SP 800-32
 

Introduction to Public Key

Technology and the Federal PKI

Infrastructure

D. Richard Kuhn 

Vincent C. Hu 

26 February 2001 W. Timothy Polk 

Shu-Jen Chang 

1 
 

 

 

 

 

 

 

 

 

National Institute of Standards and Technology, 2001. U.S. Government publication. Not subject to copyright.
 

Portions of this document have been abstracted from other U.S. Government publications, including: “Minimum
 

Interoperability Specification for PKI Components (MISPC), Version 1” NIST SP 800-15, January 1998;
 

“Certification Authority Systems”, OCC 99-20, Office of the Comptroller of the Currency, May 4, 1999; “Guideline
 

for Implementing Cryptography in the Federal Government”, NIST SP800-21, November 1999; Advances and
 

Remaining Challenges to Adoption of Public Key Infrastructure Technology, U.S. General Accounting Office,
 

GAO-01-277, February, 2001.
 

Additional portions were used with permission from “Planning for PKI: Best practices for PKI Deployment”, R.
 

Housley and T. Polk, Wiley & Sons, 2001. John Wack contributed material on PKI architectures.
 

2 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1 INTRODUCTION .......................................................................................................................................................... 5
 

1.1 GOALS....................................................................................................................................................................... 5
 

1.2 MOTIVATION ............................................................................................................................................................. 5
 

1.3 OVERVIEW ................................................................................................................................................................ 6
 

2 BACKGROUND............................................................................................................................................................. 7
 

2.1 SECURITY SERVICES.................................................................................................................................................. 7
 

2.2 NON-CRYPTOGRAPHIC SECURITY MECHANISMS ....................................................................................................... 7
 

2.2.1 Parity Bits and Cyclic Redundancy Checks...................................................................................................... 7
 

2.2.2 Digitized Signatures ......................................................................................................................................... 8
 

2.2.3 PINs and Passwords......................................................................................................................................... 8
 

2.2.4 Biometrics......................................................................................................................................................... 8
 

2.2.5 Summary - Non-Cryptographic Security Mechanisms ..................................................................................... 9
 

2.3 CRYPTOGRAPHIC SECURITY MECHANISMS................................................................................................................ 9
 

2.3.1 Symmetric Key .................................................................................................................................................. 9
 

2.3.2 Secure Hash.................................................................................................................................................... 10
 

2.3.3 Asymmetric (public key) Cryptography .......................................................................................................... 11
 

2.3.4 Summary – Cryptographic Mechanisms......................................................................................................... 12
 

2.4 SECURITY INFRASTRUCTURES ................................................................................................................................. 13
 

3 PUBLIC KEY INFRASTRUCTURES ....................................................................................................................... 15
 

3.1 PKI COMPONENTS................................................................................................................................................... 16
 

3.1.1 Certification Authorities................................................................................................................................. 17
 

3.1.2 Registration Authorities.................................................................................................................................. 17
 

3.1.3 PKI Repositories............................................................................................................................................. 18
 

3.1.4 Archives.......................................................................................................................................................... 18
 

3.1.5 PKI users ........................................................................................................................................................ 18
 

3.2 PKI ARCHITECTURES .............................................................................................................................................. 18
 

3.2.1 Enterprise PKI Architectures ......................................................................................................................... 19
 

3.2.2 Bridge PKI Architecture................................................................................................................................. 20
 

3.2.3 Physical Architecture ..................................................................................................................................... 20
 

3.3 PKI DATA STRUCTURES.......................................................................................................................................... 22
 

3.3.1 X.509 Public Key Certificates ........................................................................................................................ 22
 

3.3.2 Certificate Revocation Lists (CRLs) ............................................................................................................... 24
 

3.3.3 Attribute Certificates ...................................................................................................................................... 26
 

3.4 ADDITIONAL PKI SERVICES..................................................................................................................................... 26
 

3.5 CASE STUDY ........................................................................................................................................................... 27
 

4 ISSUES AND RISKS IN CA SYSTEM OPERATION ............................................................................................. 29
 

4.1 VERIFYING IDENTITY............................................................................................................................................... 29
 

4.2 CERTIFICATE CONTENT ........................................................................................................................................... 29
 

4.3 CERTIFICATE CREATION, DISTRIBUTION, AND ACCEPTANCE ................................................................................... 30
 

4.4 MANAGING DIGITAL CERTIFICATES ......................................................................................................................... 30
 

4.4.1 Customer Disclosures..................................................................................................................................... 30
 

4.4.2 Subscriber Service and Support ..................................................................................................................... 31
 

4.4.3 Suspending and Revoking Certificates ........................................................................................................... 31
 

4.4.4 Processing Relying Party Requests ................................................................................................................ 32
 

4.4.5 Certificate Revocation .................................................................................................................................... 32
 

5 THE FEDERAL PKI ................................................................................................................................................... 33
 

5.1 FEDERAL PKI ARCHITECTURE................................................................................................................................. 33
 

5.2 FEDERAL CERTIFICATE PROFILE(S).......................................................................................................................... 35
 

5.3 FEDERAL CRL PROFILE(S) ...................................................................................................................................... 37
 

6 DEPLOYING AN AGENCY PKI............................................................................................................................... 38
 

3 
 

 

 

 

 

 

 

 

 

 

 

6.1 ANALYZE DATA AND APPLICATIONS FOR YOUR ORGANIZATION ............................................................................. 38
 

6.2 COLLECT SAMPLE POLICIES AND BASE STANDARDS ............................................................................................... 39
 

6.3 DRAFT CERTIFICATE POLICY(S)............................................................................................................................... 39
 

6.3.1 Certificate Policies ......................................................................................................................................... 40
 

6.3.2 Computer Security Objects Registry............................................................................................................... 41
 

6.3.3 Establishing Policy Mappings and Constraints ............................................................................................. 41
 

6.3.4 Local certificate and CRL profile(s)............................................................................................................... 41
 

6.4 SELECT PKI PRODUCT OR SERVICE PROVIDER........................................................................................................ 42
 

6.5 DEVELOP CPS (CERTIFICATION PRACTICE STATEMENT)......................................................................................... 42
 

6.6 DO A PILOT.............................................................................................................................................................. 43
 

6.7 APPLY FOR CROSS CERTIFICATION WITH THE FBCA ............................................................................................... 43
 

7 SUMMARY AND CONCLUSIONS........................................................................................................................... 44
 

8 ACRONYMS AND ABBREVIATIONS..................................................................................................................... 45
 

9 GLOSSARY.................................................................................................................................................................. 46
 

10 SELECTED BIBLIOGRAPHY............................................................................................................................... 53
 

4 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1 INTRODUCTION

Public Key Infrastructures (PKIs) can speed up and simplify delivery of products and services by 

providing electronic approaches to processes that historically have been paper based. These 

electronic solutions depend on data integrity and authenticity. Both can be accomplished by 

binding a unique digital signature to an individual and ensuring that the digital signature cannot 

be forged. The individual can then digitally sign data and the recipient can verify the originator of 

the data and that the data has not been modified without the originator’s knowledge. In addition, 

the PKI can provide encryption capabilities to ensure privacy. 

As with all aspects of information technology, introducing a PKI into an organization requires 

careful planning and a thorough understanding of its relationship to other automated systems. 

This document provides a brief overview of issues related to the emerging Federal public key 

infrastructure, and its implementation within government agencies. It also reviews the risks and 

benefits of various PKI components, and some of the tradeoffs that are possible in the 

implementation and operation of PKIs within the Federal government. 

1.1 GOALS

This publication was developed to assist agency decision-makers in determining if a PKI is 

appropriate for their agency, and how PKI services can be deployed most effectively within a 

Federal agency. It is intended to provide an overview of PKI functions and their applications. 

Additional documentation will be required to fully analyze the costs and benefits of PKI systems 

for agency use, and to develop plans for their implementation. This document provides a 

starting point and references to more comprehensive publications. 

1.2 MOTIVATION

Practically every organization is looking to the Internet to deliver services, sell products, and cut 

costs. Federal agencies are under additional pressure to deliver Internet-based services to 

satisfy legislative and regulatory requirements. Two of the laws that motivate federal agencies to 

offer services electronically are the Government Paperwork Elimination Act (GPEA) [NARA 00] 

and the Health Insurance Portability and Accountability Act (HIPAA) [HCFA 01]. 

The Government Paperwork Elimination Act requires Federal agencies to offer services 

electronically. GPEA requires Federal agencies, by October 21, 2003, to provide an option to 

submit information or perform transactions electronically and to maintain records electronically. 

The law specifically establishes the legal standing of electronic records and their related 

electronic signatures. 

Agencies are required to use electronic authentication methods to verify the identity of the 

sender and the integrity of electronic content. GPEA defines electronic signature as any method 

of signing an electronic message that identifies and authenticates the person who is the source 

of the message and indicates their approval of the contents. 

The Health Insurance Portability and Accountability Act was passed in 1996. One part of this 

legislation was designed to improve efficiency through the use of uniform electronic data 

exchange mechanisms for health information. To achieve this, HIPAA required electronic 

processing and transmission of administrative and financial health care information. To address 

privacy and security concerns, HIPAA also mandates security and privacy standards to protect 

this health information. 

5 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Neither GPEA nor HIPAA mandates the use of specific technologies. Instead, they establish 

requirements to deliver services or transmit information while protecting the privacy and integrity 

of the citizen. However, the broad range of requirements established in these laws promotes 

the use of a comprehensive security infrastructure, such as PKI. Digital signatures and PKI offer 

a very strong mechanism to implement these requirements. 

1.3 OVERVIEW

This document is divided into six sections. This section describes the motivations and contents 

of the document. Section 2, Background, describes the security services, mechanisms that 

have been used historically, and the rationale for supporting these services through a public key 

infrastructure. This section also explains why traditional security mechanisms may need to be 

supplemented with PKI functions for many applications. Section 3, Public Key Infrastructures, 

describes the technology on which PKI is based, and shows how public key systems provide 

security. Section 4 is devoted to operation of a key PKI component, the certification authority. In 

this section, some of the risk/benefit tradeoffs in operating an agency PKI system are described. 

Section 5 introduces the Federal PKI (FPKI) and some of the considerations for agencies that 

plan to connect with the FPKI. Finally, Section 6 provides a brief overview of the procedures 

required to set up a PKI within a Federal agency. 

6 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

           

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2 BACKGROUND
 

This section is intended to describe the security services that may be achieved, and provide a 

comparison for the various techniques that may be used. 

2.1 SECURITY SERVICES

There are four basic security services: integrity, confidentiality, identification and authentication, 

and non-repudiation. This section describes the four services and why they may be necessary 

in a particular application. 

Data integrity services address the unauthorized or accidental modification of data. This 

includes data insertion, deletion, and modification. To ensure data integrity, a system must be 

able to detect unauthorized data modification. The goal is for the receiver of the data to verify 

that the data has not been altered. 

Confidentiality services restrict access to the content of sensitive data to only those individuals 

who are authorized to view the data. Confidentiality measures prevent the unauthorized 

disclosure of information to unauthorized individuals or processes. 

Identification and authentication services establish the validity of a transmission, message, 

and its originator. The goal is for the receiver of the data to determine its origin. 

Non-repudiation services prevent an individual from denying that previous actions had been 

performed. The goal is to ensure that the recipient of the data is assured of the sender’s identity. 

2.2 NON­CRYPTOGRAPHIC SECURITY MECHANISMS

Some of the security services described above can be achieved without the use of cryptography. 

Where illustrations may be useful, we will use Alice, Bob, and Charlie. Alice and Bob want to 

communicate in a secure manner. Charlie would like to interfere with the security services that 

Alice and Bob would like to obtain. 

2.2.1 Parity Bits and Cyclic Redundancy Checks

The simplest security mechanisms were designed to ensure the integrity of data transmitted 

between devices (e.g., computers and terminals). When devices communicate over a noisy 

channel, such as a phone line, there was a possibility that data might be altered. To guard 

against this, systems would transmit an extra bit, the parity bit, for each byte of data. The value 

of the extra bit was chosen to ensure that the number of 1s in the nine bits were odd (odd parity) 

or even (even parity). If the parity was wrong, data had been altered, and should be rejected. 

This mechanism is frequently used with modem connections. 

Parity bits are a relatively expensive form of integrity protection. They increase the size of the 

message by at least 12.5%. Worse, they may not detect multiple errors in the same byte. While 

this mechanism can be extended to detect such errors by using additional parity bits, the cost is 

increased yet again. 

Cyclic redundancy checks, or CRCs, perform the same function for larger streams of data with 

less overhead. CRCs are calculated by the sender using a mathematical function applied to the 

data to be transmitted to create a fixed size output. The CRC is appended to the transmitted 

data. The receiver calculates the CRC from the data stream and matches it against the CRC 

7 
 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

provided by the sender. If the two match, the data has not changed accidentally. This technique 

is commonly used in network protocols, such as Ethernet. 

Parity bits and CRCs protect against accidental modification of data, but do not protect against 

an attacker. If Alice sends a message to Bob, he can use these techniques as protection against 

a noisy channel, but a knowledgeable attacker could replace or modify the message without 

detection. 

2.2.2 Digitized Signatures

In the paper world, the traditional mechanism for non-repudiation is the handwritten signature. 

This signature indicates that the signer has written, approved, or acknowledged the contents of 

the paper document. A digitized signature is sometimes used as a substitute for written 

signatures when applications are computerized. 

A digitized signature is created by scanning in a handwritten signature. When someone wishes 

to sign an electronic document, they simply insert the image of their signature where 

appropriate. When the receiver views an electronic document or message, they immediately 

recognize the meaning of the digitized signature. 

Digitized signatures are one of the easiest mechanisms to use. If Bob knows Alice’s signature, 

he will recognize it right away. However, they are also one of the easiest to subvert. Charlie can 

easily cut Alice’s digitized signature from one document and insert it into another. Digitized 

signatures should not be relied upon for any security services. Digitized signatures are generally 

used in conjunction with a stronger mechanism to add usability. 

2.2.3 PI s a d PanN sswords

The traditional method for authenticating users has been to provide them with a personal 

identification number or secret password, which they must use when requesting access to a 

particular system. Password systems can be effective if managed properly, but they seldom 

are. Authentication that relies solely on passwords has often failed to provide adequate 

protection for computer systems for a number of reasons. If users are allowed to make up their 

own passwords, they tend to choose ones that are easy to remember and therefore easy to 

guess. If passwords are generated from a random combination of characters, users often write 

them down because they are difficult to remember. Where password-only authentication is not 

adequate for an application, it is often used in combination with other security mechanisms. 

PINs and passwords do not provide non-repudiation, confidentiality, or integrity. If Alice wishes 

to authenticate to Bob using a password, Bob must also know it. Since both Alice and Bob know 

the password, it is difficult to prove which of them performed a particular operation. 

2.2.4 Biometrics

Biometric authentication relies on a unique physical characteristic to verify the identity of system 

users. Common biometric identifiers include fingerprints, written signatures, voice patterns, 

typing patterns, retinal scans, and hand geometry. The unique pattern that identifies a user is 

formed during an enrollment process, producing a template for that user. 

When a user wishes to authenticate to the system, a physical measurement is made to obtain a 

current biometric pattern for the user. This pattern can then be compared against the 

enrollment template in order to verify the user’s identity. Biometric authentication devices tend 

to cost more than password or token-based systems, because the hardware required to capture 

and analyze biometric patterns is more complicated. However, biometrics provide a very high 

8 
 

 

 

       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

level of security because the authentication is directly related to a unique physical characteristic 

of the user which is more difficult to counterfeit. Recent technological advances have also 

helped to reduce the cost of biometric authentication systems. 

2.2.5 Summary ­ Non­Cryptographic Security Mechanisms

Non-cryptographic mechanisms may be used to authenticate the identity of a user or verify the 

integrity of data that has been transmitted over a communications line. None of these 

mechanisms provide confidentiality or non-repudiation. In general, cryptographic security 

mechanisms are required to achieve confidentiality or non-repudiation. 

Mechanism Data 

integrity 

Confidentialit 

y 

Identification and 

authentication 

Nonrepudiation 

Parity bits and 

CRCs 

Yes No No No 

Digitized 

signatures 

No No No No 

PINs and 

passwords 

No No Yes No 

Biometrics No No Yes No 

2.3 CRYPTOGRAPHIC SECURITY MECHANISMS

Cryptography is a branch of applied mathematics concerned with transformations of data for 

security. In cryptography, a sender transforms unprotected information (plaintext) into coded 

text (ciphertext). A receiver uses cryptography to either (a) transform the ciphertext back into 

plaintext, (b) verify the sender’s identity, (c) verify the data’s integrity, or some combination. 

In many cases, the sender and receiver will use keys as an additional input to the cryptographic 

algorithm. With some algorithms, it is critical that the keys remain a secret. If Charlie is able to 

obtain secret keys, he can pretend to be Alice or Bob, or read their private messages. One of 

the principal problems associated with cryptography is getting secret keys to authorized users 

without disclosing them to an attacker. This is known as secret key distribution. 

This document will examine three commonly used classes of cryptographic mechanisms: 

symmetric algorithms, secure hash algorithms, and asymmetric algorithms. For each class, we 

will discuss which of the four security services can be supported. In addition, we will discuss 

whether the algorithm can be used for secret key distribution. 

2.3.1 Symmetric Key

Symmetric key cryptography is a class of algorithms where Alice and Bob share a secret key. 

These algorithms are primarily used to achieve confidentiality, but may also be used for 

authentication, integrity and limited non-repudiation. 

Symmetric algorithms are ideally suited for confidentiality. Modern symmetric algorithms, such 

as AES, are very fast and very strong. To use a symmetric algorithm for confidentiality, Alice 

9 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

transforms a plaintext message to ciphertext using a symmetric algorithm and a key. Alice 

transmits the ciphertext to Bob. Bob uses the same key to transform the ciphertext back into the 

plaintext. 

Symmetric algorithms can also be used to authenticate the integrity and origin of data. Alice 

uses her key to generate ciphertext for the entire plaintext, as above. She sends the plaintext 

and a portion of the ciphertext to Bob. This portion of the ciphertext is known as a message 

authentication code, or MAC. Bob uses his copy of the key to generate the ciphertext, selects 

the same portion of the ciphertext and compares it to the MAC he received. If they match, Bob 

knows that Alice sent him the message. This does not provide non-repudiation, though. Alice 

can deny sending the message, since Bob could have generated it himself. 

Alice and Bob need to share a symmetric key before Alice encrypts or generates a MAC for a 

message. Establishing that shared key is called key management, and it is a difficult problem. 

Key management can be performed with symmetric key cryptography, but it is a classic “chicken 

vs. egg” problem. To use symmetric cryptography, Alice and Bob need to share a secret. Once 

Alice and Bob share a symmetric encryption key, the algorithm can be used to establish 

additional shared secrets. 

In general, that first shared key must be established through “out-of-band” mechanisms. This is 

acceptable if Alice communicates only with Bob. If she communicates with a larger community, 

the burden of establishing each relationship becomes a serious impediment to obtaining security 

services. 

However, this problem can become manageable through the introduction of a trusted third party 

(TTP). If Alice and the party she wishes to communicate with trust the same TTP, they can get 

a new key for this purpose from the TTP. Each party must establish a secret out of band with 

the TTP as a starting point. However, Alice will not need to repeat this process for each new 

party with which she communicates. 

2.3.2 Secure Hash

The secure hash function takes a stream of data and reduces it to a fixed size through a oneway mathematical function. The result is called a message digest and can be thought of as a 

fingerprint of the data. The message digest can be reproduced by any party with the same 

stream of data, but it is virtually impossible to create a different stream of data that produces the 

same message digest. 

A message digest can be used to provide integrity. If Alice sends a message and its digest to 

Bob, he can recompute the message digest to protect against accidental changes in the data. 

However, this does not protect Bob from an attacker. Charlie can intercept Alice’s message and 

replace it with a new message and the digest of the new message. 

A secure hash can be used to create a hash-based message authentication code, or HMAC, if 

Alice and Bob share a secret key. If Alice sends a message and its HMAC to Bob, he can 

recompute the HMAC to protect against changes in the data from any source. Charlie can 

intercept Alice’s message and replace it with a new message, but he cannot compute an 

acceptable HMAC without knowing the secret key. If Bob trusts Alice, he may accept an HMAC 

as authenticating Alice’s identity. However, the services of confidentiality and non-repudiation 

are not provided. The current Federal standard for a secure hash algorithm is SHA-1, which is 

specified in FIPS 180-1 [NIST 95]. An Internet Engineering Task Force document, RFC 2104 

[IETF 99], describes an open specification for HMAC use on the internet. The RFC 2104 HMAC 

can be used in combination with any iterated cryptographic hash, such as MD5 and SHA-1. It 

also provides for use of a secret key to calculate and verify the message authentication values. 

10 
     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.3.3 Asymmetric ( c k ) C publi ey ryptography

Asymmetric key cryptography, also known as public key cryptography, uses a class of 

algorithms in which Alice has a private key, and Bob (and others) have her public key. The 

public and private keys are generated at the same time, and data encrypted with one key can be 

decrypted with the other key. That is, a party can encrypt a message using Alice’s public key, 

then only Alice, the owner of the matching private key, can decrypt the message. Asymmetric 

algorithms are poorly suited for encrypting large messages because they are relatively slow. 

Instead, these algorithms are used to achieve authentication, integrity and non-repudiation, and 

support confidentiality through key management. Asymmetric algorithms are used to perform 

three operations explained below: digital signatures, key transport, and key agreement. 

Digital Signatures. Alice can generate a digital signature for a message using a message 

digest and her private key. To authenticate Alice as the sender, Bob generates the message 

digest as well and uses Alice’s public key to validate the signature. If a different private key was 

used to generate the signature, the validation will fail. 

In contrast to handwritten signatures, a digital signature also verifies the integrity of the data. If 

the data has been changed since the signature was applied, a different digest would be 

produced. This would result in a different signature. Therefore, if the data does not have 

integrity, the validation will fail. 

In some circumstances, the digital signature can be used to establish non-repudiation. If Bob 

can demonstrate that only Alice holds the private key, Alice cannot deny generating the 

signature. In general, Bob will need to rely on a third party to attest that Alice had the private key. 

Digital signatures are also used for authentication to systems or applications. A system can 

authenticate Alice’s identity through a challenge-response protocol. The system generates a 

random challenge and Alice signs it. If the signature is verified with Alice’s public key, it must 

have been signed by Alice. This type of authentication is useful for remote access to information 

on a server, protecting network management from masqueraders, or for gaining physical access 

to a restricted area. 

Key Transport. Some asymmetric algorithms (e.g., RSA [RSA 78]) can be used to encrypt and 

decrypt data. In practice these algorithms are never used to encrypt large amounts of data, 

because they are much slower than symmetric key algorithms. However, these algorithms are 

perfectly suited to encrypting small amounts of data – such as a symmetric key. This operation 

is called key transport or key exchange, and is used in many protocols. The following example 

might describe an electronic mail message from Alice to Bob: 

•	 Alice generates an AES [NIST 01b] key, and encrypts the message. She encrypts 

the AES key using Bob’s public key, and sends both the encrypted key and 

encrypted message to Bob. 

•	 Bob uses his private key to recover Alice’s AES key; he then uses the AES key to 

obtain the plaintext message. 

In this case, Alice uses asymmetric cryptography to achieve confidentiality for key distribution. 

This procedure does not provide any additional security services; since Alice used Bob’s public 

key, anyone could have generated the message. 

Key Agreement. Other asymmetric algorithms (e.g., Diffie-Hellman [DH 76]) may be used for 

key agreement. Assume Bob and Alice each generated a pair of Diffie-Hellman keys. Alice has 

her private key and Bob’s public key. Bob has his private key and Alice’s public key. Through a 

mathematical algorithm, Alice and Bob both generate the same secret value. Charlie may have 

both public keys, but he cannot calculate the secret value. Alice and Bob can use the secret 

value that they independently calculated as the AES key and protect their messages. 

11 
 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

There are forms of key agreement that provide implicit authentication as well. If Bob can 

retrieve the plaintext, he knows it was encrypted by Alice. She is the only one that could have 

generated the same secret value. 

2.3.4 Summary – Cryptographic Mec anih sms

Cryptographic mechanisms need to be used in concert to provide a complete suite of security 

services. Each class of algorithms has strengths and weaknesses. 

Symmetric cryptographic algorithms, such as AES, are needed to achieve confidentiality. These 

algorithms can provide some degree of integrity and authentication as well, but they are poorly 

suited to achieve non-repudiation. The Achilles heel for symmetric algorithms, however, is key 

distribution. 

The secure hash algorithm and the HMAC provide the basis for data integrity in electronic 

communications. They do not provide confidentiality, and are a weak tool for authentication or 

non-repudiation. The secure hash and HMAC cannot be used for key distribution, either. 

Symmetric cryptographic algorithms are highly effective for integrity, authentication, and key 

distribution. Digital signature algorithms, such as RSA or DSA, leverage secure hash algorithms 

for efficiency. When leveraging a trusted third party, digital signatures can be used to provide 

non-repudiation. Key transport algorithms (e.g., RSA) and key agreement algorithms (e.g., 

Diffie-Hellman) can be used to efficiently and securely distribute symmetric keys. Once again, 

leveraging a trusted third party to establish the identity of the private key holder simplifies the 

problem. 

Many applications will use these three classes of cryptographic mechanisms in concert to 

achieve the complete suite of security services. 

12 
   

   

   

       

     

       

   

         

       

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mechanism Data

integrity

Confidentiality Identification

and

authentication

NonrepudiationKey

Distribution

Symmetric key

cryptography

Encryption No Yes No No No

Message

authentication

codes

Yes No Yes No No

Key transport No No No No Yes­requires

out­of­band

initialization

step or a

TTP

Secure Hash

Functions

Message

digest

Yes No No No No

HMAC Yes No Yes No No

Asymmetric

cryptography

Digital

signatures

Yes No Yes Yes (with a

TTP)

No

Key transport No No No No Yes

Key

Agreement

No No Yes No Yes

2.4 SECURITY INFRASTRUCTURES

To achieve the broad range of security services, Alice and Bob will need to use several classes 

of cryptographic security mechanisms in concert. In particular, to achieve confidentiality they will 

need to distribute symmetric encryption keys. Distributing symmetric keys can be performed 

three ways: (1) directly between the parties using symmetric encryption; (2) using symmetric 

encryption and a trusted third party (TTP); or (3) using public key based key management with a 

TTP. 

The first mechanism is sufficient for small closed communities. If Alice communicates with just 

three or four people, she can perform an out-of-band initialization with each party. As 

communities grow, this solution fails to scale, though. What if Alice communicates with dozens 

of people? Now she needs a TTP to eliminate the out-of-band initialization step. The second 

mechanism is clearly more scalable, but it provides only limited support for authentication and 

does not support non-repudiation. 

The third mechanism is also scalable, and it also provides a comprehensive solution. If a TTP 

binds the public key to a user or system – that is, attests to the identity of the party holding the 

corresponding private key - the complete range of security services may be obtained. The user 

may obtain integrity, authentication, and non-repudiation through digital signatures. Symmetric 

13 
 

 

 

 

 

 

 

keys can be distributed using either key transport or key agreement. Those symmetric keys can 

be used to achieve confidentiality. 

Of course, a single TTP will only scale so far. To achieve security services across 

organizational boundaries, many inter-linked TTPs will be required. This set of interlinked TTPs 

forms a security infrastructure that users can rely upon to obtain security services. When this 

security infrastructure is designed to distribute public keys, it is known as a public key 

infrastructure (PKI). 

14 
   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3 PUBLIC KEY INFRASTRUCTURES

A public key infrastructure (PKI) binds public keys to entities, enables other entities to verify 

public key bindings, and provides the services needed for ongoing management of keys in a 

distributed system. 

The overall goals of modern security architectures are to protect and distribute information that 

is needed in a widely distributed environment, where the users, resources and stake-holders 

may all be in different places at different times. The emerging approach to address these 

security needs makes use of the scalable and distributed characteristics of public key 

infrastructure (“PKI”). PKI allows you to conduct business electronically with the confidence that: 

•	 The person or process identified as sending the transaction is actually the
 

originator.
 

•	 The person or process receiving the transaction is the intended recipient. 

•	 Data integrity has not been compromised. 

In conventional business transactions, customers and merchants rely on credit cards (e.g., VISA 

or MasterCard) to complete the financial aspects of transactions. The merchant may 

authenticate the customer through signature comparison or by checking identification, such as a 

driver’s license. The merchant relies on the information on the credit card and status 

information obtained from the credit card issuer to ensure that payment will be received. 

Similarly, the customer performs the transaction knowing they can reject the bill if the merchant 

fails to provide the goods or services. The credit card issuer is the trusted third party in this type 

of transaction. 

The same model is often applied in electronic commerce, even though the customer and issuer 

may never meet. The merchant cannot check the signature or request identification information. 

At best, the merchant can verify the customer’s address against the credit card issuer’s 

database. Again, the customer knows that they can reject the bill if the merchant fails to provide 

the goods or services. The credit card issuer is the trusted third party that makes consumer-tobusiness e-commerce possible. 

With electronic commerce, customer and merchant may be separated by hundreds of miles. 

Other forms of authentication are needed, and the customer’s credit card and financial 

information must be protected for transmission over the internet. Customers who do business 

with a merchant over the internet must use encryption methods that enable them to protect the 

information they transmit to the merchant, and the merchant must protect the information it 

transmits back to customers. Both customer and merchant must be able to obtain encryption 

keys and ensure that the other party is legitimate. The PKI provides the mechanisms to 

accomplish these tasks. 

Two parties who wish to transact business securely may be separated geographically, and may 

not have ever met. To use public key cryptography to achieve their security services, they must 

be able to obtain each other’s public keys and authenticate the other party’s identity. This may 

be performed out-of-band if only two parties need to conduct business. If they will conduct 

business with a variety of parties, or cannot use out-of-band means, they must rely on a trusted 

third party to distribute the public keys and authenticate the identity of the party associated with 

the corresponding key pair. 

Public key infrastructure is the combination of software, encryption technologies, and services 

that enables enterprises to protect the security of their communications and business 

transactions on networks. PKI integrates digital certificates, public key cryptography, and 

certification authorities into a complete enterprise-wide network security architecture. A typical 

enterprise’s PKI encompasses the issuance of digital certificates to individual users and servers; 

15 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

end-user enrollment software; integration with certificate directories; tools for managing, 

renewing, and revoking certificates; and related services and support. 

The term public key infrastructure is derived from public key cryptography, the technology on 

which PKI is based. Public key cryptography is the technology behind modern digital signature 

techniques. It has unique features that make it invaluable as a basis for security functions in 

distributed systems. This section provides additional background on the underlying mechanisms 

of a public key system. 

3.1 PKICOMPONENTS

Functional elements of a public key infrastructure include certification authorities, registration 

authorities, repositories, and archives. The users of the PKI come in two flavors: certificate 

holders and relying parties. An attribute authority is an optional component. 

A certification authority (CA) is similar to a notary. The CA confirms the identities of parties 

sending and receiving electronic payments or other communications. Authentication is a 

necessary element of many formal communications between parties, including payment 

transactions. In most check-cashing transactions, a driver’s license with a picture is sufficient 

authentication. A personal identification number (PIN) provides electronic authentication for 

transactions at a bank automated teller machine (ATM). 

A registration authority (RA) is an entity that is trusted by the CA to register or vouch for the 

identity of users to a CA. 

A repository is a database of active digital certificates for a CA system. The main business of 

the repository is to provide data that allows users to confirm the status of digital certificates for 

individuals and businesses that receive digitally signed messages. These message recipients 

are called relying parties. CAs post certificates and CRLs to repositories. 

An archive is a database of information to be used in settling future disputes. The business of 

the archive is to store and protect sufficient information to determine if a digital signature on an 

“old” document should be trusted. 

The CA issues a public key certificate for each identity, confirming that the identity has the 

appropriate credentials. A digital certificate typically includes the public key, information about 

the identity of the party holding the corresponding private key, the operational period for the 

certificate, and the CA’s own digital signature. In addition, the certificate may contain other 

information about the signing party or information about the recommended uses for the public 

key. A subscriber is an individual or business entity that has contracted with a CA to receive a 

digital certificate verifying an identity for digitally signing electronic messages. 

CAs must also issue and process certificate revocation lists (CRLs), which are lists of 

certificates that have been revoked. The list is usually signed by the same entity that issued the 

certificates. Certificates may be revoked, for example, if the owner’s private key has been lost; 

the owner leaves the company or agency; or the owner’s name changes. CRLs also document 

the historical revocation status of certificates. That is, a dated signature may be presumed to be 

valid if the signature date was within the validity period of the certificate, and the current CRL of 

the issuing CA at that date did not show the certificate to be revoked. 

PKI users are organizations or individuals that use the PKI, but do not issue certificates. They 

rely on the other components of the PKI to obtain certificates, and to verify the certificates of 

other entities that they do business with. End entities include the relying party, who relies on the 

certificate to know, with certainty, the public key of another entity; and the certificate holder, that 

is issued a certificate and can sign digital documents. Note that an individual or organization 

may be both a relying party and a certificate holder for various applications. 

16 
   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3.1.1 Certification Authorities

The certification authority, or CA, is the basic building block of the PKI. The CA is a collection of 

computer hardware, software, and the people who operate it. The CA is known by two 

attributes: its name and its public key. The CA performs four basic PKI functions: issues 

certificates (i.e., creates and signs them); maintains certificate status information and issues 

CRLs; publishes its current (e.g., unexpired) certificates and CRLs, so users can obtain the 

information they need to implement security services; and maintains archives of status 

information about the expired certificates that it issued. These requirements may be difficult to 

satisfy simultaneously. To fulfill these requirements, the CA may delegate certain functions to 

the other components of the infrastructure. 

A CA may issue certificates to users, to other CAs, or both. When a CA issues a certificate, it is 

asserting that the subject (the entity named in the certificate) has the private key that 

corresponds to the public key contained in the certificate. If the CA includes additional 

information in the certificate, the CA is asserting that information corresponds to the subject as 

well. This additional information might be contact information (e.g., an electronic mail address), 

or policy information (e.g., the types of applications that can be performed with this public key.) 

When the subject of the certificate is another CA, the issuer is asserting that the certificates 

issued by the other CA are trustworthy. 

The CA inserts its name in every certificate (and CRL) it generates, and signs them with its 

private key. Once users establish that they trust a CA (directly, or through a certification path) 

they can trust certificates issued by that CA. Users can easily identify certificates issued by that 

CA by comparing its name. To ensure the certificate is genuine, they verify the signature using 

the CA’s public key. As a result, it is important that the CA provide adequate protection for its 

own private key. Federal government CAs should always use cryptographic modules that have 

been validated against FIPS 140. 

As CA operation is central to the security services provided by a PKI, this topic is explored in 

additional detail in Section 5, CA System Operation. 

3.1.2 Registration Authorities

An RA is designed to verify certificate contents for the CA. Certificate contents may reflect 

information presented by the entity requesting the certificate, such as a drivers license or recent 

pay stub. They may also reflect information provided by a third party. For example, the credit 

limit assigned to a credit card reflects information obtained from credit bureaus. A certificate 

may reflect data from the company’s Human Resources department, or a letter from a 

designated company official. For example, Bob’s certificate could indicate that he has signature 

authority for small contracts. The RA aggregates these inputs and provides this information to 

the CA. 

Like the CA, the RA is a collection of computer hardware, software, and the person or people 

who operate it. Unlike a CA, an RA will often be operated by a single person. Each CA will 

maintain a list of accredited RAs; that is a list of RAs determined to be trustworthy. An RA is 

known to the CA by a name and a public key. By verifying the RA’s signature on a message, the 

CA can be sure an accredited RA provided the information, and it can be trusted. As a result, it 

is important that the RA provide adequate protection for its own private key. Federal 

government RAs should always use cryptographic modules that have been validated against 

FIPS 140. 

17 
  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3.1.3 PKI Repositories

PKI applications are heavily dependent on an underlying directory service for the distribution of 

certificates and certificate status information. The directory provides a means of storing and 

distributing certificates, and managing updates to certificates. Directory servers are typically 

implementations of the X.500 standard or subset of this standard. 

X.500 consists of a series of recommendations and the specification itself references several 

ISO standards. It was designed for directory services that could work across system, corporate, 

and international boundaries. A suite of protocols is specified for operations such as chaining, 

shadowing, and referral for server-to-server communication, and the Directory Access Protocol 

(DAP) for client to server communication. The Lightweight Directory Access Protocol (LDAP) 

was later developed as an alternative to DAP. Most directory servers and clients support LDAP, 

though not all support DAP. 

To be useful for the PKI applications, directory servers need to be interoperable; without such 

interoperability, a relying party will not be able to retrieve the needed certificates and CRLs from 

remote sites for signature verifications. To promote interoperablility among Federal agency 

directories and thus PKI deployments, the Federal PKI Technical Working Group is developing a 

Federal PKI Directory Profile [Chang] to assist agencies interested in participating in the FBCA 

demonstration effort. It is recommended that agencies refer to this document for the minimum 

interoperability requirements before standing up their agency directories. 

3.1.4 Archives

An archive accepts the responsibility for long term storage of archival information on behalf of 

the CA. An archive asserts that the information was good at the time it was received, and has 

not been modified while in the archive. The information provided by the CA to the archive must 

be sufficient to determine if a certificate was actually issued by the CA as specified in the 

certificate, and valid at that time. The archive protects that information through technical 

mechanisms and appropriate procedures while in its care. If a dispute arises at a later date, the 

information can be used to verify that the private key associated with the certificate was used to 

sign a document. This permits the verification of signatures on old documents (such as wills) at 

a later date. 

3.1.5 PKI users

PKI Users are organizations or individuals that use the PKI, but do not issue certificates. They 

rely on the other components of the PKI to obtain certificates, and to verify the certificates of 

other entities that they do business with. End entities include the relying party, who relies on the 

certificate to know, with certainty, the public key of another entity; and the certificate holder, that 

is issued a certificate and can sign digital documents. Note that an individual or organization 

may be both a relying party and a certificate holder for various applications. 

3.2 PKIARCHITECTURES

Certificate holders will obtain their certificates from different CAs, depending upon the 

organization or community in which they are a member. A PKI is typically composed of many 

CAs linked by trust paths. A trust path links a relying party with one or more trusted third parties, 

such that the relying party can have confidence in the validity of the certificate in use. Recipients 

of a signed message who have no relationship with the CA that issued the certificate for the 

18 
 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

sender of the message can still validate the sender’s certificate by finding a path between their 

CA and the one that issued the sender’s certificate. 

The initial challenge is deploying a PKI that can be used throughout an enterprise (e.g., a 

company or government agency). There are two traditional PKI architectures to support this 

goal, hierarchical and mesh enterprise architectures. More recently, enterprises are seeking to 

link their own PKIs to those of their business partners. A third approach, bridge CA 

architecture, has been developed to address this problem. These three architectures are 

described below. 

3.2.1 Enterprise PKI Architectures

CAs may be linked in a number of ways. Most enterprises that deploy a PKI will choose either a 

“mesh” or a “hierarchical” architecture: 

•	 Hierarchical: Authorities are arranged hierarchically under a “root” CA that issues 

certificates to subordinate CAs. These CAs may issue certificates to CAs below 

them in the hierarchy, or to users. In a hierarchical PKI, every relying party knows 

the public key of the root CA. Any certificate may be verified by verifying the 

certification path of certificates from the root CA. Alice verifies Bob’s certificate, 

issued by CA 4, then CA 4’s certificate, issued by CA 2, and then CA 2’s certificate 

issued by CA 1, the root, whose public key she knows. 

•	 Mesh: Independent CA’s cross certify each other (that is issue certificates to each 

other), resulting in a general mesh of trust relationships between peer CAs. Figure 1 

(b) illustrates a mesh of authorities. A relying party knows the public key of a CA 

“near” himself, generally the one that issued his certificate. The relying party verifies 

certificate by verifying a certification path of certificates that leads from that trusted 

CA. CAs cross certify with each other, that is they issue certificates to each other, 

and combine the two in a crossCertificatePair. So, for example, Alice knows the 

public key of CA 3, while Bob knows the public key of CA 4. There are several 

certification paths that lead from Bob to Alice. The shortest requires Alice to verify 

Bob’s certificate, issued by CA 4, then CA 4’s certificate issued by CA 5 and finally 

CA 5’s certificate, issued by CA 3. CA 3 is Alice’s CA and she trusts CA 3 and 

knows its public key. 

Figure 1 illustrates these two basic PKI architectures. 

Figure 1. Traditional PKI Architectures 

19 
     

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

3.2.2 riB dge P I ArK chi ectt ure

The Bridge CA architecture was designed to connect enterprise PKIs regardless of the 

architecture. This is accomplished by introducing a new CA, called a Bridge CA, whose sole 

purpose is to establish relationships with enterprise PKIs. 

Unlike a mesh CA, the Bridge CA does not issue certificates directly to users. Unlike a root CA 

in a hierarchy, the Bridge CA is not intended for use as a trust point. All PKI users consider the 

Bridge CA an intermediary. The Bridge CA establishes peer-to-peer relationships with different 

enterprise PKIs. These relationships can be combined to form a bridge of trust connecting the 

users from the different PKIs. 

If the trust domain is implemented as a hierarchical PKI, the Bridge CA will establish a 

relationship with the root CA. If the domain is implemented as a mesh PKI, the bridge will 

establish a relationship with only one of its CAs. In either case, the CA that enters into a trust 

relationship with the Bridge is termed a principal CA. 

In Figure 2, the Bridge CA has established relationships with three enterprise PKIs. The first is 

Bob’s and Alice’s CA, the second is Carol’s hierarchical PKI, and the third is Doug’s mesh PKI. 

None of the users trusts the Bridge CA directly. Alice and Bob trust the CA that issued their 

certificates; they trust the Bridge CA because the Fox CA issued a certificate to it. Carol’s trust 

point is the root CA of her hierarchy; she trusts the Bridge CA because the root CA issued a 

certificate to it. Doug trusts the CA in the mesh that issued his certificate; he trusts the Bridge 

CA because there is a valid certification path from the CA that issued him a certificate to the 

Bridge CA. Alice (or Bob) can use the bridge of trust that exists through the Bridge CA to 

establish relationships with Carol and Doug. 

Figure 2. Bridge CA and Enterprise PKIs 

3.2.3 hysP ical Archi ectt ure

There are numerous ways in which a PKI can be designed physically. It is highly recommended 

that the major PKI components be implemented on separate systems, that is, the CA on one 

system, the RA on a different system, and directory servers on other systems. Because the 

systems contain sensitive data, they should be located behind an organization's Internet firewall. 

20 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The CA system is especially important because a compromise to that system could potentially 

disrupt the entire operations of the PKI and necessitate starting over with new certificates. 

Consequently, placing the CA system behind an additional organizational firewall is 

recommended so that it is protected both from the Internet and from systems in the organization 

itself. Of course, the organizational firewall would permit communications between the CA and 

the RA as well as other appropriate systems. 

If distinct organizations wish to access certificates from each other, their directories will need to 

be made available to each other and possibly to other organizations on the Internet. However, 

some organizations will use the directory server for much more than simply a repository for 

certificates. The directory server may contain other data considered sensitive to the 

organization and thus the directory may be too sensitive to be made publicly available. A typical 

solution would be to create a directory that contains only the public keys or certificates, and to 

locate this directory at the border of the organization - this directory is referred to as a border 

directory. A likely location for the directory would be outside the organization’s firewall or 

perhaps on a protected DMZ segment of its network so that it is still available to the public but 

better protected from attack. Figure 3 illustrates a typical arrangement of PKI-related systems. 

The main directory server located within the organization's protected network would periodically 

refresh the border directory with new certificates or updates to the existing certificates. Users 

within the organization would use the main directory server, whereas other systems and 

organizations would access only the border directory. When a user in organization A wishes to 

send encrypted e-mail to a user in organization B, user A would then retrieve user B's certificate 

from organization B's border directory, and then use the public key in that certificate to encrypt 

the e-mail. 

RA 

CA 

Main

Directory

Main 

Directory 

Border

Directory

Border 

Directory 

Main

Firewall

Main 

Firewall 

Inner

Firewall

Inner 

Firewall 

Internet 

Gateway 

Figure 3. PKI Physical Topology 

21 
   

 

 

 

       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3.3 PKI DATA STRUCTURES

Two basic data structures are used in PKIs. These are the public key certificate and the 

certificate revocation lists. A third data structure, the attribute certificate, may be used as 

an addendum 

3.3.1 X.509 Public Key Certificates

The X.509 public key certificate format [IETF 01] has evolved into a flexible and powerful 

mechanism. It may be used to convey a wide variety of information. Much of that information is 

optional, and the contents of mandatory fields may vary as well. It is important for PKI 

implementers to understand the choices they face, and their consequences. Unwise choices 

may hinder interoperability or prevent support for critical applications. 

The X.509 public key certificate is protected by a digital signature of the issuer. Certificate users 

know the contents have not been tampered with since the signature was generated if the 

signature can be verified. Certificates contain a set of common fields, and may also include an 

optional set of extensions. 

There are ten common fields: six mandatory and four optional. The mandatory fields are: the 

serial number, the certificate signature algorithm identifier, the certificate issuer name, the 

certificate validity period, the public key, and the subject name. The subject is the party that 

controls the corresponding private key. There are four optional fields: the version number, two 

unique identifiers, and the extensions. These optional fields appear only in version 2 and 3 

certificates. 

Version. The version field describes the syntax of the certificate. When the version field is 

omitted, the certificate is encoded in the original, version 1, syntax. Version 1 certificates do not 

include the unique identifiers or extensions. When the certificate includes unique identifiers but 

not extensions, the version field indicates version 2. When the certificate includes extensions, 

as almost all modern certificates do, the version field indicates version 3. 

Serial number. The serial number is an integer assigned by the certificate issuer to each 

certificate. The serial number must be unique for each certificate generated by a particular 

issuer. The combination of the issuer name and serial number uniquely identifies any certificate. 

Signature. The signature field indicates which digital signature algorithm (e.g., DSA with SHA-1 

or RSA with MD5) was used to protect the certificate. 

Issuer. The issuer field contains the X.500 distinguished name of the TTP that generated the 

certificate. 

Validity. The validity field indicates the dates on which the certificate becomes valid and the 

date on which the certificate expires. 

Subject. The subject field contains the distinguished name of the holder of the private key 

corresponding to the public key in this certificate. The subject may be a CA, a RA, or an end 

entity. End entities can be human users, hardware devices, or anything else that might make 

use of the private key. 

Subject public key information. The subject public key information field contains the 

subject’s public key, optional parameters, and algorithm identifier. The public key in this field, 

along with the optional algorithm parameters, is used to verify digital signatures or perform key 

management. If the certificate subject is a CA, then the public key is used to verify the digital 

signature on a certificate. 

22 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Issuer unique ID and subject unique ID. These fields contain identifiers, and only appear in 

version 2 or version 3 certificates. The subject and issuer unique identifiers are intended to 

handle the reuse of subject names or issuer names over time. However, this mechanism has 

proven to be an unsatisfactory solution. The Internet Certificate and CRL profile does not 

[HOUS99] recommend inclusion of these fields. 

Extensions. This optional field only appears in version 3 certificates. If present, this field 

contains one or more certificate extensions. Each extension includes an extension identifier, a 

criticality flag, and an extension value. Common certificate extensions have been defined by 

ISO and ANSI to answer questions that are not satisfied by the common fields. 

Subject type. This field indicates whether a subject is a CA or an end entity. 

Names and identity information. This field aids in resolving questions about a user’s identity, 

e.g., are “alice@gsa.gov” and “c=US; o=U.S. Government; ou=GSA; cn=Alice Adams” the same 

person? 

Key attributes. This field specifies relevant attributes of public keys, e.g., whether it can be 

used for key transport, or be used to verify a digital signature. 

Policy information. This field helps users determine if another user’s certificate can be trusted, 

whether it is appropriate for large transactions, and other conditions that vary with organizational 

policies. 

Certificate extensions allow the CA to include information not supported by the basic certificate 

content. Any organization may define a private extension to meet its particular business 

requirements. However, most requirements can be satisfied using standard extensions. 

Standard extensions are widely supported by commercial products. Standard extensions offer 

improved interoperability, and they are more cost effective than private extensions. 

Extensions have three components: extension identifier, a criticality flag, and extension 

value. The extension identifier indicates the format and semantics of the extension value. The 

criticality flag indicates the importance of the extension. When the criticality flag is set, the 

information is essential to certificate use. Therefore, if an unrecognized critical extension is 

encountered, the certificate must not be used. Alternatively, an unrecognized non-critical 

extension may be ignored. 

The subject of a certificate could be an end user or another CA. The basic certificate fields do 

not differentiate between these types of users. The basic constraints extension appears in CA 

certificates, indicating this certificate may be used to build certification paths. 

The key usage extension indicates the types of security services that this public key can be 

used to implement. These may be generic services (e.g., non-repudiation or data encryption) or 

PKI specific services (e.g., verifying signatures on certificates or CRLs). 

The subject field contains a directory name, but that may not be the type of name that is used by 

a particular application. The subject alternative name extension is used to provide other name 

forms for the owner of the private key, such as DNS names or email addresses. For example, 

the email address alice@gsa.gov.gov could appear in this field. 

CAs may have multiple key pairs. The authority key identifier extension helps users select the 

right public key to verify the signature on this certificate. 

Users may also have multiple key pairs, or multiple certificates for the same key. The subject 

key identifier extension is used to identify the appropriate public key. 

23 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Organizations may support a broad range of applications using PKI. Some certificates may be 

more trustworthy than others, based on the procedures used to issue them or the type of user 

cryptographic module. The certificate policies extension contains a globally unique identifier 

that specifies the certificate policy that applies to this certificate. 

Different organizations (e.g., different companies or government agencies) will use different 

certificate policies. Users will not recognize policies from other organizations. The policy 

mappings extension converts policy information from other organizations into locally useful 

policies. This extension appears only in CA certificates. 

The CRL distribution points extension contains a pointer to the X.509 CRL where status 

information for this certificate may be found. (X.509 CRLs are described in the following 

section.) 

When a CA issues a certificate to another CA, it is asserting that the other CA's certificates are 

trustworthy. Sometimes, the issuer would like to assert that a subset of the certificates should 

be trusted. There are three basic ways to specify that a subset of certificates should be trusted: 

The basic constraints extension (described above) has a second role, indicating whether this 

CA is trusted to issue CA certificates, or just user certificates. 

The name constraints extension can be used to describe a subset of certificates based on the 

names in either the subject or subject alternative name fields. This extension can be used to 

define the set of acceptable names, or the set of unacceptable names. That is, the CA could 

assert “names in the NIST directory space are acceptable” or “names in the NIST directory 

space are not acceptable.” 

The policy constraints extension can be used to describe a subset of certificates based on the 

contents of the policy extension. If policy constraints are implemented, users will reject 

certificates without a policy extension, or where the specified policies are unrecognized. 

3.3.2 Certificate R n L s ( evocati io s Ct RLs)

Certificates contain an expiration date. Unfortunately, the data in a certificate may become 

unreliable before the expiration date arrives. Certificate issuers need a mechanism to provide a 

status update for the certificates they have issued. One mechanism is the X.509 certification 

revocation list (CRL). 

CRLs are the PKI analog of the credit card hot list that store clerks review before accepting large 

credit card transactions. The CRL is protected by a digital signature of the CRL issuer. If the 

signature can be verified, CRL users know the contents have not been tampered with since the 

signature was generated. CRLs contain a set of common fields, and may also include an 

optional set of extensions. 

The CRL contains the following fields: 

Version. The optional version field describes the syntax of the CRL. (In general, the version 

will be two.) 

Signature. The signature field contains the algorithm identifier for the digital signature algorithm 

used by the CRL issuer to sign the CRL. 

Issuer. The issuer field contains the X.500 distinguished name of the CRL issuer. 

This update. The this-update field indicates the issue date of this CRL. 

24 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Next update. The next-update field indicates the date by which the next CRL will be issued. 

Revoked certificates. The revoked certificates structure lists the revoked certificates. The 

entry for each revoked certificate contains the certificate serial number, time of revocation, and 

optional CRL entry extensions. 

The CRL entry extensions field is used to provide additional information about this particular 

revoked certificate. This field may only appear if the version is v2. 

CRL Extensions. The CRL extensions field is used to provide additional information about the 

whole CRL. Again, this field may only appear if the version is v2. 

ITU-T and ANSI X9 have defined several CRL extensions for X.509 v2 CRLs. They are 

specified in [X509 97] and [X955]. Each extension in a CRL may be designated as critical or 

non-critical. A CRL validation fails if an unrecognized critical extension is encountered. 

However, unrecognized non-critical extensions may be ignored. The X.509 v2 CRL format 

allows communities to define private extensions to carry information unique to those 

communities. Communities are encouraged to define non-critical private extensions so that 

their CRLs can be readily validated by all implementations. 

The most commonly used CRL extensions include the following: 

The CRL number extension is essentially a counter. In general, this extension is provided so 

that users are informed if an emergency CRL was issued. 

As noted in the previous section, CAs may have multiple key pairs. When appearing in a CRL, 

the authority key identifier extension helps users select the right public key to verify the 

signature on this CRL. 

The issuer field contains a directory name, but that may not be the type of name that is used by 

a particular application. The issuer alternative name extension is used to provide other name 

forms for the owner of the private key, such as DNS names or email addresses. For example, 

the email address CA1@nist.gov could appear in this field. 

The issuing distribution points extension is used in conjunction with the CRL distribution 

points extension in certificates. This extension is used to confirm that this particular CRL is the 

one described by the CRL distribution points extension and contains status information for 

certificate in question. This extension is required when the CRL does not cover all certificates 

issued by a CA, since the CRL may be distributed on an insecure network. 

The extensions described above apply to the entire CRL. There are also extensions that apply 

to a particular revoked certificate. 

Certificates may be revoked for a number of different reasons. The user’s crypto module may 

have been stolen, for example, or the module may simply have been broken. The reason code 

extension describes why a particular certificate was revoked. The relying party may use this 

information to decide if a previously generated signature may be accepted. 

Sometimes a CA does not wish to issue its own CRLs. It may delegate this task to another CA. 

The CA that issues a CRL may include the status of certificates issued by a number of different 

CAs in the same CRL. The certificate issuer extension is used to specify which CA issued a 

particular certificate, or set of certificates, on a CRL. 

25 
   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     

 

 

 

 

3.3.3 Attribute Certificates

The public key certificates described in 3.1.1 are focused on the binding between the subject 

and the public key. The relationship between the subject and public key is expected to be a 

long-lived relationship. Most end entity certificates include a validity period of a year or two 

years. 

Organizations seek improved access control. Public key certificates can be used to authenticate 

the identity of a user, and this identity can be used as an input to access control decision 

functions. However, in many contexts, the identity is not the criterion used for access control 

decisions. The access control decision may depend upon role, security clearance, group 

membership, or ability to pay. 

Authorization information, such as membership in a group, often has a shorter lifetime than the 

binding of the identity and the public key. Authorization information could be placed in a public 

key certificate extension. However, this is not a good strategy for two reasons. First, the 

certificate is likely to be revoked because the authorization information needs to be updated. 

Revoking and reissuing the public key certificate with updated authorization information is quite 

expensive. Second, the CA that issues public key certificates is not likely to be authoritative for 

the authorization information. This results in additional steps for the CA to contact the 

authoritative authorization information source. 

The X.509 attribute certificate (AC) binds attributes to an AC holder [X509 97]. This definition is 

being profiled for use in Internet applications. Since the AC does not contain a public key, the 

AC is used in conjunction with a public key certificate. An access control function may make 

use of the attributes in an AC, but it is not a replacement for authentication. The public key 

certificate must first be used to perform authentication, then the AC is used to associate 

attributes with the authenticated identity. 

ACs may also be used in the context of a data origin authentication service and a nonrepudiation service. In these contexts, the attributes contained in the AC provide additional 

information about the signing entity. This information can be used to make sure that the entity is 

authorized to sign the data. This kind of checking depends either on the context in which the 

data is exchanged or on the data that has been digitally signed. 

An X.509 AC resembles the X.509 public key certificate. The AC is an ASN.1 DER encoded 

object, and is signed by the issuer. An AC contains nine fields: version, holder, issuer, signature 

algorithm identifier, serial number, validity period, attributes, issuer unique identifier, and 

extensions. The AC holder is similar to the public key certificate subject, but the holder may be 

specified with a name, the issuer and serial number of a public key certificate, or the one-way 

hash of a certificate or public key. The attributes describe the authorization information 

associated with the AC holder. The extensions describe additional information about the 

certificate and how it may be used. 

3.4 ADDITIONAL PKI SERVICES

In addition to the security services described previously (non-repudiation, identification and 

authentication, confidentiality and integrity), PKIs can also offer other services. Two important 

services that may be offered by a PKI are key recovery and authorization. These services are 

described below. 

26 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Key Recovery. If a user’s key is lost, agencies and businesses must still be able to recover 

data that the employee had encrypted, which can only be done by recovering the encryption key. 

Reasons for key recovery may include an employee forgetting a password to unlock an 

encrypted file, the death of an employee who has encrypted some information, or someone 

attempting to hide criminal activity from law enforcement officials. To ensure the ability to 

recover encrypted data, encryption keys must be backed up and stored securely. 

Note, however, that signing keys, i.e., keys used for digital signatures, should not be backed up, 

since doing so prevents the PKI from ensuring non-repudiation. If anyone other than a particular 

user has a copy of a signing key, then that user can claim that someone else supplied the 

signature on a contested document. If a user loses a signing key, a new key and associated 

certificate can be easily generated. The PKI must keep track of the user’s possession of a key, 

but not the key itself. 

Privilege/Authorization. Certificates can be used to vouch for a user’s identity and also specify 

privileges the user has been granted. Privileges might include authority to view classified 

information or permission to modify material on a Web server among other privileges. 

3.5 CASE STUDY

A PKI can support a wide variety of internet and e-commerce applications, including secure 

electronic mail, virtual private networks, secure web access, and custom applications. The 

following procurement example illustrates how the services listed above are achieved, and the 

role played by the major PKI components. For the purposes of this example, we will assume 

RSA for signatures and key transport, SHA-1 for the hash, and AES for encryption. 

Alice is a buyer for the Alpha company. She wishes to obtain widgets from the Beta corporation 

for Alpha’s new product line. The Alpha company does not want its competitors to know 

anything about the new products – not even that widgets are a subcomponent. Alice wishes to 

negotiate with Bob, the head of sales at Beta, electronically. 

Alice generates two public-private key pairs, one for signature the other for encryption. She 

goes to the Alpha company RA and provides the signature public key and a valid Alpha 

company photo-id. The RA verifies the buyer’s identity based on the photo-id and vouches for 

the identity of the buyer to Alpha’s CA, who then issues the signature certificate to Alice. The 

Alpha CA posts the new certificate in the Alpha company directory. Alice returns to her desk 

and uses her signature to authenticate her request for a key exchange certificate. In this 

request, Alice provides both her public and private encryption keys. The Alpha CA issues a key 

transport certificate to Alice and escrows her encryption private key. The Alpha CA also 

periodically posts a CRL. 

The Beta corporation also has a CA. Bob has previously generated his key pairs and obtained 

certificates. The Beta CA has posted the certificates to its directory. Like the Alpha CA, the 

Beta CA posts CRLs as well. 

Alice constructs a message describing the number of widgets she needs and the schedule for 

delivery. She signs the message using the private key associated with the public key in her 

signing certificate. She generates an AES key and encrypts the message. Next, Alice obtains 

Bob’s key transport public key from the Beta directory. Alice verifies that this certificate has not 

been revoked, and encrypts her AES key using Bob’s public key. Alice sends Bob the signed 

and encrypted message along with the encrypted AES key. 

Bob uses his key transport private key to decrypt encrypted AES key. He then uses the AES 

key to decrypt the message. Now he can read the contents, but is unsure if it really originated 

from Alice or if it has been changed since Alice created it. Bob obtains Alice’s signature 

certificate from the Alpha directory along with the current CRL. After verifying the certificate has 

27 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

not been revoked, Bob can verify the digital signature on the message. If the signature 

validates, Bob knows the message came from Alice and has not been modified. He can 

concentrate on providing an accurate quote to the Alpha company. 

Alice and Bob negotiate a deal and the widgets are delivered. The bill arrives, and the Alpha 

company disputes the cost. This dispute must be settled immediately, or the new product line 

will be delayed. The CEO needs to see the messages Bob sent to Alice. Unfortunately, Alice is 

on an adventure vacation and cannot be contacted, so she can not decrypt the messages. The 

CEO of the Alpha company requests Alice’s private encryption key from the escrow server. 

After determining the request is authentic, the key is returned to the CEO. The CEO uses the 

key to decrypt the message from the negotiation between Alice and Bob, and the dispute is 

settled. 

We could extend this scenario further to include attribute certificates for authorization. The 

attribute authority would have issued an attribute certificate to the CEO authorizing him to 

perform key recovery operations. The escrow server would retrieve and verify the CEO’s 

attribute certificate before approving the escrow request. 

28 
           

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4 ISSUES AND RISKS IN CASYSTEM OPERATION

To issue digital certificates, a CA must verify subscribers’ identities; determine the appropriate 

content of digital certificates; create, distribute, and ensure acceptance of digital certificates; and 

ensure internal security. Each of these actions introduces some risk to the parties involved. 

This section discusses some of these risks and tradeoffs that can be made to reduce or spread 

the risk1

. 

CA systems may be characterized as primarily open or closed. A fully closed system has 

contracts defining the rights and obligations of all participants for authenticating messages or 

transactions. This type of system offers the CA operators less risk exposure because there is 

little uncertainty regarding obligations. Conversely, a fully open system would not have formal 

contracts defining the rights and obligations of relying parties in the system. In such a system, 

the firms that perform the CA activities could be exposed to an uncertain level of risk for each 

authenticated message or transaction. It is likely during early stages of development that most 

CA systems will be neither fully open nor fully closed, with contracts defining the rights and 

responsibilities of at least some, but not all, of the system participants. 

4.1 VERIFYING IDENTITY

To confirm the identity of a subscriber, the CA either reviews the subscriber’s credentials 

internally or contracts with a registration authority (RA). The decision to outsource and the 

choice of RA expose the CA to risk. If the CA or RA confirms an identity that is false, or 

somehow inaccurate, the CA may suffer loss of business or even expose itself to legal actions. 

Moreover, the CA’s outstanding certificates may become suspect if there is a pattern of 

insufficient due diligence in verifying identities for issuing certificates. The risk exposure from 

falsely identifying a subscriber may be reduced when a CA issues digital certificates for use 

within a closed system, because there are contracts in place between some or all of the 

participants in the system. 

4.2 CERTIFICATE CONTENT

Certificates’ content varies by CA system. Content and a certificate’s limitations are a source of 

strategic risk to the issuing CA. Standard certificates identify the subscriber and the issuing CA. 

Another important element of a standard certificate is the expiration date. The X.509 standards 

for certificate content require that digital certificates contain the distinguished (i.e., unique) name 

of the certificate issuer (the signer), an issuer-specific serial number, the issuer’s signature 

algorithm identifier, and a validity period. The more limited the life of a certificate, the lower the 

risk exposure for the issuing CA. A certificate’s security has both physical and logical 

vulnerabilities that are outgrowths of the software used to generate a digital signature. The 

longer such software is in use, the greater the likelihood that it will be corrupted or that someone 

will gain unauthorized access. 

Certificate extensions provide information in addition to the identity of the subscriber and the 

issuing CA. Additional information may include suggested limitations on uses of the certificate, 

such as the number of and type of transactions or messages that subscribers are authorized to 

sign. Any such limitation reduces the transaction and reputation risk of the issuing CA. The CA 

also may use extensions to establish classes of digital certificates for use with financial 

transactions or for transmitting highly sensitive information. Such certificates may be for a single 

1 Portions of this discussion were derived from “Certification Authority Systems”, Office of the 

Comptroller of the Currency, OCC 99-20, May 4, 1999. 

29 
 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

message or transaction, used only with a specific relying party, or limited to a maximum financial 

amount. 

4.3 CERTIFICATE CREATION, DISTRIBUTION, AND ACCEPTANCE

The process of creating, distributing, and documenting acceptance of a subscriber’s certificate 

exposes a CA to transaction, strategic, and reputation risk. In certificate creation, the 

transaction and reputation risk exposures arise from possible errors occurring in the systems 

that match appropriate certificate limitations to each subscriber’s unique signing capabilities. 

Risk exposures are associated with the policies and procedures that control the process. 

Certificate distribution and acceptance often are not solely the responsibility of the CA. The 

subscriber likely will obtain the technology to create digital signatures from a software provider or 

other technology firm. However, the certificate is not complete until the CA acknowledges the 

subscriber’s signing capability with its own digital signature to create the certificate of record. In 

a closed CA system, the CA risk exposure may be modified by the contract establishing the 

exact roles and responsibilities of the parties. Some of the transaction risk may be allocated to 

a lead organization, individual subscribers and relying parties, or another entity maintaining the 

database of certificates. However, the CA still may have a reputation risk exposure if problems 

with the technology are attributed to the CA. 

Generally, a digital certificate will not be operational until the subscriber accepts the signed 

certificate. Certificate acceptance implies that the subscriber agrees to the terms and conditions 

established by the CA for the overall system as well as any specific conditions that apply to the 

subscriber. Errors in the communication process with subscribers regarding acceptance, from 

either inadequate policies and procedures or technical difficulties, expose the CA to both 

transaction and reputation risk. 

4.4 MANAGING DIGITALCERTIFICATES

When a CA issues certificates to support subscribers’ digital signatures, the CA usually is 

interacting only with subscribers or a representative or agent acting on behalf of the subscribers. 

However, if the CA also chooses to manage outstanding certificates, i.e., act as a repository, the 

CA will transact with relying parties that receive messages. The following discussion outlines 

the risk exposures that arise with respect to repository services for both subscribers and relying 

parties. It is organized to address four aspects of managing digital certificates: 

• Customer disclosures 

• Subscriber service and support; 

• Suspending and revoking certificates; and 

• Processing the requests of relying parties. 

4.4.1 Customer Disclosures

Although there is no legal disclosure requirement at present, a CA will need to provide some 

information concerning the basic services provided and the rights and responsibilities of 

subscribers and relying parties. The nature of the disclosures will have an impact both on the 

transaction and reputation risk exposure of a CA. For example, if disclosures clearly describe 

the CA error resolution procedures and privacy policy, there may be less confusion on the part of 

subscribers. Further, if the CA provides technical documentation on the use of the software 

associated with certificates, subscribers will be better able to distinguish between problems 

resulting from the software rather than the CA, shifting some of the reputation risk exposure 

away from the CA. 

30 
       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4.4.2 Subscriber Servi e a d S nc upport

Like many new information technology products and services, a CA requires customer support, 

which is a source of reputation risk. A CA may consider establishing a help desk or some other 

form of direct interaction with subscribers and relying parties. The policies, procedures and 

operation of the help desk are a potential source of transaction and strategic risk. Resolving 

problems or errors that subscribers and relying parties encounter from lack of familiarity with the 

use of the underlying technology will require substantial resources from the CA or a customer 

service contractor. Although the CA typically will not supply software for creating a digital 

signature, there may be some circumstances in which subscribers attribute all difficulties in 

using the technology to the CA. 

Subscribers may have technical problems because of software configurations on their personal 

computer systems that may not become apparent until they attempt to sign a message or 

transaction. Because an organization providing CA services ultimately may wish to maintain the 

customer relationship, the practical decision may be to provide customer service either internally 

or to contract with a firm with appropriate expertise. Some technology firms now provide smart 

cards to hold subscriber certificates. Instead of downloading the software to the PC hard drive, 

the subscriber would have a smart card reader attached to his PC. The smart card and reader 

would be pre-programmed to load the certificate information appropriately for the subscriber. 

Some of the transaction and reputation risk of subscriber service and support may be reduced 

by the simplicity of the use of hardware rather than requiring PC users to load the software from 

another source. 

Because the subscriber is responsible for maintaining the security of the signature capability, the 

potential exists that the system may be compromised and made available for unauthorized use. 

Thus, the CA may be required to suspend or revoke a certificate. If the CA (or another 

responsible party within the system) does not monitor and take such action in a timely manner, 

the CA may authenticate messages or transactions carrying expired digital signatures. Thus, 

CA systems that render a subscriber’s digital certificate invalid are potentially exposed to 

substantial transaction, strategic, and reputation risks. Poorly designed policies and procedures 

are a source of strategic risk, and improperly implemented ones expose the CA to transaction 

and reputation risk. The timing of necessary repository updates may differ with the type of 

certificates involved; a delay in the suspension of a certificate used for sensitive messages or 

transactions carries relatively high risk. 

A digital certificate may be rendered invalid in one of two ways. The CA may revoke a certificate 

if it is certain that a subscriber has compromised his signing capability. The most likely 

compromise would be if the subscriber did not keep his private key secure. If a subscriber’s 

private key became known, unauthorized individuals could sign messages and transactions. If 

there is some question as to the status of the certificate, the CA instead may suspend the 

certificate until its status is determined. Transaction and reputation risk may result from errors in 

processing both requests for revocation and suspension of certificates. For example, a 

subscriber whose certificate is erroneously invalidated and hence is unable to sign messages 

could potentially experience losses and may pursue legal action, damaging the CA’s reputation 

in the process. Conversely, the CA may suffer exposure if a relying party accepts a message or 

transaction that is signed by a subscriber whose certificate should have been revoked or 

suspended. 

4.4.3 Suspending a d R n evoking Certificates

31 
       

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4.4.4 Processing Relying Party Requests

Substantial transaction, strategic, and reputation risk exposure is associated with processing 

requests by relying parties regarding the status of individual certificates. Although the CAsubscriber contractual relationship may define obligations to subscribers and others, such 

contracted protection may not exist for transactions with relying parties, particularly in open 

systems. For example, if the CA represents a revoked certificate as operational to a relying 

party, the CA may be exposed to reputation damage or a lawsuit. There is an additional risk in 

an open system that the circumstances of an individual subscriber or class of subscribers have 

changed during the valid period of a circulating certificate. Any delays in processing certificate 

revocation requests as a result of inadequate policies and procedures or technical processing 

may result in such errors. If the repository processes requests in batch mode as opposed to 

real time, the risk exposure is greater. As the volume of transactions processed by the 

repository increases and as more certificates are placed in circulation with varying limitations 

and expiration dates, risk exposures also may increase. 

4.4.5 Certificate Revocation

There are two recognized methods for responding to a request about the validity of an individual 

certificate. The most well known method requires the repository to retrieve a lengthy list of 

invalid certificates, the Certificate Revocation List (CRL), to check the validity of a single 

certificate. Inaccuracies in the CRL are a source of transaction risk for the CA system. In 

addition, the scheduled frequency for generating the CRL will affect the risk exposure of the 

repository. More frequent generation of CRLs will reduce a CA’s transaction and reputation risk 

exposure. There is also an issue as to whether certificate status is “pushed” out by the CA 

repository to interested relying parties, or “pulled” from the repository by the relying parties in 

question. 

There are different transaction and reputation risk exposures associated with each method. The 

“pull” method allows the CA repository to transfer any reputation risk exposure successfully to 

the relying party with respect to accepting an invalid certificate. On the other hand, the “push” 

method places the responsibility clearly on the CA if the CRL is not accurate or is not distributed 

on a timely basis. Because of the risks and cost inefficiencies of the CRL approach, the industry 

is developing a second method. Several technology firms have developed software that allows 

a repository to search its records for the validity of a single certificate in real time. Another 

source of repository transaction risk relates to the ability of a relying party to understand 

certificate extensions. 

32 
     

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5 THE FEDERAL PKI
 

5.1 FEDERAL PKIARCHITECTURE

Many Federal agencies have initiated efforts to set up independent CAs to support applications 

such as purchasing, grants, travel, or other functions that are needed to support the agency 

mission. For these applications, the use of public key technology must be justified in terms of its 

direct benefit to a specific agency application. Another option is for the agency to use 

commercial CA service providers to issue certificates and facilitate delivery of services. Agency 

projects would then pay the commercial CA service provider. 

The main issue for the Federal PKI is how to create certification paths between Federal 

agencies that will provide for reliable and broad propagation of trust. A Bridge CA (BCA) 

provides systematic certification paths between CAs in agencies, and outside the government. 

Federal CAs that meet certain standards and requirements will be eligible to cross-certify with 

the FBCA, thereby gaining the certification paths needed to establish interoperation between the 

Federal and commercial PKIs. Figure 4 illustrates how bridge CAs will provide interoperability 

between Federal and other PKIs. 

Figure 4. Interoperation Between Federal and Non-Federal PKIs 

The Federal BCA will be the unifying element to link otherwise unconnected agency CAs into a 

systematic overall Federal PKI. It is important to note that the BCA is not a root CA, but is a 

“bridge of trust”. It does not begin certification paths, but connects trust domains through cross 

certificates between the FBCA and designated principal CAs. A federal policy management 

authority (FPMA) will supervise FBCA operation and establish the requirements for cross 

33 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

certifying with the FBCA. These trust domains may be within the government or outside the 

government. 

Federal (or non-federal) CAs that operate in trust domains that meet the requirements 

established by the FPMA will be eligible to cross certify with the FBCA. The FBCA will then 

connect them to the overall trust network of the Federal PKI. This will provide relying parties and 

certificate holders (in their trust domains) with connectivity to the larger Federal PKI. This will be 

simpler and more effective than trying to manage an ad hoc collection of many cross 

certifications with CAs in other trust domains. 

To provide maximum flexibility to Federal agencies and not intrude upon their prerogatives, 

agencies will not be required to adopt the FBCA’s policies. Rather, agencies will retain the 

option to use other policies defined by their own internal PMAs, or by commercial certificate 

service providers. Agencies will not be required to use the FBCA to interoperate with other 

Federal agencies or organizations outside the Federal government. Alternatively, Federal 

agencies may communicate directly with an agency/organization to establish requirements for 

interoperating. Components of the Federal PKI architecture are: 

Federal Policy Management Authority (FPMA): this management authority sets the overall 

policies of the Federal PKI and approves the policies and procedures of trust domains within the 

Federal PKI. It operates a Federal Bridge CA, and a repository. 

Trust Domains: In the Federal context, a trust domain is a portion of the Federal PKI that 

operates under the management of a single policy management authority. One or more CAs 

exist within each trust domain. Each trust domain has a single principal CA, but may have many 

other CAs. Each trust domain has a domain repository. 

Domain Policy Management Authority (DPMA): a policy management authority approves the 

certification practice statements of the CAs within a trust domain and monitors their operation. 

The DPMAs operate or supervise a domain repository. 

Federal Bridge CA (FBCA): The Federal Bridge CA is operated by the FPMA. Its purpose is to 

be a bridge of trust that provides trust paths between the various trust domains of the Federal 

PKI, as well as between the Federal PKI and non-Federal trust domains. FPMA- approved trust 

domains designate a principal CA that is eligible to cross-certify with the Federal FBCA. The 

FBCA is not a root CA because it does not typically begin certification paths. 

Principal CA: A CA within a trust domain that cross-certifies with the Federal BCA. Each trust 

domain has one principal CA. In a domain with hierarchical certification paths, it will be the root 

CA of the domain. In a mesh-organized domain, the principal CA may be any CA in the domain. 

However it will typically be one operated by or associated with the DPMA. 

Peer CA: A CA in a mesh domain, a peer CA has a self-signed certificate that is distributed to 

its certificate holders and used by them to initiate certification paths. Peer CAs cross-certify with 

other CAs in their trust domain. 

Root CA: In a hierarchical trust domain, the root CA is the CA that initiates all trust paths. 

Certificate holders and relying parties are given the self-signed root CA certificate by some 

authenticated means and all trust paths are initiated from that point. For hierarchical trust 

domains, the root CA is also the principle CA for that domain. 

Subordinate CA: A CA in a hierarchical domain that does not begin trust paths. Trust initiates 

from some root CA. In a hierarchical trust domain, a subordinate CA receives a certificate from 

its superior CA. A subordinate CA may have subordinate CAs of its own to which it issues 

certificates. 

34 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5.2 FEDERALCERTIFICATE PROFILE(S)

In the interest of establishing commonality and interoperability with PKI communities outside the 

Federal government, it was decided that the FPKI profile should be based on a “standard PKI 

profile” but still contain the unique parameter settings for Federal systems2

. The only widely 

accepted PKI profile currently on track to become a standard is the Internet Engineering Task 

Force (IETF) Public Key Infrastructure (PKIX) profile (RFC 2459) developed by the PKIX 

working group. The profile can be found at http://www.ietf.org/rfc/rfc2459. The PKIX profile, 

Internet X.509 Public Key Infrastructure Certificate and CRL Profile, identifies the format and 

semantics of certificates and CRLs for the Internet PKI. Procedures are described for 

processing and validating certification paths in the Internet environment. Encoding rules are 

provided for cryptographic algorithms and all fields are profiled in both the version 3 X.509 and 

version 2 CRL. 

The FPKI profile complements the current PKIX profile and stipulates any differences between 

the two. If an organization needs to implement a subset of the FPKI-compliant certificate and/or 

CRL, the organization should tailor its X.509 certificate and/or CRL using the parameters 

stipulated in the Federal PKI together with the parameters stipulated in PKIX. Parameters 

stipulated in the Federal PKI document should take precedence. An organization deciding to 

tailor its FPKI-compliant X.509 certificate and/or CRLs to meet its specific needs must document 

the intended subset profile (referencing FPKI profile as a basis) so that the certificate generation 

element will know how to populate the program-specific certificates [Fed PKI profile]. 

The federal certificate Profile describes the contents of five classes of X.509 certificates: end 

entity digital signature certificates; end entity key management certificates; CA certificates, 

Bridge CA certificates, and self-signed CA certificates. 

•	 End entity digital signature certificates contain a public key that is designated for
 

validation of digital signatures on objects other than certificates and CRLs.
 

•	 End entity key management certificates contain a public key that is designated for
 

key transport.
 

•	 CA certificates are any certificates issued by an agency CA to another CA. 

•	 Bridge CA certificates are issued by the federal Bridge CA to agency principal CAs. 

•	 Self-signed CA certificates are generated by CAs for secure distribution to their
 

subscribers.
 

The following table identifies mandatory and optional extensions for each class of certificate. 

Further details are contained in the profile; agencies should obtain a copy of the current profile, 

as well as RFC 2459, before deploying their PKI. 

2 Portions of this discussion were derived from the Federal Certificate Profile, http://csrc.nist.gov/pki/twg. 

35 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Certificate 

extension 

Mandatory or Optional Contents 

Key usage Appears in all certificates Specifies the set of generic uses for 

this key. For users or systems, the 

key may be used for digital signatures 

on documents, authentication, key 

management, or data encipherment. 

For CAs, the keys may be used for 

verifying signatures on certificates or 

CRLs. 

Basic constraints Appears in all CA certificates Indicates the subject is a CA. May 

define the maximum number of 

additional certificates in a path. This 

is used to limit trust to end entity 

certificate issued by the subject. 

Authority key 

identifier 

Appears in all certificates SHA-1 hash of the public key which 

verifies the signature on the 

certificate; used to select the right key 

Subject key 

identifier 

Appears in all CA certificates. 

May optionally appear in end 

entity certificates. 

SHA-1 hash of this public key; this 

matches the authority key identifier in 

certificates and CRLs issued by the 

subject. 

Subject 

alternative name 

Optional, but commonly used. Used to specify a user’s email 

address, or the DNS name or IP 

address of an Internet host. This 

information supports the S/MIME 

application and IPsec protocols, 

respectively. 

Certificate policies Appears in all certificates In an end entity certificate, the 

policies listed describe the level of 

trust in this certificate. In a CA 

certificate, this list describes the 

range of policies for which its 

certificates may be trusted. 

Policy mapping Appears only in CA certificates. 

Appears only when the issuer and 

subject issue certificates under 

different policies. 

Describes which of the subject CA’s 

policies correspond to the issuer’s 

local policies. 

Name constraints Appears optionally in CA 

certificates. 

Used to limit the set of trusted 

certificates by name. 

Policy constraints Appears optionally in CA 

certificates. 

Used to impose policy requirements 

and/or prohibit policy mapping. 

CRL distribution 

points 

Optional. Appears in all 

certificates whose revocation 

status is distributed by indirect 

CRLs or segmented CRLs. 

Used to identify the CRL that covers 

this particular certificate. 

Certificate extensions in the federal PKI 

36 
37

5.3 FEDERAL CRL PROFILE(S) 

The Federal PKI Profile specifies two CRL profiles - one for the FBCA, and a second for all other 

CAs in the Federal PKI. Both profiles are based on the X.509 Version 2 CRL. The version 2 

CRL may include two different types of extensions. 

The first type is the CRL entry extension, which provides additional information about a particular 

revoked certificate. The CRL entry extensions identified in the CRL profiles are the certificate 

issuer and reason code extensions. By default, all of the certificates listed on a CRL were 

issued by the CA that generated the CRL. If this is not true, the CRL will use the certificate 

issuer to identify the CA that issued the certificate. The reason code extension describes the 

circumstances for revocation. A relying party may accept a certificate that has been 

“superseded,” but reject a certificate that was revoked because of “key compromise.” 

The second type is the CRL extension, which provides additional information about the entire 

CRL. The CRL extensions identified by the federal profile are authority key identifier, issuer 

alternative name, CRL number, and issuing distribution point. 

The following tables identify mandatory and optional extensions for each class of CRLs. As 

noted above, further details are contained in the profile. Agencies should obtain a copy of the 

current profile, as well as RFC 2459, before deploying their PKI. 

CRL Extension Mandatory 

or Optional 

Contents 

CRL number Appears in all 

CRLs 

Monotonically increasing integer; may be used to detect 

emergency CRL generation 

Authority key 

identifier 

Appears in all 

CRLs 

SHA-1 hash of the public key which verifies the signature 

on the CRL; used to select the right key 

Issuer alternative 

name 

optional Can be used to specify the CA’s email address 

Issuing 

distribution point 

Appears in all 

indirect and 

segmented 

CRLs 

Contents must match the CRL distribution points 

extension in the certificates covered by the CRL 

CRL Extensions in the Federal PKI 

CRL entry 

extension 

Mandatory 

or Optional 

Contents 

Reason code Appears for 

every 

certificate, 

unless the 

CA has no 

information 

Specifies if the certificate was revoked because the key 

were compromised, the subject’s affiliation changed, or it 

was superseded by a newer certificate. 

Certificate issuer Appears in all 

indirect CRLs 

Identifies the issuer for a subset of the certificates listed in 

this CRL. 

CRL Entry Extensions in the Federal PKI
38

6 DEPLOYING AN AGENCY PKI 

Agency PKIs will, in most cases, be developed in some degree of isolation. The PKI will be 

developed to meet internal requirements, where both the subscriber and the relying party are 

users within that agency. As the agency’s confidence and experience increases, it may become 

cost effective to integrate the agency PKI into the Federal PKI. Adequate planning can ensure 

that an agency PKI is ready to join the broader Federal PKI, providing access to security 

services with a growing community of users. This section recommends steps for setting up an 

agency PKI. 

There are numerous impediments and challenges in creating an agency PKI that will 

interoperate with other federal PKI's. Therefore, it is important to as much as possible follow 

existing standards and coordinate with the Federal PKI Steering Committee (FPKISC). Some of 

the challenges an agency will face include the significant cost that can be associated with 

implementing a PKI, including the cost of creating and distributing certificates, purchasing or 

creating client software, and in maintaining and supporting users of the PKI. As well, different 

vendors may implement standards differently, and therefore interoperation of directories and 

other associated software between PKI's will be tricky. 

Accordingly, NIST recommends that agencies follow the steps in this chapter when deploying an 

agency PKI or PKI pilot. In particular, NIST recommends following the FBCA policy and 

adhering to the federal certificate profile and CRL extensions profile. It would be best if 

agencies assume that at some point their PKI will cross-certify with the federal bridge CA, 

therefore coordinating the development of an agency PKI with the FPKISC is highly 

recommended. The following sections contain information and steps for developing an agency 

PKI such that interoperability problems will be minimized. 

6.1 ANALYZE DATA AND APPLICATIONS FOR YOUR ORGANIZATION

Installing a PKI can have a significant impact on the security model of an information technology 

operation. As with most security planning, the agency PKI must be designed using the familiar 

principles of Risk Management. Planning should begin with risk analysis. In addition to 

comparing the initial and operating costs of the PKI with anticipated cost-reductions, the costbenefit analysis should attempt to identify larger risks from not implementing a PKI. 

The next step is to identify the data and applications that need to be secured within the agency’s 

computing system. Data may include data used during execution, stored data on magnetic 

media, printed data, archival data, update logs, audit records, and documentation. Applications 

may include local/network communications, access controls, and Internet applications. The 

analysis should also determine the impact if security is compromised, and the degree of risk will 

determine the appropriate level of assurance for the agency PKI. For example, the more limited 

the life of a certificate, the lower the risk exposure for the issuing CA. More than one policy may 

be required if there is a wide variation of risk associated with different applications. 

Record retention is always a concern for government agencies, and the movement toward 

paperless operations will result in significant changes in long-term storage of records. The 

National Archives and Records Administration (NARA) has developed guidance on records 

management for agencies implementing electronic signature technologies [NARA00]. The 

NARA guidance should be consulted during the planning phase to ensure proper record 

retention practices. 
39

6.2 COLLECT SAMPLE POLICIES AND BASE STANDARDS

It is efficient to begin the development of the PKI by collecting sample policies and using them as 

templates to develop the agency’s own policy(s). Collections of standards are also required for 

writing your policy(s) because standards are the basis for achieving interoperability between 

agencies. The following is a list the standards and their references. 

FBCA policy FBCA policy is used for determining which level(s) are consistent with 

the agency’s requirements. Current FBCA policy can be found in 

http://csrc.nist.gov/pki/twg/y2000/doc_reg_00.htm – Bridge Certification Authorities. 

Final policy will be posted at http://csrc.nist.gov/csor.

• FPKI X.509 Certificate and CRL Extensions Profile. As part of FBCA policy, this 

document specifies the version 3 (v3) X.509 certificate and version 2 (v2) Certificate 

Revocation List (CRL) profiles for Federal Public Key Infrastructure (FRKI) systems. The 

profiles serve to identify unique parameter settings for FPKI. The Federal certificate 

profile is located at http://csrc.nist.gov/pki/twg/y2000/papers/twg-00-18.xls —Federal 

Public Key Infrastructure (PKI) X.509 Certificate and CRL Extensions Profile. 

• X.500 directories The certificates or other digitally signed instruments issued by the 

FBCA and, where applicable, agency CAs, will reside in the FBCA X.500 directory. 

Directory references are listed at http://csrc.nist.gov/pki/twg/directory_references.htm. 

6.3 DRAFT CERTIFICATE POLICY(S)

The first requirement for an agency developing a PKI is to establish appropriate certificate 

policy(s) (CP). The policy(s) must reflect the types of applications that will be secured by the 

PKI. An effective strategy is to adapt and reuse existing policies (especially FBCA) to create 

policy(s) for the agency. Certificate policies should be at a sufficiently high level that the policies 

will not change too frequently. The format and content of these policies is discussed in further 

detail in section 6.3.1. 

The set of policies under which a CA issues certificates is termed its trust domain or policy 

domain. The agency needs to obtain an object identifier (OID) for each of the policies in its 

trust domain. These OIDs will be used to differentiate the appropriate set of applications for a 

particular certificate. An X.509 v3 certificate may state one or more certificate policies in the 

certificatePolicy extension. A certificatePolicy extension contains one or more 

policyIdentifiers. A policyIdentifier is a unique, registered OID that represents a certificate 

policy in a certificate. Applications may use these policies to decide whether or not to trust a 

certificate for a particular purpose. The registration process follows the procedures specified in 

ISO/IEC, (i.e., joint International Organization for Standardization and International 

Electrotechnical Commission), and International Telecommunications Union (ITU) standards. 

The application that registers the OID also needs a textual specification of the certificate policy, 

for examination by certificate users and other applications. If an agency issues under a single 

policy, it should still obtain an OID for that policy. When the CA joins the federal PKI, it will be 

used to differentiate its certificates from the many policies used in the federal government. 

Procedures to obtain an OID will be covered in section 6.3.2. 
40

6.3.1 Certificate Policies 

Policies are generally written in standard format. RFC 2527, the Certificate Policy and 

Certification Practices Framework, defines the accepted standard CP format. RFC 2527 

includes a standard outline with eight major sections and 185 second and third level 

subsections. Most CPs are written to this outline, since the standard format has a number of 

distinct advantages. The FBCA CP is consistent with RFC 2527. 

By adhering to a well-defined format, the CP writer is less likely to forget something important. It 

would be easy to overlook a few of the 185 topics identified in RFC 2527 if the author changed 

the outline. Adhering to the standard format will also simplify cross-certification with other CAs. 

The cross-certification process should always include a comparison of the other CA’s certificate 

policies. This information is used to determine the contents of the policy mappings and policy 

constraints extensions to be included in the CA certificates. The eight major sections are 

summarized below; for details the reader should obtain RFC 2527. 

• The INTRODUCTION explains how to identify certificates issued under this policy 

(i.e., the OID that will appear in the policy extension), defines the community for 

these certificates (e.g., NIST employees, or financial managers,) and provides 

contact information for the people who administer the CA and maintain the policy. 

• The GENERAL PROVISIONS captures broadly applicable legal and general 

practices information. For example, this section identifies the various participants in 

the PKI (e.g., CA, RAs, subscriber, and relying party) and their various obligations 

and liabilities. It identifies the applicable laws, fees, and auditing requirements. 

This section also describes what information (if any) will be considered confidential 

and the circumstances that would justify disclosure (e.g., a subpoena.). 

• IDENTIFICATION AND AUTHENTICATION describes the procedures used to 

authenticate requests for certificates, or for certificate revocation. 

• OPERATIONAL REQUIREMENTS describes the operations that must be 

performed by the CA, RAs, end entities, or other parties under this policy. Specific 

actions are identified that must be performed when requesting or generating new 

certificates, revoking certificates, creating and protecting audit logs, archiving 

records, changing the CA’s key, disaster recovery, and terminating the CA’s 

operations. 

• PHYSICAL, PROCEDURAL, AND PERSONNEL SECURITY CONTROLS

describes how the PKI uses physical security (e.g., guards, guns, and gates), 

procedures (e.g., separation of duty), and personnel requirements (e.g., background 

checks and procedures) to complement the technical security controls. 

• TECHNICAL SECURITY CONTROLS describes the security measures used to 

protect cryptographic keys (e.g., a FIPS 140-1 validated hardware module), protect 

critical security parameters (such as the list of trusted RAs), and provide quality 

assurance for the systems (e.g., a NIAP evaluation), and protect the CA from 

network-based attacks. 

• CERTIFICATE AND CRL PROFILES specifies the certificate and CRL profile. This 

section specifies the cryptographic algorithms that will be used to sign the 

certificates, the length of the signing key, and the name forms that will appear in 

certificates. It describes the extensions that are included in certificates and CRLs. 

For federal agencies, this should correspond closely to the federal profile discussed 

above. 

• SPECIFICATION ADMINISTRATION is the final section, and it describes how the 

policy will be maintained. It describes the procedures that will be followed if the 

specification is changed, how those modifications will be published, and the 

approval procedures. 
41

CP authors should not develop their documents in a vacuum. CP authors should search the 

available CPs and identify CPs with similar scope and requirements. These CPs should be 

used as inputs to the CP development process. Example Certificate Policies are available from 

a number of sources, including U.S. Federal CPs that may be found at 

http://csrc.nist.gov/csor/pkireg.htm. 

6.3.2 Computer Security Objects Registry 

As noted above, government agencies should include an appropriate policy OID in all certificates 

that it issues. NIST maintains the Computer Security Objects Register (CSOR), one of the 

CSOR’s functions is the assignment of object identifiers for PKI certificate policies. 

The CSOR has allocated the following registration branch for Public Key Infrastructure (PKI) 

objects: 

csor-certpolicy={joint-iso-ccitt(2) country(16) us(840) organization(1) gov(101) csor(3) pki(2) 

cert-policy(1)}. 

Government agencies may obtain OIDs in this arc by submitting a RFC 2527-formatted 

Certificate Policy to csor@nist.gov. 

In addition, NIST has defined a set of OIDs that may be used during pilot testing of PKIs. The 

policies associated with these OIDs have no meaning, but may be used to verify the correct 

operation of certificate policy mechanisms. 

6.3.3 Establishing Policy Mappings and Constraints 

As agencies progress from a single isolated CA to more sophisticated architectures, they will 

establish trust relationships between certification authorities. These relationships are 

manifested as CA certificates. Where CAs issue certificates under common policies, the 

contents of these certificates are straightforward. The certificate policy extension contains the 

OIDs for each of the policies shared by the two CAs. There is no need for policy mapping, or 

policy constraints. 

When two CAs issue certificates in different policy domains, procedures become more 

complicated. Each CA must review the other CA’s policies, and determine which of their own 

policies (if any) they satisfy. These policy relationships are encoded in the policy mapping 

extensions. If one of the other CA’s policies is deemed entirely unacceptable, a CA may include 

policy constraints in the CA certificate it issues. This extension permits a CA to specify a limited 

set of policies that are acceptable. As a result, certificates issued under the unacceptable policy 

will be rejected. 

6.3.4 Local certificate and CRL profile(s) 

To maximize interoperability, the local certificate and CRL profile must be consistent with the 

Federal Certificate and CRL Profile described above. That is, the certificate issued by a federal 

agency must contain all required fields and extensions. The federal agency may mandate 

inclusion of optional features and add private extensions. However, the private extensions must 

never be marked as critical. Certificates with unrecognized critical extensions are ignored, so 

marking private extensions as critical would limit interoperability. 
42

The following table describes consistency: 

Federal Profile A Consistent Local Profile 

Basic certificate 

fields 

Does not use unique identifiers Does not use unique identifiers 

Mandatory extensions Mandatory; must match in criticality 

Optional extensions Mandatory, optional, or never 

populated. If present must match 

criticality setting in federal profile. 

Standard (ISO or 

IETF) extensions 

Omitted extensions Must not be used if critical 

Private 

extensions 

None Must be noncritical 

6.4 SELECT PKI PRODUCT OR SERVICE PROVIDER

The next step after planning is to select the appropriate PKI product or service provider. The 

agency needs to review the candidate products or service providers to determine which can 

implement the agency’s policy(s). The following lists aspects to be considered for the selection. 

• Compatibility and interoperability with other PKI products/service providers. 

• Ease of adoption to open standards. 

• Minimum proprietary application program interface (API) dependency. 

• Ease of supporting applications such as virtual private networks, access control, 

secure e-commerce, smartcard management, smartcards and hardware, 

directories, secure messaging, secure forms, enterprise, and others. 

• Ease of deployment. 

• Flexibility of administration. 

• Scalability and portability of installation. 

6.5 DEVELOP CPS (CERTIFICATION PRACTICE STATEMENT) 

After the selection of vendor product or service provider the agency need to develop a highly 

specific document, a CPS (Certification Practice Statements), that describes how an agency (or 

service provider) will implement the policy(s) developed in 6.3. A CPS is a statement of the 

practices that a particular CA employs in issuing certificates. A CPS describes the details of the 

system used and the practices employed by a CA to issue certificates, and it details the 

procedures used to implement the policies identified in the certificates issued by a CA, including 

the means used to identify certificate subjects. The CPS also states the means used to protect 

the private key of the CA, and the other operational practices followed by the CA to ensure 

security. Each Federal CA will post its CPS in the BCA Repository, and also post the CPS in any 

repository associated with the CA. The basic list of topics to be covered is the same for both a 

certificate policy (CP) and a CPS as stated in Section 6.3.1. 
43

6.6 DO A PILOT

PKIs are non-trivial. It is recommended that the agency start by supporting a limited number of 

users and use it for internal applications first. During the pilot the following operations need to be

exercised: 

• Set up test accounts (users) for all application that will use the PKI. 

• Test all the administration operations to make sure they all work properly. 

• Shut down the system, bring it back up, and check that everything works correctly. 

• Test all PKI functions of the applications locally and remotely (through network if 

applicable). 

• Make sure that the agency has the physical security and personnel controls in place 

to support the PKI. 

• Repeat 6.3, 6.4, and 6.5 to incorporate lessons learned. 

6.7 APPLY FOR CROSS CERTIFICATION WITH THE FBCA 

Once the PKI and critical applications work well internally the agency may choose to crosscertify directly with the agencies it commonly deals with. However, the most efficient mechanism 

for joining the federal PKI is cross-certification with the FBCA. To join the FBCA, one of the 

important steps is to establish policy mappings and constraints as described in 6.3.1 

If the agency wishes to cross-certify its PKI with the FBCA, the first step is to select a principal 

CA. If the agency PKI has only one CA, it is the principal CA. If the agency PKI is a hierarchy, 

the principal CA must be the root CA. If the agency PKI is a mesh, the agency is free to select 

any of its CAs as the principal CA. 

The FBCA Policy Management Authority processes requests to cross-certify with the bridge. 

While procedures are still being finalized, the FBCA will require an agency to submit its CP(s) 

and may request an independent audit to ensure the CP is being implemented faithfully. The 

FBCA will determine a mapping between its four certificate policies, which specify four 

increasing levels of assurance, and the policies that comprise the agency PKI’s trust domain. 

Once an agency PKI cross-certifies with the FBCA, that agency is given a seat in the FBCA 

Policy Authority. 
44

7 SUMMARY AND CONCLUSIONS 

As Federal agency operations are moved online, information technology security services based 

on cryptography become essential. Public key cryptography can play an important role in 

providing needed security services including confidentiality, integrity, authentication, and digital 

signatures. Public key cryptography uses two electronic keys: a public key and a private key. 

The public key can be known by anyone while the private key is kept secret by its owner. 

Public key cryptography is straightforward to implement for a pair of users and a single 

application. This technology will scale easily to support a few applications or a small community 

of users. However, as the community grows, it becomes difficult to distribute the public keys 

and keep track of the user that owns the corresponding private key. To use public key 

cryptography on a broad scale, users need the support of a security infrastructure to manage 

public keys. 

A public key infrastructure (PKI) allows public key cryptography to be employed on a broad 

scale. With a PKI, parties who have not met in person are able to engage in verifiable 

transactions. The identity of the originator of a message can be traced to the owner of the 

private key as long as there is strong binding between the owner and the owner’s public key. A 

PKI provides the means to bind public keys to their owners and helps in the reliable distribution 

of public keys in large heterogeneous networks. Public keys are bound to their owners by public 

key certificates. These certificates contain information such as the owner’s name and the 

associated public key and are issued by a reliable Certification Authority (CA). 

A PKI is often composed of many CAs linked by trust paths. The CAs may be linked in several 

ways. They may be arranged hierarchically under a "root CA" that issues certificates to 

subordinate CAs. The CAs can also be arranged independently in a network. Recipients of a 

signed message with no relationship with the CA that issued the certificate for the sender of the 

message can still validate the sender’s certificate by finding a path between their CA and the 

one that issued the sender’s certificate. 

The confidence that can be placed on the binding between a public key and its owner depends 

much on the confidence that can be placed on the CA that issued the certificate that binds them. 

Provisions in the X.509 standard enable the identification of policies that indicate the strength of 

mechanisms used and the do’s and don’ts of certificate handling. The rules expressed by 

certificate policies are reflected in certification practice statements (CPSs) that detail the 

operational rules and system features of CAs and other PKI components. By examining the 

policies associated with a sender’s certificate, the recipient of a signed or encrypted message 

can determine whether the binding between the sender and the sender’s key is acceptable and 

thus accept or reject the message. By examining a CA’s CPS, users can determine whether to 

obtain certificates from it, based on their security requirements. Other CAs can also use the 

CPS to determine if they want to cross-certify with that CA. 

This publication may be used to assist Federal agency decision-makers in determining if a PKI 

is needed by their agency, and how PKI services can be deployed most effectively within an 

agency. It provides an overview of PKI functions and their applications. Additional 

documentation will be required to fully analyze the costs and benefits of PKI systems for agency 

use, and to develop plans for their implementation. This document provides a starting point and 

references to more comprehensive publications. 
45

8 ACRONYMSANDABBREVIATIONS 

ACES Access Certificates for Electronic Services 

API Application programming interface 

ARL Authority Revocation List 

CA Certification Authority 

CP Certificate Policy 

CPS Certification Practice Statement 

CRL Certificate Revocation List 

CSOR Computer Security Object Registry 

DN Distinguished Name 

DSA Digital Signature Algorithm 

DSS Digital Signature Standard 

ECA External certification authority 

ERC Enhanced Reliability Check 

FAR Federal Acquisition Regulations 

FBCA Federal Bridge Certification Authority 

FBCA OA Federal Bridge Certification Authority Operational Authority 

FED-STD Federal Standard 

FIPS PUB Federal Information Processing Standard Publication 

FPKISC Federal PKI Steering Committee 

FPKIPA Federal PKI Policy Authority 

GITSB Government Information Technology Services Board 

GPEA Government Paperwork Elimination Act of 1998

IETF Internet Engineering Task Force 

ISO International Organization for Standardization 

ITU International Telecommunications Union 

ITU-T International Telecommunications Union – Telecommunications Sector 

ITU-TSS International Telecommunications Union – Telecommunications System 

Sector 

MOA Memorandum of Agreement (as used in the context of this CP, between an 

Agency and the FPKIPA allowing interoperation between the FBCA and 

Agency Principal CA) 

NIST National Institute of Standards and Technology 

NSA National Security Agency 

OID Object Identifier 

PIN Personal Identification Number 

PKI Public Key Infrastructure 

PKIX Public Key Infrastructure X.509 

RA Registration Authority 

RFC Request For Comments 

RSA Rivest-Shamir-Adleman 

SHA-1 Secure Hash Algorithm, Version 1 

SSL Secure Sockets Layer 

URL Uniform Resource Locator 

U.S.C. United States Code 

WWW World Wide Web 
46

9 GLOSSARY 

Access Ability to make use of any information system (IS) resource. 

Access 

Control 

Process of granting access to information system resources only to authorized users, 

programs, processes, or other systems. 

Accreditation Formal declaration by a Designated Approving Authority that an Information System 

is approved to operate in a particular security mode using a prescribed set of 

safeguards at an acceptable level of risk. 

Activation Data Private data, other than keys, that are required to access cryptographic modules (i.e., 

unlock private keys for signing or decryption events). 

Agency Any department, subordinate element of a department, or independent organizational 

entity that is statutorily or constitutionally recognized as being part of the Executive 

Branch of the Federal Government. 

Agency CA A CA that acts on behalf of an Agency, and is under the operational control of an 

Agency. 

Applicant The subscriber is sometimes also called an "applicant" after applying to a certification 

authority for a certificate, but before the certificate issuance procedure is completed. 

Archive Long-term, physically separate storage. 

Attribute 

Authority 

An entity, recognized by the Federal PKI Policy Authority or comparable Agency body 

as having the authority to verify the association of attributes to an identity. 

Audit Independent review and examination of records and activities to assess the 

adequacy of system controls, to ensure compliance with established policies and 

operational procedures, and to recommend necessary changes in controls, policies, 

or procedures. [NS4009] 

Audit Data Chronological record of system activities to enable the reconstruction and 

examination of the sequence of events and changes in an event. [NS4009, "audit 

trail"] 

Authenticate To confirm the identity of an entity when that identity is presented. 

Authentication Security measure designed to establish the validity of a transmission, message, or 

originator, or a means of verifying an individual's authorization to receive specific 

categories of information. [NS4009] 

Backup Copy of files and programs made to facilitate recovery if necessary. [NS4009] 

Binding Process of associating two related elements of information. [NS4009] 

Biometric A physical or behavioral characteristic of a human being. 

Certificate A digital representation of information which at least (1) identifies the certification 

authority issuing it, (2) names or identifies its subscriber, (3) contains the subscriber's 

public key, (4) identifies its operational period, and (5) is digitally signed by the 

certification authority issuing it. [ABADSG]. As used in this CP, the term “Certificate” 

refers to certificates that expressly reference the OID of this CP in the “Certificate 

Policies” field of an X.509 v.3 certificate. 
47

Certification 

Authority (CA) 

An authority trusted by one or more users to issue and manage X.509 Public Key 

Certificates and CARLs or CRLs. 

Certification 

Authority 

Revocation 

List (CARL) 

A signed, time-stamped list of serial numbers of CA public key certificates, including 

cross-certificates, that have been revoked. 

CA Facility The collection of equipment, personnel, procedures and structures that are used by a 

Certification Authority to perform certificate issuance and revocation. 

Certificate A digital representation of information which at least (1) identifies the certification 

authority issuing it, (2) names or identifies it’s Subscriber, (3) contains the 

Subscriber’s public key, (4) identifies it’s operational period, and (5) is digitally signed 

by the certification authority issuing it. [ABADSG]

Certificate 

Management 

Authority 

(CMA) 

A Certification Authority or a Registration Authority. 

Certification 

Authority 

Software 

Key Management and cryptographic software used to manage certificates issued to 

subscribers. 

Certificate 

Policy (CP) 

A Certificate Policy is a specialized form of administrative policy tuned to electronic 

transactions performed during certificate management. A Certificate Policy 

addresses all aspects associated with the generation, production, distribution, 

accounting, compromise recovery and administration of digital certificates. Indirectly, 

a certificate policy can also govern the transactions conducted using a 

communications system protected by a certificate-based security system. By 

controlling critical certificate extensions, such policies and associated enforcement 

technology can support provision of the security services required by particular 

applications. 

Certification 

Practice 

Statement 

(CPS) 

A statement of the practices that a CA employs in issuing, suspending, revoking and 

renewing certificates and providing access to them, in accordance with specific 

requirements (i.e., requirements specified in this CP, or requirements specified in a 

contract for services). 

CertificateRelated 

Information 

Information, such as a subscriber's postal address, that is not included in a 

certificate. May be used by a CA managing certificates. 

Certificate 

Revocation 

List (CRL) 

A list maintained by a Certification Authority of the certificates which it has issued that 

are revoked prior to their stated expiration date. 

Certificate 

Status 

Authority 

A trusted entity that provides on-line verification to a Relying Party of a subject 

certificate's trustworthiness, and may also provide additional attribute information for 

the subject certificate. 

Client 

(application) 

A system entity, usually a computer process acting on behalf of a human user, that 

makes use of a service provided by a server. 

Common 

Criteria 

A set of internationally accepted semantic tools and constructs for describing the 

security needs of customers and the security attributes of products. 
48

Compromise Disclosure of information to unauthorized persons, or a violation of the security policy 

of a system in which unauthorized intentional or unintentional disclosure, 

modification, destruction, or loss of an object may have occurred. 

Computer 

Security 

Objects 

Registry 

(CSOR) 

Computer Security Objects Registry operated by the National Institute of Standards 

and Technology. 

Confidentiality Assurance that information is not disclosed to unauthorized entities or processes. 

[NS4009] 

CrossCertificate 

A certificate used to establish a trust relationship between two Certification 

Authorities. 

Cryptographic 

Module 

The set of hardware, software, firmware, or some combination thereof that 

implements cryptographic logic or processes, including cryptographic algorithms, and 

is contained within the cryptographic boundary of the module. [FIPS1401] 

Cryptoperiod Time span during which each key setting remains in effect. 

Data Integrity Assurance that the data are unchanged from creation to reception. 

Digital 

Signature 

The result of a transformation of a message by means of a cryptographic system 

using keys such that a Relying Party can determine: (1) whether the transformation 

was created using the private key that corresponds to the public key in the signer’s 

digital certificate; and (2) whether the message has been altered since the 

transformation was made. 

Dual Use 

Certificate 

A certificate that is intended for use with both digital signature and data encryption 

services. 

Duration A field within a certificate that is composed of two subfields; “date of issue” and “date 

of next issue”. 

E-commerce The use of network technology (especially the internet) to buy or sell goods and 

services. 

Employee Any person employed by an Agency as defined above. 

Encrypted 

Network 

A network on which messages are encrypted (e.g. using DES, AES, or other 

appropriate algorithms) to prevent reading by unauthorized parties. 

Encryption 

Certificate 

A certificate containing a public key that is used to encrypt electronic messages, files, 

documents, or data transmissions, or to establish or exchange a session key for 

these same purposes. 

End Entity Relying Parties and Subscribers. 

Federal Bridge 

Certification 

Authority 

(FBCA) 

The Federal Bridge Certification Authority consists of a collection of Public Key 

Infrastructure components (Certificate Authorities, Directories, Certificate Policies and 

Certificate Practice Statements) that are used to provide peer to peer interoperability 

among Agency Principal Certification Authorities. 

Federal Bridge 

Certification 

Authority 

Membrane 

The Federal Bridge Certification Authority Membrane consists of a collection of Public 

Key Infrastructure components including a variety of Certification Authority PKI 

products, Databases, CA specific Directories, Border Directory, Firewalls, Routers, 

Randomizers, etc. 
49

FBCA 

Operational 

Authority 

The Federal Bridge Certification Authority Operational Authority is the organization 

selected by the Federal Public Key Infrastructure Policy Authority to be responsible 

for operating the Federal Bridge Certification Authority. 

Federal Public 

Key 

Infrastructure 

Policy 

Authority (FPKI 

PA) 

The Federal PKI Policy Authority is a federal government body responsible for 

setting, implementing, and administering policy decisions regarding interagency PKI 

interoperability that uses the FBCA. 

Firewall Gateway that limits access between networks in accordance with local security 

policy. [NS4009] 

High 

Assurance 

Guard (HAG) 

An enclave boundary protection device that controls access between a local area 

network that an enterprise system has a requirement to protect, and an external 

network that is outside the control of the enterprise system, with a high degree of 

assurance. 

Information 

System 

Security 

Officer (ISSO) 

Person responsible to the designated approving authority for ensuring the security of 

an information system throughout its lifecycle, from design through disposal. 

[NS4009] 

Inside threat An entity with authorized access that has the potential to harm an information system 

through destruction, disclosure, modification of data, and/or denial of service. 

Integrity Protection against unauthorized modification or destruction of information. [NS4009]. 

A state in which information has remained unaltered from the point it was produced 

by a source, during transmission, storage, and eventual receipt by the destination. 

Intellectual 

Property 

Useful artistic, technical, and/or industrial information, knowledge or ideas that 

convey ownership and control of tangible or virtual usage and/or representation. 

Intermediate 

CA 

A CA that is subordinate to another CA, and has a CA subordinate to itself. 

Key Escrow A deposit of the private key of a subscriber and other pertinent information pursuant 

to an escrow agreement or similar contract binding upon the subscriber, the terms of 

which require one or more agents to hold the subscriber's private key for the benefit 

of the subscriber, an employer, or other party, upon provisions set forth in the 

agreement. [adapted from ABADSG, "Commercial key escrow service"] 

Key Exchange The process of exchanging public keys in order to establish secure communications. 

Key 

Generation 

Material 

Random numbers, pseudo-random numbers, and cryptographic parameters used in 

generating cryptographic keys. 

Key Pair Two mathematically related keys having the properties that (1) one key can be used 

to encrypt a message that can only be decrypted using the other key, and (ii) even 

knowing one key, it is computationally infeasible to discover the other key. 

Local 

Registration 

Authority 

(LRA) 

A Registration Authority with responsibility for a local community. 

Memorandum Agreement between the Federal PKI Policy Authority and an Agency allowing 
50

of Agreement 

(MOA) 

interoperability between the Agency Principal CA and the FBCA. 

Mission 

Support 

Information 

Information that is important to the support of deployed and contingency forces. 

Mutual 

Authentication 

Occurs when parties at both ends of a communication activity authenticate each 

other (see authentication). 

Naming 

Authority 

An organizational entity responsible for assigning distinguished names (DNs) and for 

assuring that each DN is meaningful and unique within its domain. 

NonRepudiation 

Assurance that the sender is provided with proof of delivery and that the recipient is 

provided with proof of the sender's identity so that neither can later deny having 

processed the data. [NS4009] Technical non-repudiation refers to the assurance a 

Relying Party has that if a public key is used to validate a digital signature, that 

signature had to have been made by the corresponding private signature key. Legal 

non-repudiation refers to how well possession or control of the private signature key 

can be established. 

Object 

Identifier (OID) 

A specialized formatted number that is registered with an internationally recognized 

standards organization. The unique alphanumeric/numeric identifier registered under 

the ISO registration standard to reference a specific object or object class. In the 

federal government PKI they are used to uniquely identify each of the four policies 

and cryptographic algorithms supported. 

Out-of-Band Communication between parties utilizing a means or method that differs from the 

current method of communication (e.g., one party uses U.S. Postal Service mail to 

communicate with another party where current communication is occurring online). 

Outside Threat An unauthorized entity from outside the domain perimeter that has the potential to 

harm an Information System through destruction, disclosure, modification of data, 

and/or denial of service. 

Physically 

Isolated 

Network 

A network that is not connected to entities or systems outside a physically controlled 

space. 

PKI Sponsor Fills the role of a Subscriber for non-human system components that are named as 

public key certificate subjects, and is responsible for meeting the obligations of 

Subscribers as defined throughout this CP. 

Policy 

Management 

Authority 

(PMA) 

Body established to oversee the creation and update of Certificate Policies, review 

Certification Practice Statements, review the results of CA audits for policy 

compliance, evaluate non-domain policies for acceptance within the domain, and 

generally oversee and manage the PKI certificate policies. For the FBCA, the PMA is 

the Federal PKI Policy Authority. 

Principal CA The Principal CA is a CA designated by an Agency to interoperate with the FBCA. 

An Agency may designate multiple Principal CAs to interoperate with the FBCA. 

Privacy Restricting access to subscriber or Relying Party information in accordance with 

Federal law and Agency policy. 

Private Key (1) The key of a signature key pair used to create a digital signature. (2) The key of 

an encryption key pair that is used to decrypt confidential information. In both cases, 

this key must be kept secret. 

Public Key (1) The key of a signature key pair used to validate a digital signature. (2) The key of 
51

an encryption key pair that is used to encrypt confidential information. In both cases, 

this key is made publicly available normally in the form of a digital certificate. 

Public Key 

Infrastructure 

(PKI) 

A set of policies, processes, server platforms, software and workstations used for the 

purpose of administering certificates and public-private key pairs, including the ability 

to issue, maintain, and revoke public key certificates. 

Registration 

Authority (RA) 

An entity that is responsible for identification and authentication of certificate 

subjects, but that does not sign or issue certificates (i.e., a Registration Authority is 

delegated certain tasks on behalf of an authorized CA). 

Re-key (a 

certificate) 

To change the value of a cryptographic key that is being used in a cryptographic 

system application; this normally entails issuing a new certificate on the new public 

key. 

Relying Party A person or Agency who has received information that includes a certificate and a 

digital signature verifiable with reference to a public key listed in the certificate, and is 

in a position to rely on them. 

Renew (a 

certificate) 

The act or process of extending the validity of the data binding asserted by a public 

key certificate by issuing a new certificate. 

Repository A database containing information and data relating to certificates as specified in this 

CP; may also be referred to as a directory. 

Responsible 

Individual 

A trustworthy person designated by a sponsoring organization to authenticate 

individual applicants seeking certificates on the basis of their affiliation with the 

sponsor. 

Revoke a 

Certificate 

To prematurely end the operational period of a certificate effective at a specific date 

and time. 

Risk An expectation of loss expressed as the probability that a particular threat will exploit 

a particular vulnerability with a particular harmful result. 

Risk Tolerance The level of risk an entity is willing to assume in order to achieve a potential desired 

result. 

Root CA In a hierarchical PKI, the CA whose public key serves as the most trusted datum (i.e., 

the beginning of trust paths) for a security domain. 

Server A system entity that provides a service in response to requests from clients. 

Signature 

Certificate 

A public key certificate that contains a public key intended for verifying digital 

signatures rather than encrypting data or performing any other cryptographic 

functions. 

Subordinate 

CA 

In a hierarchical PKI, a CA whose certificate signature key is certified by another CA, 

and whose activities are constrained by that other CA. (See superior CA). 

Subscriber A Subscriber is an entity that (1) is the subject named or identified in a certificate 

issued to that entity, (2) holds a private key that corresponds to the public key listed 

in the certificate, and (3) does not itself issue certificates to another party. This 

includes, but is not limited to, an individual or network device 

Superior CA In a hierarchical PKI, a CA who has certified the certificate signature key of another 

CA, and who constrains the activities of that CA. (See subordinate CA). 

System 

Equipment 

A comprehensive accounting of all system hardware and software types and settings. 
52

Configuration 

Technical nonrepudiation 

The contribution public key mechanisms to the provision of technical evidence 

supporting a non-repudiation security service. 

Threat Any circumstance or event with the potential to cause harm to an information system 

in the form of destruction, disclosure, adverse modification of data, and/or denial of 

service. [NS4009] 

Trust List Collection of trusted certificates used by Relying Parties to authenticate other 

certificates. 

Trusted Agent Entity authorized to act as a representative of an Agency in confirming Subscriber 

identification during the registration process. Trusted Agents do not have automated 

interfaces with Certification Authorities. 

Trusted 

Certificate 

A certificate that is trusted by the Relying Party on the basis of secure and 

authenticated delivery. The public keys included in trusted certificates are used to 

start certification paths. Also known as a "trust anchor". 

Trusted 

Timestamp 

A digitally signed assertion by a trusted authority that a specific digital object existed 

at a particular time. 

Trustworthy 

System 

Computer hardware, software and procedures that: (1) are reasonably secure from 

intrusion and misuse; (2) provide a reasonable level of availability, reliability, and 

correct operation; (3) are reasonably suited to performing their intended functions; 

and (4) adhere to generally accepted security procedures. 

Update (a 

certificate) 

The act or process by which data items bound in an existing public key certificate, 

especially authorizations granted to the subject, are changed by issuing a new 

certificate. 
53

10 SELECTED BIBLIOGRAPHY

[BDNP 98] Burr, W., D. Dodson, N. Nazario, W.T. Polk. Minimum Interoperability Specification 

for PKI Components (MISPC), Version 1. NIST SP 800-15. National Institute of Standards and 

Technology, January 1998. 

http://csrc.nist.gov/publications/nistpubs/800-15/SP800-15.PDF 

[CJB 01] Chang, S., Johnson, R., and W. Burr, "Federal PKI Directory Profile”, working 

document, http://csrc.nist.gov/pki/twg/y2001/doc_reg_01.htm. 

[COOP 99] Cooper, D.A. ”A model of certificate revocation,” Proceedings of the Fifteenth Annual 

Computer Security Applications Conference, pages 256-264, December 1999. 

[DH 76] W. Diffie, M.E. Hellman, “New Directions in Cryptography,” IEEE Transactions on 

Information Theory, v. IT-22, n. 6, (Nov 1976), pp. 644-654. 

[ENTR 00] Entrust Technologies. “The PKI: Paving the Way for Secure Electronic Service 

Delivery,” January, 2000. 

[HCFA 01] The Health Insurance Portability and Accountability Act of 1996 (HIPAA) Page, 

http://www.hcfa.gov/hipaa/hipaahm.htm 

[HP 01] Housley, R., and W.T. Polk. Planning for PKI: Best practices for PKI Deployment, Wiley 

& Sons, 2001. 

[IETF 99] RFC 2104 HMAC: Keyed-Hashing for Message Authentication. 

http://www.ietf.org/rfc/rfc2104.txt 

[IETF 01] Public-Key Infrastructure (X.509) (pkix)

http://www.ietf.org/html.charters/pkix-charter.html

[LEE 99] Lee, A. Guideline for Implementing Cryptography in the Federal Government, NIST SP 

800-21. National Institute of Standards and Technology, November, 1999. 

http://csrc.nist.gov/publications/nistpubs/800-21/800-21.pdf 

[LYON 00] Lyons-Burke, K. “Federal Agency Use of Public Key Technology for Digital 

Signatures and Authentication,” NIST Special Publication 800-25, October 2000. 

http://csrc.nist.gov/publications/nistpubs/800-12/ 

[MOSE 99] Moses, T. “Trust Management in the Public Key Infrastructure,” Entrust 

Technologies, January 14, 1999. http://www.entrust.com/resources/pdf/trustmodels.pdf 

[NARA 00] National Archives and Records Administration, “Records Management Guidance for 

Agencies Implementing Electronic Signature Technologies”, October 18, 2000”, 

http://www.nara.gov/records/policy/gpea.html 

[NIST 01] National Institute of Standards and Technology. Certificate Issuing and Management 

Components Protection Profile, 

http://csrc.nist.gov/pki/documents/CIMC_PP_final-corrections_20010126.pdf 

[NIST 97] National Institute of Standards and Technology. Public Key Infrastructure Technology, 

ITL Bulletin, July 1997. http://www.nist.gov/itl/lab/bulletns/archives/july97bull.htm 

[NIST 94] National Institute of Standards and Technology. FIPS 140-1, Security Requirements 

for Cryptographic Modules, Jan. 1994. 

http://csrc.nist.gov/publications/fips/fips1401.htm
54

[NIST 95] National Institute of Standards and Technology. FIPS 180-1, Secure Hash Standard, 

April 1995. http://csrc.nist.gov/publications/fips/fips180-1/fip180-1.pdf 

[NIST 00] National Institute of Standards and Technology. “X.509 Certificate Policy for the 

Federal Bridge Certification Authority (FBCA), Version 1.4R, March 4, 2000. 

http://csrc.nist.gov/pki/fbca/FBCA_CP_20001227.doc 

[NIST 01b] “Draft FIPS for the AES”, http://csrc.nist.gov/encryption/aes/index.html. 

[RSA 78] R.L. Rivest, A. Shamir, L.M. Adleman, “A Method for Obtaining Digital Signatures and 

Public-Key Cryptosystems,” Communications of the ACM, v. 21, n. 2 (Feb 1978), pp. 120-126. 

[TREA 99] Office of the Comptroller of the Currency. “Certification Authority Systems”, OCC 99-

20, May 4, 1999.