PKI (public-key infrastructure) enables the secure exchange of data over otherwise unsecured media, such as the Internet. PKI is the underlying cryptographic security mechanism for digital certificates and certificate directories, which are used to authenticate a message sender. Because PKI is the standard for authenticating commercial electronic transactions, Understanding PKI, Second Edition, provides network and security architects with the tools they need to grasp each phase of the key/certificate life cycle, including generation, publication, deployment, and recovery.
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About the Authors.
I. CONCEPTS.1. Introduction.
Symmetric versus Asymmetric Ciphers.
New Directions: Public Key.
Services of Public-Key Cryptography.
Security between Strangers.
ECDSA and ECDH.
Summary.3. The Concept of an Infrastructure.
Public-Key Infrastructure Defined.
Key Backup and Recovery.
Automatic Key Update.
Support for Non-repudiation.
Summary.4. Core PKI Services: Authentication, Integrity, and Confidentiality.
Online versus Offline Operation.
Commonality of Underlying Algorithms.
Summary.5. PKI-Enabled Services.
Secure Time Stamping.
Connection with Other Services.
Need for Secure Data Archive.
Complexity of This Service.
The Human Factor.
Authentication and Authorization.
Connection with the PKI.
Mechanisms Required to Create PKI-Enabled Services.
Digital Signatures, Hashes, MACs, and Ciphers.
Trusted Time Sources.
Privilege Policy Creation Mechanism.
Privilege Policy Processing Engines.
Privilege Management Infrastructure Mechanisms.
Trusted Time Delivery Mechanism.
Physically Secure Archive Facilities.
Privacy Certificates and Identity Mapping.
Comprehensive PKI and Current Practice.
Summary.6. Certificates and Certification.
Certificate Structure and Semantics.
Alternative Certificate Formats.
Summary.7. Key and Certificate Management.
Key/Certificate Life-Cycle Management.
Summary.8. Certificate Revocation.
Periodic Publication Mechanisms.
Certificate Revocation Lists (CRLs).
Certification Authority Revocation Lists (CARLs).
End-Entity Public-Key Certification Revocation Lists (EPRLs).
CRL Distribution Points.
Delta and Indirect Delta CRLs.
Certificate Revocation Trees (CRTs).
Online Query Mechanisms.
Online Certificate Status Protocol (OCSP).
Simple Certificate Validation Protocol (SCVP).
Other Revocation Options.
Performance, Scalability, and Timeliness.
Summary.9. Trust Models.
Strict Hierarchy of Certification Authorities.
Loose Hierarchy of Certification Authorities.
Distributed Trust Architecture.
Four-Corner Trust Model.
Certificate Path Processing.
Trust Anchor Considerations.
Summary.10. Multiple Certificates per Entity.
Multiple Key Pairs.
Key Pair Uses.
Relationship between Key Pairs and Certificates.
Independent Certificate Management.
Support for Non-repudiation.
Summary.11. PKI Information Dissemination: Repositories and Other Techniques.
Publication and Repositories.
Locating Repositories 162Tradeoffs.
Interdomain Repository Issues and Options.
In-band Protocol Exchange.
Summary.12. PKI Operational Considerations.
User Key Compromise.
Disaster Preparation and Recovery.
Relying Party Notification.
Summary.13. Electronic Signature Legislation and Considerations.
Electronic Signature Legislation.
Digital Signatures in Context.
EU Electronic Signature Directive.
The Significance of Electronic Signature Initiatives.
Legal Considerations for PKIs.
Roles and Responsibilities.
Private Enterprise PKIs.
Other Contractual-Based Frameworks.
Summary.14. PKI in Practice.
What PKI Does.
What PKI Does Not Do.
The Value of PKI.
When Certificates and People Meet.
An E-mail Scenario.
A Web Scenario.
Summary.15. The Future of PKI.
How the World Is Changing.
A Recognized Authoritative Body.
Reasons for Cautious Optimism.
Summary.16. Conclusions and Further Reading.
Suggestions for Further Reading.
II. STANDARDS.17. Introduction.
Summary.19. Standardization Status and Road Map.
Current Standardization Status.
Toolkit Requirements (APIs and Mechanisms).
Ongoing Standardization Work.
Summary.20. Standards: Necessary but Not Sufficient.
The Role of Standards, Profiles, and Interoperability Testing.
Profiles and Interoperability Testing.
Automotive Network eXchange.
Bridge CA Demonstration.
Minimum Interoperability Specification.
National Automated Clearing House Association.
Securities Industry Root CA Proof of Concept.
EEMA PKI Challenge.
Summary.21. Conclusions and Further Reading.
Suggestions for Further Reading.
Certificate/CRL Syntax and Life-Cycle Management Protocols.
Certificate/CRL Storage and Retrieval.
Standards Bodies' Web Sites.
III. DEPLOYMENT CONSIDERATIONS.22. Introduction.
Business Case Considerations.
Deployment: Now or Later?
Summary.24. Deployment Issues and Decisions.
Trust Models: Hierarchical versus Distributed.
In-sourcing versus Out-sourcing.
Build versus Buy.
Closed versus Open Environment.
X.509 versus Alternative Certificate Formats.
Targeted Applications versus Comprehensive Solution.
Standard versus Proprietary Solutions.
Certificate and CRL Profiles.
Multiple Industry-Accepted Standards.
Policy/Business Control Issues.
On-line versus Off-line Operations.
Disaster Planning and Recovery.
Summary.25. Barriers to Deployment.
Lack of Industry-Accepted Standard.
Scalability and Performance.
Summary.26. Typical Business Models.
Internal Communications Business Model.
External Communications Business Model.
Internal/External Business Model Hybrids.
Business Model Influences.
VeriSign Trust Network.
GTE CyberTrust/Baltimore Technologies OmniRoot.
Other Trust Networks.
Summary.27. Conclusions and Further Reading.
Suggestions for Further Reading.References.
Without doubt, the promise of public-key infrastructure (PKI) technology has attracted a significant amount of attention in the last few years. Hardly a week goes by without some facet of PKI being addressed in a newspaper, trade journal, or conference paper. We hear and read about the promise of authentication and non-repudiation services provided through the use of digital signature techniques and about confidentiality and key management services based on a combination of symmetric and asymmetric cryptography—all facilitated through the realization of a supporting technology referred to as PKI. In fact, many people consider the widespread deployment of PKI technology to be an important enabler of secure global electronic commerce.
Although the foundation for PKI was established over two decades ago with the invention of public-key cryptography, PKI technology has been offered as a commercially viable solution only within the last few years. But what started as a handful of technology vendors a few years ago has seen the birth of dozens, perhaps hundreds, of products that offer one form or another of PKI-related service. Further, the commercial demand for PKI-based services remains strong, and available evidence suggests that this will continue for the foreseeable future.
Still, as a technology, PKI is fairly new. And to many, PKI technology is shrouded in mystery to some extent. This situation appears to be exacerbated by the proliferation of conflicting documentation, standards, and vendor approaches. Furthermore, there are few comprehensive books devoted to PKI that provide a good introduction to its critical concepts and technology fundamentals.
Thus, the authors share a common motivation in writing this book: to provide a vendor-neutral source of information that can be used to establish a baseline for understanding PKI. In this book, we provide answers to many of the fundamental PKI-related questions, including
These are just some of the questions we explore in this book.
It is important to recognize that PKI is not simply a "neat" technology without tangible benefits.When deployed judiciously, PKI offers certain fundamental advantages to an organization, including the potential for substantial cost savings. PKI can be used as the underlying technology to support authentication, integrity, confidentiality, and non-repudiation. This is accomplished through a combination of symmetric and asymmetric cryptographic techniques enabled through the use of a single, easily managed infrastructure rather than multiple security solutions. (See Chapter 2, Public-Key Cryptography; Chapter 3, The Concept of an Infrastructure; Chapter 4, Core PKI Services: Authentication, Integrity, and Confidentiality; and Chapter 5, PKI-Enabled Services.) PKI offers scalable key management in that the overhead associated with the distribution of keying material to communicating parties is reduced significantly when compared with solutions based solely on symmetric cryptography. (See Chapter 2 for a description of symmetric and asymmetric cryptographic techniques.) Ultimately, however, the primary motivations from a business standpoint are not technical but economic: How can PKI give a positive return on investment? To that end, judicious deployment of a single, unifying PKI technology can help, among other things
Not only does PKI technology have the potential to realize cost savings, but in some cases it also might even be a source of revenue for an organization (through support for new services that might otherwise not be offered). Benefits and related business considerations associated with PKI technology are discussed further in Part III, Deployment Considerations.
The world, and PKI's place in the world, has evolved somewhat since the first edition of this book was written. Like many technologies, PKI has experienced the highs and lows of media attention and analyst focus: In three short years, the descriptions have covered the spectrum from "silver bullet" to "snake oil." There is still confusion regarding naming of entities and the use of PKI in real-world business applications such as e-mail. Occasionally, the long-term viability of PKI is questioned in journals or trade publications. In this second edition, two new chapters have been added to address precisely these areas:
For the most part, however, the roller coaster of public opinion has now largely stabilized. There is general consensus that PKI is one viable option for a good, solid authentication technology with a number of appealing benefits compared with other technologies. In conjunction with this, PKI itself has matured and evolved to better meet the needs of the environments that might deploy it and rely on it for various services. In this edition, changes and additions have been made throughout the book to capture and explain this evolution. Some specific examples include the following:
The main purpose of this book is to provide a fairly comprehensive overview that will help the reader better understand the technical and operational considerations behind PKI technology. You will benefit from this book if you are responsible for the planning, deployment, and/or operation of an enterprise PKI. Those who are simply interested in the basic principles behind a PKI should also find this book useful.
We hope that this book will become an educational tool for many and a handy reference guide for others. This book is not intended to resolve extremely detailed implementation questions, although it can serve as a primer for someone who will eventually be more interested in the finer implementation details.
The book is organized into three parts. Part I provides essential background information necessary to better understand the concepts and principles behind PKI. Part II addresses standards and related activities (for example, industry-sponsored interoperability initiatives) related to PKI. There are two primary purposes for including this section in the book:
Part III discusses PKI deployment considerations, providing guidance for some of the initial and fundamental decisions that must be made prior to any PKI deployment.
Part I of this book deals with fundamental PKI concepts. This includes background information (for example, a primer on cryptography is included), as well as detailed information with respect to public-key certificates and certificate revocation schemes.
Chapter 1, Introduction, introduces Part I and provides a list of the contents of Part I on a chapter-by-chapter basis.
Chapter 2, Public-Key Cryptography, provides a brief, nonmathematical introduction to the concepts of public-key cryptography relevant to the material presented throughout the remainder of the book. It includes the distinction between symmetric and public-key ciphers, the concept of a key pair, the services of this technology, terminology, and sample algorithms.
Chapter 3, The Concept of an Infrastructure, discusses an infrastructure, highlighting its usefulness as an application enabler, its role in secure single sign-on, and its capability to provide end-user transparency and comprehensive security. This chapter also provides a working definition of PKI.
Chapter 4, Core PKI Services: Authentication, Integrity, and Confidentiality, and Chapter 5, PKI-Enabled Services, examine services that a PKI can provide. Chapter 4 discusses the core services of authentication, integrity, and confidentiality; Chapter 5 looks at PKI-enabled services such as digital time stamping, notarization, non-repudiation, and privilege management.
Chapter 6, Certificates and Certification, introduces the concept of a certificate and discusses the process of certification. Certificate contents and format are described, along with the role of a Certification Authority (CA) and a Registration Authority (RA).
Chapter 7, Key and Certificate Management, looks at the whole area of key/certificate lifecycle management, including generation, publication, update, termination, key history, key backup, and key recovery.
Chapter 8, Certificate Revocation, discusses common techniques for certificate revocation, including both periodic publication mechanisms and on-line query mechanisms.
Chapter 9, Trust Models, examines the concept of trust models. Strict hierarchies, loose hierarchies, policy-based hierarchies, distributed architectures, the four-corner model, the Web model, user-centric trust, and cross-certification are presented and compared. We also discuss certificate path processing in this chapter.
Chapter 10, Multiple Certificates per Entity, includes an examination of key pair uses, support for non-repudiation, and independent certificate management.
Chapter 11, PKI Information Dissemination: Repositories and Other Techniques, looks at the area of certificate dissemination and repositories. Options for sharing public-key-related information between two or more cooperating PKI domains are discussed.
Chapter 12, PKI Operational Considerations, discusses client-side software, on-line requirements, physical security, and disaster planning/recovery, along with tradeoffs between system security and ease of use.
Chapter 13, Electronic Signature Legislation and Considerations, discusses some of the recent legislation and directives that pertain to electronic signatures and clarifies some of the terminology associated with various forms of electronic signatures, including digital signatures. Some of the requirements and obligations that may apply to Certification Authorities (CAs), subscribers, and relying parties are briefly discussed.
Chapter 14, PKI in Practice, focuses on the use of PKI in the real world and tries to clarify some common misunderstandings and sources of confusion about what PKI can do and what it can't do (and was never intended to do).
Chapter 15, The Future of PKI, considers this oft-posed question: Why has PKI not "taken off" yet? This chapter offers an opinion about why PKI adoption has been slower than many people expected and discusses—with a view to emerging trends in the industry—the future of PKI.
Chapter 16, Conclusions and Further Reading, concludes Part I and suggests some sources to consult for further reading in this area.
Part II of this book addresses standards activities and interoperability initiatives.
Chapter 17, Introduction, introduces Part II and provides a list of the contents of Part II on a chapter-by-chapter basis.
Chapter 18, Major Standards Activities, discusses some of the most prominent activities taking place within formal standards bodies, as well as related efforts being undertaken outside the standards bodies.
Chapter 19, Standardization Status and Road Map, provides the current and projected nearterm standardization status of some of the most significant specifications.
Chapter 20, Standards: Necessary but Not Sufficient, considers the fact that the existence of a "standard," whether it is the product of a formal standards body or not, is necessary but not sufficient to guarantee that the products of different vendors will interoperate with one another. Some of the reasons for this are given, along with a discussion of the usefulness of profiling activities and interoperability pilots.
Finally, Chapter 21, Conclusions and Further Reading, provides concluding remarks and some suggestions for further reading.
Part III of this book addresses deployment. Not intended to be a deployment handbook, this part of the book primarily identifies many of the deployment questions that should be asked (and answered) when considering any large-scale enterprise PKI deployment.
Chapter 22, Introduction, introduces Part III and provides a list of the contents of Part III on a chapter-by-chapter basis.
Chapter 23, Benefits and Costs of a PKI, discusses the benefits realized through the deployment of a PKI. It also discusses cost considerations. This chapter helps identify sound business reasons for deploying a PKI in the enterprise environment.
Chapter 24, Deployment Issues and Decisions, discusses a number of issues that should be resolved before initial deployment occurs. Essentially, this chapter provides a basic foundation for product selection.
Chapter 25, Barriers to Deployment, addresses some of the more common hurdles to deployment,issues that one must consider in terms of long-term strategy.
Chapter 26, Typical Business Models, explains some of the more common business models one may want to implement. It also provides a brief discussion of some initiatives that can be used as a basis to establish interdomain trust.
Chapter 27, Conclusions and Further Reading, concludes Part III and offers suggestions for further reading.
We would like to emphasize that we have made every attempt to ensure that this book is as vendor neutral as possible. In fact, some of the original text has been modified at the request of one or more reviewers when (unintentionally) it even remotely appeared that we were advocating one approach over another. As authors, we are describing in this book our "vision" of what constitutes a comprehensive PKI. Although this viewpoint occasionally aligns more closely with some environments and certain specific vendor products than others, we hasten to point out that we are not aware of any one vendor that offers all the services that are described within this book.
We also recognize that some environments are necessarily more closely aligned with a subset of the components and services described herein (because of their specific requirements and target users), and we fully understand that these environments may never need to fully align with what we refer to as a comprehensive PKI. This is as it should be. This book is not about the "Internet PKI," nor is it meant to be limited to the "enterprise PKI"—although, arguably, the enterprise environment is closer today to our notion of the comprehensive PKI than many alternative deployment environments. This book attempts to describe all aspects of a PKI; specific environments will implement subsets as needed. We have provided a discussion of some of today's PKI variations at the end of Chapter 5 in order to clarify these concepts.
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