About This Guide
Chapter 1 Introduction to Certificate Management System
Chapter 2 Default Demo Installation
Chapter 3 Planning Your Deployment
Chapter 4 Installation Worksheet
Chapter 5 Installation and Configuration
Appendix A Migrating from Certificate Server 1.x
Appendix B Certificate Extensions
Appendix C Certificate Download Specification
Appendix D Using SSL with iPlanet Web Server, Enterprise Edition
Appendix E Export Control Information
Appendix B Certificate Extensions
This appendix explains both the standard certificate extensions defined by X.509 v3 and the extensions defined by Netscape that were used in versions of products released before X.509 v3 was finalized. It also provides recommendations for extensions to use with specific kinds of certificates, including both PKIX Part 1 recommendations and Netscape extensions that must be supported for compatibility with early versions of Netscape products.This appendix contains the following sections:
|Introduction to Certificate Extensions|
An X.509 v3 certificate contains an extensions field that permits any number of additional fields to be added to the certificate. Certificate extensions provide a way of adding information such as alternative subject names and usage restrictions to certificates. Older versions of Netscape browsers and servers taht were developed before PKIX part 1 standards were defined require Netscape-specific extensions.
The X.509 v1 certificate specification was originally designed to bind public keys to names in an X.500 directory. As certificates began to be used on the Internet and extranets, and directory lookups could not always be performed, problem areas such as the following emerged that were not foreseen in the original specification:|
|Recommendations for Extension Use|
Most deployments will use some or all of these extensions:
authorityKeyIdentifier. Identifies the public key corresponding to the private key used to sign a certificate.
basicConstraints. Identifies CA certificates and optionally specifies a maximum certificate chain path length.
cRLDistributionPoints. Defines how CRL information for the certificate is to be obtained.
extKeyUsage. Indicates purpose or purposes for which the certificate may be used, either in addition to or instead of the purposes indicated by the keyUsage extension.
keyUsage. Indicates the purpose or purposes for which the public key certified by the certificate may be used.
netscape-cert-type. Indicates the purpose or purposes for which the certificate may be used. Required only for compatibility with some Netscape products that were released before by X.509 v3 was finalized.
subjectAltName. Specifies one or more alternative names for the identity bound by the CA to the certified public key.
subjectKeyIdentifier. Identifies the public key certified by the certificate.
These extensions, plus others, are described in detail in later sections of this appendix. Additional extensions may be useful for a variety of purposes. However, the extensions listed above are either required or recommended for various kinds of certificates issued by Certificate Management System.
Table B.1 summarizes guidelines for using these extensions. The table provides a summary only. Each extension is explained in detail later in the Appendix. Click the boldface name of each extension in the table to go to more detailed information. Keep the following in mind as you use the table:
|Standard X.509 v3 Certificate Extensions|
This section summarizes the extension types that are defined as part of the Internet X.509 Version 3 standard, as of September 1998, and indicates which types are recommended by the PKIX working group.
This section summarizes important information about each certificate. For complete details, see both the X.509 v3 standard (available from the ITU) and the Internet X.509 Public Key Infrastructure - Certificate and CRL Profile (RFC 2459),. The descriptions of extensions reference the RFC and section number of the standard draft that discusses the extension; the object identifier (OID) for each extensions is also provided.
Each extension in a certificate can be designated as critical or noncritical. A certificate-using system, such as browser software, must reject the certificate if it encounters a critical extension it does not recognize; however, a noncritical extension can be ignored if it is not recognized.
The descriptions below contain recommendations for use of the extension from Netscape and Microsoft. The Microsoft recommendations were taken from "Structuring X.509 Certificates for Use with Microsoft Products", dated December 4, 1997.
Certificate Management System (CMS) version support is listed for each extension. "Supported" means that the indicated version of CMS ships with built-in support for the extension via a policy plug-in. "Not supported" means that the indicated version of CMS does not ship a policy plug-in for the extension (although the extension can be used if a custom plug-in is written).
These are the standard X.509 v3 extensions described in the sections that follow:
The Authority Information Access extension indicates how and where to access information about the issuer of the certificate. The extension contains an accessMethod and an accessLocation field. The accessMethod specifies (by an OID) the type and format of information about the issuer found at the accessLocation.PKIX Part 1 defines one accessMethod (id-ad-caIssuers) to get a list of CAs that have issued certificates higher in the CA chain than the issuer of the certificate using the extension. The accessLocation field then typically contains a URL indicating the location and protocol (LDAP, HTTP, FTP) used to retrieve the list. The Online Certificate Status Protocol (RFC 2560) defines an accessMethod (id-ad-ocsp) for using OCSP to verify certificates. The accessLocation field then contains a URL indicating the location and protocol used to access an OCSP responder that can validate the certificate. CMS Version Support Netscape Recommendation OCSP signing certificates and CA signing certificates should only use the authorityInfoAccess extension to point to an OCSP responder if that responder has been configured to verify them. For example, if there is a hierarchy of responders, a subordinate responder may point to its parent for verification. If a CA signing certificate points to an OCSP responder, that responder's signing certificate should be signed by a different CA (for example, the CA that issued the CA signing certificate in question). Microsoft Recommendation authorityKeyIdentifier Reference Criticality Discussion
The Authority Key Identifier extension identifies the public key corresponding to the private key used to sign a certificate. This extension is useful when an issuer has multiple signing keys (for example, due to CA certificate renewal).The extension consists of either or both of the following:
Netscape recommends that this extension be present in all certificates and that the authorityCertIssuer and authorityCertSerialNumber fields be specified. This extension is not supported by Navigator 3.x, but its presence in a certificate won't interfere with Navigator 3.x.Microsoft Recommendation basicConstraints Reference Criticality Discussion
This extension is used during the certificate chain verification process to identify CA certificates and to apply certificate chain path length constraints. The cA component should be set to true for all CA certificates. PKIX recommends that this extension should not appear in end-entity certificates.If the pathLenConstraint component is present, its value must be greater than the number of CA certificates that have been processed so far (starting with the end-entity certificate and moving up the chain). If pathLenConstraint is omitted, then all of the higher level CA certificates in the chain must not include this component when the extension is present. See CA Certificates and Extension Interactions regarding the interaction of the this extension with the Netscape Certificate Type extension. CMS Version Support Netscape Recommendation Microsoft Recommendation certificatePolicies References Criticality Discussion
The Certificate Policies extension defines one or more policies, each of which consists of an OID and optional qualifiers. The extension can include a URI to the issuer's Certificate Practice Statement or can embed issuer policy information, such as a user notice in text form. This information can be used by certificate-enabled applications.If this extension is present, PKIX Part 1 recommends that policies be identified with an OID only, or if necessary only certain recommended qualifiers. CMS Version Support Netscape Recommendation Microsoft Recommendation cRLDistributionPoints Reference Criticality Discussion If the extension contains a DistributionPointName of type URI, the URI is assumed to be a pointer to the current CRL for the associated reasons and will be issued by the associated cRLIssuer. The expected values for the URI are those defined for the subjectAltName extension. If the distributionPoint omits reasons, the CRL must include revocations for all reasons. If the distributionPoint omits cRLIssuer, the CRL must be issued by the CA that issued the certificate. PKIX recommends that this extension be supported by CAs and applications. CMS Version Support Netscape Recommendation Microsoft Recommendation extKeyUsage Reference Criticality
If this extension is marked critical, the certificate must be used for one of the indicated purposes only. If it is not marked critical, it is treated as an advisory field that may be used to identify keys but does not restrict the use of the certificate to the indicated purposes.Discussion
The Extended Key Usage extension indicates one or more purposes for which the certified public key may be used. These purposes may be in addition to or in place of the basic purposes indicated in the key usage extension.The Extended Key Usage extension must include OCSP Signing in an OCSP responder's certificate (unless the CA signing key that signed the certificates validated by the responder is also the OCSP signing key). The OCSP responder's certificate must be issued directly by the CA that signs certificates the responder will validate. The Key Usage, Extended Key Usage, and Basic Constraints extensions act together to define the purposes for which the certificate is intended to be used. Applications can use these extensions to disallow the use of a certificate in inappropriate contexts. Table B.2 lists the uses defined by PKIX for this extension, and Table B.3 lists uses privately defined by Microsoft or Netscape.
Netscape recommends that this extension be supported for all certificates, and requires it for all certificates that support step-up, or Server Gated Crypto (SGC). OCSP Signing should be included in all certificates issued to OCSP responders.Microsoft Recommendations
Microsoft products interpret this extension as follows. If the extension is not present, the certificate is considered to be valid for any usage (to support backward compatibility with certificates that did not use this extension). Otherwise, interpretation depends on usage, as follows:
Microsoft products do not examine this extension. Microsoft recommends that authorityKeyIdentifier be used rather than issuerAltName or the certificate's issuer name for the purposes of building certificate chains.keyUsage Reference Criticality Discussion
The Key Usage extension defines the purpose of the key contained in the certificate. The Key Usage, Extended Key Usage, Basic Constraints, and Netscape Certificate Type extensions act together to specify the purposes for which a certificate can be used. For more information on interactions between these extensions in CA certificates, see CA Certificates and Extension Interactions.If this extension is included at all, set the bits as follows:
Microsoft recommends this extension for all certificates if their intended purpose or purposes are known. If the extension is absent, Microsoft products will assume the certificate is valid for all usages. If the extension is present, Microsoft products will interpret the extension in the same way whether marked critical or not. If the extension is present, the actual usage must conform to the specified usage.The only Microsoft application that currently enforces this extension is Microsoft Outlook. nameConstraints Reference Criticality Discussion CMS Version Support Netscape Recommendation Microsoft Recommendation OCSPNocheck Reference Criticality Discussion
The extension is meant to be included in an OCSP responder's signing certificate. The extension tells an OCSP client that the signing certificate can be trusted without querying the OCSP responder (since the reply would again be signed by the OCSP responder, and the client would again request the validity status of the signing certificate). This extension is null-valued: its meaning is determined by its presence or absence.Since the presence of this extension in a certificate will cause OCSP clients to trust responses signed with that certificate, use of this extension should be managed carefully. If the OCSP signing key is compromised, the entire process of validating certificates in the PKI will be compromised for the duration of the validity period of the certificate. Therefore, certificates using OCSPNocheck should be issued with short lifetimes and be renewed frequently. CMS Version Support Netscape Recommendation
Netscape recommends using this extension in OCSP responder signing certificates. The validity period should be short enough to minimize the potential impact of a compromised OCSP responder signing key to your organization.Microsoft Recommendation policyConstraints References Criticality Discussion
This extension, which is for CA certificates only, constrains path validation in two ways. It can be used to prohibit policy mapping or to require that each certificate in a path contain an acceptable policy identifier.PKIX requires that, if present, this extension must never consist of a null sequence. At least one of the two available fields must be present. CMS Version Support Netscape Recommendations Microsoft Recommendations policyMappings References Criticality Discussion
The Policy Mappings extension is used in CA certificates only. It lists one or more pairs of OIDs used to indicate that the corresponding policies of one CA are equivalent to policies of another CA. It may be useful in the context of cross-certification.This extension may be supported by CAs and/or applications. CMS Version Support Netscape Recommendation Microsoft Recommendations privateKeyUsagePeriod Reference Discussion
The Private Key Usage Period extension allows the certificate issuer to specify a different validity period for the private key than for the certificate itself. This extension is intended for use with digital signature keys.PKIX Part 1 recommends against the use of this extension. CAs conforming to PKIX Part 1 must not generate certificates with this extension. CMS Version Support Netscape Recommendation Microsoft Recommendation subjectAltName Reference Criticality Discussion
The Subject Alternative Name extension includes one or more alternative (non-X.500) names for the identity bound by the CA to the certified public key. It may be used in addition to the certificate's subject name or as a replacement for it. Defined name forms include Internet electronic mail address (SMTP, as defined in RFC-822), DNS name, IP address, and uniform resource identifier (URI).PKIX requires this extension for entities that are identified by name forms other than the X.500 distinguished name (DN) used in the subject field. PKIX Part 1 describes additional rules for the relationship between this extension and the subject field. Email addresses may be provided either in the Subject Alternative Name extension, the certificate subject name field, or both. If the email address is provided as part of the subject name, it must be in the form of the EmailAddress attribute defined by PKCS-9. Software that supports S/MIME must be able to read an email address from either the Subject Alternative Name extension or from the subject name field. CMS Version Support Netscape Recommendation Netscape products read only the first alternative name in this extension, and ignore the rest. For S/MIME certificates, Netscape software first checks the first alternative name in this extension (if the extension is present) for the EmailAddress attribute. If the first alternative name is not an EmailAddress attribute, Netscape software looks for the e= attribute of the DN. If the e= attribute is not present, Netscape software looks for the mail= attribute of the DN. Microsoft Recommendation
Microsoft recommends the use of this extension whenever X.500 guidelines are insufficient for naming purposes. Currently, no Microsoft products require the use of Subject Alternative Name. All Microsoft products that support S/MIME are capable of reading email names from this extension or from the subject name. Future versions of Microsoft Exchange Server will issue certificates with X.500 names that do not contain the Email Address attribute, and will place the SMTP address in the Subject Alternative Name extension.subjectDirectoryAttributes Reference Criticality Discussion
The Subject Directory Attributes extension conveys any desired directory attribute values for the subject of the certificate. It is not recommended as an essential part of the proposed PKIX standard, but may be used in local environments.CMS Version Support Netscape Recommendation Microsoft Recommendation subjectKeyIdentifier Reference Criticality Discussion
The Subject Key Identifier extension identifies the public key certified by this certificate. This extension provides a way of distinguishing public keys if more than one is available for a given subject name, for example after the certificate has been renewed with a new key.The value of this extension should be calculated by performing a SHA-1 hash of the certificate's DER-encoded subjectPublicKeyInfo, as recommended by PKIX. This extension is used with the form of the authorityKeyIdentifier extension in which the issuer's public key is specified by a hash. In this case the verifier does not need to compute the hash, since it's only necessary to compare the issuer's Subject Key Identifier with the subject's Authority Key Identifier. PKIX Part 1 requires this extension for all CA certificates and recommends it for all other certificates. CMS Version Support Netscape Recommendation Microsoft Recommendation
|Standard X.509 v3 CRL Extensions|
In addition to certificate extensions, the X.509 v3 proposed standard defines extensions to CRLs, which provide methods for associating additional attributes with Internet CRLs. These are of two kinds: extensions to the CRL itself, and extensions to individual certificate entries in the CRL.
Extensions for CRLs
These are the CRL extensions described in the sections that follow:
The Authority Key Identifier extension for a CRL identifies the public key corresponding to the private key used to sign the CRL. For details, see the discussion under certificate extensions at authorityKeyIdentifier.CRLNumber Reference Criticality Discussion PKIX requires that all CRLs have this extension. deltaCRLIndicator Reference Criticality Discussion
The Delta CRL Indicator extension identifies a delta-CRL. The use of delta-CRLs allows changes to be added to the local database while ignoring unchanged information that is already in the local database. This can significantly improve processing time for applications that store revocation information in a format other than the CRL structure.This extension is used only with delta-CRLs, which are not supported by Certificate Management System. issuerAltName Reference Discussion issuingDistributionPoint Reference Criticality Discussion PKIX Part I does not require this extension. CRL Entry Extensions
The sections that follow lists the CRL entry extension types that are defined as part of the Internet X.509 v3 Public Key Infrastructure proposed standard, as of September 1998. All of these extensions are noncritical.These are the CRL entry extensions described in the sections that follow: certificateIssuer Reference Discussion This extension is used only with indirect CRLs, which are not supported by Certificate Management System. holdInstructionCode Reference Discussion invalidityDate Reference Discussion reasonCode Reference Discussion
|Netscape-Defined Certificate Extensions|
Netscape has defined certain certificate extensions for use with Navigator and Communicator. Some of the extensions that have been defined are now obsolete, and others can be superseded by the extensions defined in the X.509 proposed standard. All Netscape extensions should be tagged as noncritical, so that their presence in a certificate does not make that certificate incompatible with other clients.
The specifications for all Netscape-defined extensions are defined at http://home.netscape.com/eng/security/comm4-cert-exts.html. For most CMS deployments, only netscape-cert-type and netscape-comment need to be supported to maintain compatibility with Navigator 3.x. Therefore, only these two Netscape certificate extensions are described here.
The Netscape Certificate Type extension can be used to limit the purposes for which a certificate can be used. It has been replaced by the X.509 v3 extensions extKeyUsage and basicConstraints, but must still be supported in deployments that include Navigator 3.x clients.If the extension exists in a certificate, it limits the certificate to the uses specified in it. If the extension is not present, the certificate can be used for all applications except object signing. The value is a bit-string, where the individual bit positions, when set, certify the certificate for particular uses as follows:
|Adding Extensions in Certificate Management System|
When Certificate Management System creates a certificate in response to a certificate request, it can add extensions according to the policiesd defined in extension-specific policy modules. Policy modules are available with the distribution that can be used to add certificate extensions. For more information about policies and extensions, see Chapter 16, "Introduction to Policy," in Netscape Certificate Management System Administrator's Guide.
By default, only noncritical extensions are added to certificates. This ensures that the resulting certificates can be used with all clients. If you add a critical extension, the resulting certificate can only be used by clients that support that extension.
You can write a policy module to add any extension that Certificate Management System supports. For a summary of the certificate extensions supported by CMS policy modules, see Policy Modules in Chapter 1.
|CA Certificates and Extension Interactions|
Netscape recommends that all CA certificates contain the basicConstraints extension, as this is the standard way to identify a CA certificate. In addition, to ensure support for Navigator 3.x, CAs should also use netscape-cert-type. These two extensions can interact with each other. The following table describes what different combinations of the two extensions mean.