nonrepudiation: Secret Keys Can't Do It.

Review.

8. Problems with Secret Key Exchange.

The Problem and the Traditional Solution.

Using a Trusted Third Party.

Key Distribution Center and Key Recovery.

Problems with Using a Trusted Third Party.

Growth in the Number of Secret Keys.

Trust and Lifetime.

Review.

II. PUBLIC KEY CRYPTOGRAPHY.

9. Pioneering Public Key: Public Exchange of Secret Keys.

The Search for an Innovative Key Delivery Solution.

Developing an Innovative Secret Key Delivery Solution.

First Attempt: A Database of Key/Serial Number Pairs.

Second Attempt: An Encrypted Database of Key/Serial Number Pairs.

Merkle's Insight: Individually Encrypted Key/Serial Number Pairs.

Black Hat's Frustrating Problem.

The Key to Public Key Technology.

A New Solution: Diffie-Hellman-Merkle Key Agreement.

Alice and Bob Openly Agree on a Secret Key.

Problems with the Diffie-Hellman Method.

Separate Encryption and Decryption Keys.

Review.

10. Confidentiality Using Public Keys.

New Twists on Old Security Issues.

Confidentiality Assurances.

Distribution of Public Keys.

Two-Way Confidentiality.

Review.

11. Making Public Keys: Math Tricks.

Alice's Easy Problem.

Grade School Math Tricks.

More Grade School Math.

Division and Remainders: Modular Math.

Modular Inverses.

Using Modular Inverses to Make a Public Key.

Putting It All Together.

Giving BlackHat a Difficult, Time-Consuming Problem.

Trapdoor to the Easy Problem.

Knapsack Cryptography.

Modulo Calculations.

Exercise: Find Which Numbers Sum to 103.

Review.

12. Creating Digital Signatures Using the Private Key.

Written and Digital Signature Assurances.

Reviewing and Comparing Authentication.

Secret Key Authentication.

Private Key Authentication 117

Authentication and Integrity Using Private and Secret Keys.

Private Key Authentication Methods.

RSA.

DSA.

Signing Terminology.

Nonrepudiation.

Assurances in Both Directions.

Summary of Public Key Assurances.

Public Key Means Public / Private Key.

Assurance Initiated.

Compressing before Signing.

Review.

13. Hashes: Non-keyed Message Digests.

Detecting Unintentional Modifications.

Detecting Intentional Modifications.

Signing the Message Digest.

Detecting BlackHat's Forgery.

Replay Attacks.

Supplement: Unsuccessfully Imitating a Message Digest.

Review.

14. Message Digest Assurances.

Two Message Digest Flavors.

Non-keyed Message Digest Assurances.

One-wayness.

Collision Resistance.

Weak Collision Resistance.

Examples of One-way and Weak Collision Resistance.

Strong Collision Resistance.

Non-keyed Digest Implementations.

Keyed Message Digest Assurances.

A MAC Made with DES.

DES-MAC Security.

Message Digest Compression.

Digest Speed Comparisons.

Hashed MAC.

Review.

15. Comparing Secret Key, Public Key, and Message Digests.

Encryption Speed.

Key Length.

Ease of Key Distribution.

Cryptographic Assurances.

Symmetric (Secret) Key.

Asymmetric (Public) Key.

Review.

III. DISTRIBUTION OF PUBLIC KEYS.

16. Digital Certificates.

Verifying a Digital Certificate.

Attacking Digital Certificates.

Attacking the Creator of the Digital Certificate.

Malicious Certificate Creator.

Attacking the Digital Certificate User.

The Most Devastating Attack.

Understanding Digital Certificates: A Familiar Comparison.

Issuer and Subject.

Issuer Authentication.

Transfer of Trust from the Issuer to the Subject.

Issuer's Limited Liability.

Time Limits.

Revoking Trust.

More than One Certificate.

Fees for Use.

The Needs of Digital Certificate Users.

Getting Your First Public Key.

Certificates Included in Your Browser.

Review.

17. X.509 Public Key Infrastructure.

Why Use X.509 Certificate Management?

What Is a Certificate Authority?

Application, Certification, and Issuance.

Certificate Revocation.

Polling and Pushing: Two CRL Delivery Models.

Building X.509 Trust Networks.

Root Certificates.

More Risks and Precautions.

Distinguished Names.

Certification Practice Statement.

X.509 Certificate Data.

Challenge Response Protocol.

Review.

18. Pretty Good Privacy and the Web of Trust.

The History of PGP.

Comparing X.509 and PGP Certificates.

Building Trust Networks.

Bob Validates Alice's Key.

Casey Validates Alice's Key Sent by Bob.

Dawn Validates Alice's Key Sent by Casey via Bob.

Web of Trust.

PGP Certificate Repositories and Revocation.

Compatibility of X.509 and PGP

Review.

IV. REAL-WORLD SYSTEMS.

E-mail Cryptographic Parameters.

Negotiation of SSL and IPsec Cryptographic Parameters.

User Initiation of Cryptographic E-mail, SSL, and Ipsec.

19. Secure E-mail.

Generic Cryptographic E-mail Messages.

Invoking Cryptographic Services.

Confidentiality and Authentication.

Choosing Services.

Positioning Services.

Deterring E-mail Viruses.

Review.

20. Secure Socket Layer and Transport.

Layer Security.

History of SSL.

Overview of an SSL Session.

An SSL Session in Detail.

Hello and Negotiate Parameters.

Key Agreement (Exchange).

Authentication.

Confidentiality and Integrity.

TLS Variations.

Anonymous Diffie-Hellman.

Fixed and Ephemeral Diffie-Hellman.

Comparing TLS, SSL v3, and SSL v2.

A Big Problem with SSL v2.

A Possible Problem with TLS and SSL.

Generating Shared Secrets.

Bob Authenticates Himself to AliceDotComStocks.

Review.

21. IPsec Overview.

Enhanced Security.

Key Management.

Manual Distribution.

Automated Distribution.

IPsec Part 1: User Authentication and Key Exchange Using IKE.

SSL/TLS and IPsec Key Agreement.

Security Association.

Phases.

IKE Nomenclature.

Benefits of Two-Phase Key Exchange.

IPsec Part 2: Bulk Data Confidentiality and Integrity for Message or File Transport.

Protocol and Mode.

ESP Examples.

AH Examples.

Management Control.

Implementation Incompatibilities and Complications.

Review.

22. Cryptographic Gotchas.

Replay Attack.

Man-in-the-Middle Attack.

Finding Your Keys in Memory.

Does Confidentiality Imply Integrity?

Example 1.

Example 2: Cut-and-Paste Attack.

Public Key as a Cryptanalysis Tool.

Example 1: The Chosen Plaintext Attack.

Public Key Cryptographic Standards.

Example 2: The Bleichenbacher Attack.

BlackHat Uses Bob's RSA Private Key.

Review.

23. Protecting Your Keys.

Smart Cards.

Types of Smart Cards.

What's Inside a Smart Card.

Protections and Limitations.

Smart Card Attacks.

Review.

Epilogue.

Appendix A. Public Key Mathematics (and Some Words on Random Numbers).

Appendix B. (A Few) IPsec Details.

Bibliography.

Index. 0201616475T04062001

Preface

A Tool for Everyone

In the past, cryptography was used mainly to secure the communications of the powerful and influential, the military and royalty. But the widespread use of computers, and the attacks to which they are vulnerable, has expanded the need for secure communications around the globe. This book describes the protection afforded by modern computer cryptographic systems and explains how the pace of modern technology requires continuing attention to the security of those systems.

The advent of computers changed a great many things, but not the fundamentals of cryptography. Through stories and pictures, Cryptography Decrypted presents cryptography's evolution into a modern-day science, laying out patterns from the past that are applicable today. It also gives you a thorough understanding of terms that are destined to become as much a part of our language and life as megabyte and Internet. As you begin to think about controlling various aspects of your life using wired or wireless communication, on line all the time, your understanding of cryptography—its benefits and its pitfalls—will make you feel a little more in control of a rapidly changing world.

Because rapid advances in the speed of hardware will continue to threaten the security of current cryptographic methods, it's essential that you choose appropriate techniques and perform ongoing assessment if you want to maintain your digital security. You can make such choices and assessments only if you know the basic concepts of cryptography. Cryptography Decrypted offers you that knowledge through visual representation of difficult concepts, an easy-to-use reference for reviewing key cryptographic terminology, and instructive historical information.

You need little or no background in cryptography to read this book. Neither does it require technical or math genius. It's designed so that anyone from CIOs to self-taught computer enthusiasts—and everyone in between—can pick up this book without any knowledge of encryption and find it fascinating, understandable, and instructive.

If you have some understanding of computer cryptography, Cryptography Decrypted is systematic and comprehensive enough to solidify your knowledge. It provides a simple description of the component parts of secret key and public key cryptography. (Those who already understand and don't wish to cover any more material about secret key cryptography may choose to read only Parts II through IV, bypassing Part I.)

Throughout the book, we use images to clarify cryptographic terms. After explaining the basic cryptographic components, we describe real-world cryptographic systems, some possible attacks on those systems, and ways to protect your keys.

The book provides a historical framework on which to build your understanding of how and why computer cryptography works. After a discussion of how cryptography has evolved into an essential Internet tool, we analyze secret key exchange problems and then explain the evolution of public key cryptography, with its solution to the key exchange problem. Along the way we explain some simple background on the math tricks that make public key cryptography secure. Traditionally, those who have thoroughly understood cryptography have been trained as mathematicians or scientists. Our goal here is to explain computer cryptography with rather little discussion of math. If the esoteric details aren't of immediate concern to you, you can skip Chapter 11 ('Making Public Keys: Math Tricks'), Chapter 14 ('Message Digest Assurances'), and the appendixes without diminishing your understanding of the basic concepts. Appendix A describes some aspects of public key mathematics, including inverses, primes, the Fermat test, Diffie-Hellman, DSA, elliptic curve, and pseudo-random number generation. Appendix B provides details of IPsec, a security system introduced in Chapter 21.

Editorial Reviews

Analyzes secret key exchange problems, and describes the evolution of public key cryptography, with its solution to the key exchange problem. The authors then explain how keys are distributed through digital certificates, and show how three real-world systems<-->secure mail, secure socket layer, and Internet protocol security<-->use cryptographic keys and methods. Annotation c. Book News, Inc., Portland, OR (booknews.com)

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