The Internet Encyclopedia (Volume 3)

P1: JDW

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POLICIES ANDPROCEDURES 163

for the requesting, using, and handling of certificates and

keys. The CP asserts that this security policy shall be im-

plemented from certificate generation until its expiration

or revocation. It does not specify how the policy shall be

implemented. For example, a CP might state the follow-

ing: “All subscribers shall be authenticated in person by

an RA before a certificate is issued.” The CP excludes all

operational details, because these may evolve over time.

The CP should not identify the physical location of the CA

or the products used in the CA. By excluding these details,

the CP is a stable and high-level document. Multiple CAs

may operate under a single CP. This is often the case when

multiple CAs are maintained by a single enterprise, jointly

supporting a single community.

Different people will use the CP for different reasons.

For example, the CP will be used to guide the development

of the CPS for each CA that operates under its provisions.

CAs from other enterprise PKIs will review the CP before

cross-certification. Auditors and accreditors will use the

CP as the basis for their review of CA operations. Applica-

tion owners will review a CP to determine whether these

certificates are appropriate for their application.

The CPS is a highly detailed document that describes

how a particular CA implements a specific CP. The CPS

identifies the CP and specifies the mechanisms and proce-

dures that are used to achieve the security policy. The CPS

asserts that the specified products will be used in com-

bination with the specified procedures. The CPS might

state the following: “Users will receive their certificates

and smartcards from the RA after presenting the fol-

lowing credentials in person: (a) current driver’s license,

(b) work identification card, (c) blood sample, and (d) hair

sample.” A CPS includes sufficient operational details to

demonstrate that the CP can be satisfied by this combina-

tion of mechanisms and procedures.

Each CPS applies to a single CA. The CPS may be con-

sidered the overall operations manual for the CA. Specific

portions of the CPS may be extracted to form theCA Op-

erator’s Guide, RA Manual, PKI Users Guide,or other role-

specific documentation. Auditors and accreditors will use

the CPS to supplement the CP during their review of CA

operations. Note that a CPS does not need to be published.

The combination of a CP and the results of an accredita-

tion process should be sufficient for external parties.

RFC 2527 proposes an outline with eight major sec-

tions and 185 second- and third-level topics. RFC 2527

established an outline with the following major sections:

Introduction

General Provisions

Identification and Authentication

Operational Requirements

Physical, Procedural, and Personnel Security Controls

Technical Security Controls

Certificate and CRL Profiles

Specification Administration

Privilege Management

Organizations seek improved access control. Public

key certificates can be used to authenticate the identity of

version

serial number

signature

issuer

validity

issuerUniqueID

extensions

attributes

holder

Figure 6: X.509 attribute certificate structure.

a user, and this identity can be used as an input to access

control decision functions. In many contexts, however, the

identity is not the criterion used for access control deci-

sions. The access control decision may depend on role,

security clearance, group membership, or ability to pay.

Authorization information often has a shorter lifetime

than the binding of the subject identity and the public key.

Authorization information could be placed in a public key

certificate extension; however, this is not usually a good

strategy. First, the certificate is likely to be revoked be-

cause the authorization information needs to be updated.

Revoking and reissuing the public key certificate with up-

dated authorization information can be expensive. Sec-

ond, 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 author-

itative authorization information source.

The X.509attribute certificate(AC) binds attributes to

anAC holder. Because the AC does not contain a public

key, the AC is used in conjunction with a public key certi-

ficate. An access control function may make use of the

attributes in an AC, but it is not a replacement for au-

thentication. 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 non-repudiation service. In

these contexts, the attributes contained in the AC provide

additional information about the signing entity. This in-

formation can be used to make sure that the entity is au-

thorized 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.

Figure 6 illustrates an attribute certificate for Alice.

This is a version 2 AC, and the AC holder is Alice. The AC

was issued by the Hawk DataAttribute Authority,and was

signed with DSA and SHA-1. The serial number is 4801,

and the AC is valid from 8 a.m. on April 2, 2002, until

noon that same day. The attributes indicate that Alice

is VPN administrator. The AC extensions indicate that

this certificate is targeted toward the Hawk VPN server,

and that revocation information is not available for this

certificate. ACs often have no revocation information.