5.2. Defining the Certificate Authority Hierarchy

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The CA is the center of the PKI, so the relationship of CA systems, both to each other (CA hierarchy) and to other subsystems (security domain) is vital to planning a Certificate System PKI.
When there are multiple CAs in a PKI, the CAs are structured in a hierarchy or chain. The CA above another CA in a chain is called an root CA; a CA below another CA in the chain is called a subordinate CA. A CA can also be subordinate to a root outside of the Certificate System deployment; for example, a CA which functions as a root CA within the Certificate System deployment can be subordinate to a third-party CA.
A Certificate Manager (or CA) is subordinate to another CA because its CA signing certificate, the certificate that allows it to issue certificates, is issued by another CA. The CA that issued the subordinate CA signing certificate controls the CA through the contents of the CA signing certificate. The CA can constrain the subordinate CA through the kinds of certificates that it can issue, the extensions that it is allowed to include in certificates, the number of levels of subordinate CAs the subordinate CA can create, and the validity period of certificates it can issue, as well as the validity period of the subordinate CAs signing certificate.


Although a subordinate CA can create certificates that violate these constraints, a client authenticating a certificate that violates those constraints will not accept that certificate.
A self-signed root CA signs its own CA signing certificate and sets its own constraints as well as setting constraints on the subordinate CA signing certificates it issues.
A Certificate Manager can be configured as either a root CA or a subordinate CA. It is easiest to make the first CA installed a self-signed root, so that it is not necessary to apply to a third party and wait for the certificate to be issued. Before deploying the full PKI, however, consider whether to have a root CA, how many to have, and where both root and subordinate CAs will be located.

5.2.1. Subordination to a Public CA

Chaining the Certificate System CA to a third-party public CA introduces the restrictions that public CAs place on the kinds of certificates the subordinate CA can issue and the nature of the certificate chain. For example, a CA that chains to a third-party CA might be restricted to issuing only Secure Multipurpose Internet Mail Extensions (S/MIME) and SSL/TLS client authentication certificates, but not SSL/TLS server certificates. There are other possible restrictions with using a public CA. This may not be acceptable for some PKI deployments.
One benefit of chaining to a public CA is that the third party is responsible for submitting the root CA certificate to a web browser or other client software. This can be a major advantage for an extranet with certificates that are accessed by different companies with browsers that cannot be controlled by the administrator. Creating a root CA in the CA hierarchy means that the local organization must get the root certificate into all the browsers which will use the certificates issued by the Certificate System. There are tools to do this within an intranet, but it can be difficult to accomplish with an extranet.

5.2.2. Subordination to a Certificate System CA

The Certificate System CA can function as a root CA, meaning that the server signs its own CA signing certificate as well as other CA signing certificates, creating an organization-specific CA hierarchy. The server can alternatively be configured as a subordinate CA, meaning the server's CA signing key is signed by another CA in an existing CA hierarchy.
Setting up a Certificate System CA as the root CA means that the Certificate System administrator has control over all subordinate CAs by setting policies that control the contents of the CA signing certificates issued. A subordinate CA issues certificates by evaluating its own authentication and certificate profile configuration, without regard for the root CA's configuration.

5.2.3. Linked CA

The Certificate System Certificate Manager can function as a linked CA, chaining up to many third-party or public CAs for validation; this provides cross-company trust, so applications can verify certificate chains outside the company certificate hierarchy. A Certificate Manager is chained to a third-party CA by requesting the Certificate Manager's CA signing certificate from the third-party CA.
Related to this, the Certificate Manager also can issue cross-pair or cross-signed certificates. Cross-pair certificates create a trusted relationship between two separate CAs by issuing and storing cross-signed certificates between these two CAs. By using cross-signed certificate pairs, certificates issued outside the organization's PKI can be trusted within the system.
These are also called bridge certificates, related to the Federal Bridge Certification Authority (FBCA) definition.

5.2.4. CA Cloning

Instead of creating a hierarchy of root and subordinate CAs, it is possible to create multiple clones of a Certificate Manager and configure each clone to issue certificates within a range of serial numbers.
A cloned Certificate Manager uses the same CA signing key and certificate as another Certificate Manager, the master Certificate Manager.


If there is a chance that a subsystem will be cloned, then it is easiest to export its key pairs during the configuration process and save them to a secure location. The key pairs for the original Certificate Manager have to be available when the clone instance is configured, so that the clone can generate its certificates from the original Certificate Manager's keys.
It is also possible to export the keys from the security databases at a later time, using the pk12util or the PKCS12Export commands.
Because clone CAs and original CAs use the same CA signing key and certificate to sign the certificates they issue, the issuer name in all the certificates is the same. Clone CAs and the original Certificate Managers issue certificates as if they are a single CA. These servers can be placed on different hosts for high availability failover support.
The advantage of cloning is that it distributes the Certificate Manager's load across several processes or even several physical machines. For a CA with a high enrollment demand, the distribution gained from cloning allows more certificates to be signed and issued in a given time interval.
A cloned Certificate Manager has the same features, such as agent and end-entity gateway functions, of a regular Certificate Manager.
The serial numbers for certificates issued by clones are distributed dynamically. The databases for each clone and master are replicated, so all of the certificate requests and issued certificates, both, are also replicated. This ensures that there are no serial number conflicts while serial number ranges do not have to be manually assigned to the cloned Certificate Managers.
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