Chapter 8. Publishing Certificates and CRLs
Red Hat Certificate System includes a customizable publishing framework for the Certificate Manager, enabling certificate authorities to publish certificates, certificate revocation lists (CRLs), and other certificate-related objects to any of the supported repositories: an LDAP-compliant directory, a flat file, and an online validation authority. This chapter explains how to configure a Certificate Manager to publish certificates and CRLs to a file, to a directory, and to the Online Certificate Status Manager.
The general process to configure publishing is as follows:
- Configure publishing to a file, LDAP directory, or OCSP responder.There can be a single publisher or multiple publishers, depending on how many locations will be used. The locations can be split by certificates and CRLs or narrower definitions, such as certificate type. Rules determine which type to publish and to what location by being associated with the publisher.
- Set rules to determine what certificates are published to the locations. Any rule which a certificate or CRL matches is activated, so the same certificate can be published to a file and to an LDAP directory by matching a file-based rule and matching a directory-based rule.Rules can be set for each object type: CA certificates, CRLs, user certificates, and cross-pair certificates. Disable all rules that will not be used.
- Configure CRLs. CRLs must be configured before they can be published. See Chapter 7, Revoking Certificates and Issuing CRLs.
- Enable publishing after setting up publishers, mappers, and rules. Once publishing is enabled, the server starts publishing immediately. If the publishers, mappers, and rules are not completely configured, publishing may not work correctly or at all.
8.1. About Publishing
The Certificate System is capable of publishing certificates to a file or an LDAP directory and of publishing CRLs to a file, an LDAP directory, or to an OCSP responder.
For additional flexibility, specific types of certificates or CRLs can be published to a single format or all three. For example, CA certificates can be published only to a directory and not to a file, and user certificates can be published to both a file and a directory.
An OCSP responder only provides information about CRLs; certificates are not published to an OCSP responder.
Different publishing locations can be set for certificates files and CRL files, as well as different publishing locations for different types of certificates files or different types of CRL files.
Similarly, different types of certificates and different types of CRLs can be published to different places in a directory. For example, certificates for users from the West Coast division of a company can be published in one branch of the directory, while certificates for users in the East Coast division can be published to another branch in the directory.
When publishing is enabled, every time a certificate or a CRL is issued, updated, or revoked, the publishing system is invoked. The certificate or CRL is evaluated by the rules to see if it matches the type and predicate set in the rule. The type specifies if the object is a CRL, CA certificate, or any other certificate. The predicate sets more criteria for the type of object being evaluated. For example, it can specify user certificates, or it can specify West Coast user certificates. To use predicates, a value needs to be entered in the predicate field of the publishing rule, and a corresponding value (although formatted somewhat differently) needs to be contained in the certificate or certificate request to match. The value in the certificate or certificate request may be derived from information in the certificate, such as the type of certificate, or may be derived from a hidden value that is placed in the request form. If no predicate is set, all certificates of that type are considered to match. For example, all CRLs match the rule if
CRLis set as the type.
Every rule that is matched publishes the certificate or CRL according to the method and location specified in that rule. A given certificate or CRL can match no rules, one rule, more than one rule, or all rules. The publishing system attempts to match every certificate and CRL issued against all rules.
When a rule is matched, the certificate or CRL is published according to the method and location specified in the publisher associated with that rule. For example, if a rule matches all certificates issued to users, and the rule has a publisher that publishes to a file in the location
/etc/CS/certificates, the certificate is published as a file to that location. If another rule matches all certificates issued to users, and the rule has a publisher that publishes to the LDAP attribute
userCertificate;binaryattribute, the certificate is published to the directory specified when LDAP publishing was enabled in this attribute in the user's entry.
For rules that specify to publish to a file, a new file is created when either a certificate or a CRL is issued in the stipulated directory.
For rules that specify to publish to an LDAP directory, the certificate or CRL is published to the entry specified in the directory, in the attribute specified. The CA overwrites the values for any published certificate or CRL attribute with any subsequent certificate or CRL. Simply put, any existing certificate or CRL that is already published is replaced by the next certificate or CRL.
For rules that specify to publish to an Online Certificate Status Manager, a CRL is published to this manager. Certificates are not published to an Online Certificate Status Manager.
For LDAP publishing, the location of the user's entry needs to be determined. Mappers are used to determine the entry to which to publish. The mappers can contain an exact DN for the entry, some variable that associates information that can be gotten from the certificate to create the DN, or enough information to search the directory for a unique attribute or set of attributes in the entry to ascertain the correct DN for the entry.
When a certificate is revoked, the server uses the publishing rules to locate and delete the corresponding certificate from the LDAP directory or from the filesystem.
When a certificate expires, the server can remove that certificate from the configured directory. The server does not do this automatically; the server must be configured to run the appropriate job. For details, see Chapter 12, Setting Automated Jobs.
Setting up publishing involves configuring publishers, mappers, and rules.
Publishers specify the location to which certificates and CRLs are published. When publishing to a file, publishers specify the filesystem publishing directory. When publishing to an LDAP directory, publishers specify the attribute in the directory that stores the certificate or CRL; a mapper is used to determine the DN of the entry. For every DN, a different formula is set for deriving that DN. The location of the LDAP directory is specified when LDAP publishing is enabled. When publishing a CRL to an OCSP responder, publishers specify the hostname and URI of the Online Certificate Status Manager.
Mappers are only used in LDAP publishing. Mappers construct the DN for an entry based on information from the certificate or the certificate request. The server has information from the subject name of the certificate and the certificate request and needs to know how to use this information to create a DN for that entry. The mapper provides a formula for converting the information available either to a DN or to some unique information that can be searched in the directory to obtain a DN for the entry.
Rules for file, LDAP, and OCSP publishing tell the server whether and how a certificate or CRL is to be published. A rule first defines what is to be published, a certificate or CRL matching certain characteristics, by setting a type and predicate for the rule. A rule then specifies the publishing method and location by being associated with a publisher and, for LDAP publishing, with a mapper.
Rules can be as simple or complex as necessary for the PKI deployment and are flexible enough to accommodate different scenarios.
8.1.4. Publishing to Files
The server can publish certificates and CRLs to flat files, which can then be imported into any repository, such as a relational database. When the server is configured to publish certificates and CRLs to file, the published files are DER-encoded binary blobs, base-64 encoded text blobs, or both.
- For each certificate the server issues, it creates a file that contains the certificate in either DER-encoded or base-64 encoded format. Each file is named either
.b64. The serial_number is the serial number of the certificate contained in the file. For example, the filename for a DER-encoded certificate with the serial number
- Every time the server generates a CRL, it creates a file that contains the new CRL in either DER-encoded or base-64 encoded format. Each file is named either issuing_point_name-this_update
.b64, depending on the format. The issuing_point_name identifies the CRL issuing point which published the CRL, and this_update specifies the value derived from the time-dependent update value for the CRL contained in the file. For example, the filename for a DER-encoded CRL with the value
This Update: Friday January 28 15:36:00 PST 2020, is
8.1.5. OCSP Publishing
There are two forms of Certificate System OCSP services, an internal service for the Certificate Manager and the Online Certificate Status Manager. The internal service checks the internal database of the Certificate Manager to report on the status of a certificate. The internal service is not set for publishing; it uses the certificates stored in its internal database to determine the status of a certificate. The Online Certificate Status Manager checks CRLs sent to it by Certificate Manager. A publisher is set for each location a CRL is sent and one rule for each type of CRL sent.
For detailed information on both OCSP services, see Section 7.6, “Using the Online Certificate Status Protocol (OCSP) Responder”.
8.1.6. LDAP Publishing
In LDAP publishing, the server publishes the certificates, CRLs, and other certificate-related objects to a directory using LDAP or LDAPS. The branch of the directory to which it publishes is called the publishing directory.
- For each certificate the server issues, it creates a blob that contains the certificate in its DER-encoded format in the specified attribute of the user's entry. The certificate is published as a DER encoded binary blob.
- Every time the server generates a CRL, it creates a blob that contains the new CRL in its DER-encoded format in the specified attribute of the entry for the CA.
The server can publish certificates and CRLs to an LDAP-compliant directory using the LDAP protocol or LDAP over SSL (LDAPS) protocol, and applications can retrieve the certificates and CRLs over HTTP. Support for retrieving certificates and CRLs over HTTP enables some browsers to import the latest CRL automatically from the directory that receives regular updates from the server. The browser can then use the CRL to check all certificates automatically to ensure that they have not been revoked.
For LDAP publishing to work, the user entry must be present in the LDAP directory.
If the server and publishing directory become out of sync for some reason, privileged users (administrators and agents) can also manually initiate the publishing process. For instructions, see Section 8.12.2, “Manually Updating the CRL in the Directory”.