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Chapter 5. Planning the Certificate System
Each Red Hat Certificate System subsystem can be installed on the same server machine, installed on separate servers, or have multiple instances installed across an organization. Before installing any subsystem, it is important to plan the deployment out: what kind of PKI services do you need? What are the network requirements? What people need to access the Certificate System, what are their roles, and what are their physical locations? What kinds of certificates do you want to issue and what constraints or rules need to be set for them?
This chapter covers some basic questions for planning a Certificate System deployment. Many of these decisions are interrelated; one choice impacts another, like deciding whether to use smart cards determines whether to install the TPS and TKS subsystems.
5.1. Deciding on the Required Subsystems
The Certificate System subsystems cover different aspects of managing certificates. Planning which subsystems to install is one way of defining what PKI operations the deployment needs to perform.
Certificates, like software or equipment, have a lifecycle with defined stages. At its most basic, there are three steps:
- It is requested and issued.
- It is valid.
- It expires.
However, this simplified scenario does not cover a lot of common issues with certificates:
- What if an employee leaves the company before the certificate expires?
- When a CA signing certificate expires, all of the certificates issued and signed using that certificate also expire. So will the CA signing certificate be renewed, allowing its issued certificates to remain valid, or will it be reissued?
- What if an employee loses a smart card or leaves it at home. Will a replacement certificate be issued using the original certificates keys? Will the other certificates be suspended or revoked? Are temporary certificates allowed?
- When a certificate expires, will a new certificate be issued or will the original certificate be renewed?
This introduces three other considerations for managing certificates: revocation, renewal, and replacements.
Other considerations are the loads on the certificate authority. Are there a lot of issuance or renewal requests? Is there a lot of traffic from clients trying to validate whether certificates are valid? How are people requesting certificates supposed to authenticate their identity, and does that process slow down the issuance process?
5.1.1. Using a Single Certificate Manager
The core of the Certificate System PKI is the Certificate Manager, a certificate authority. The CA receives certificate requests and issues all certificates.
Figure 5.1. CA Only Certificate System
All of the basic processing for requests and issuing certificates can be handled by the Certificate Manager, and it is the only required subsystem. There can be a single Certificate Manager or many Certificate Managers, arranged in many different relationships, depending on the demands of the organization.
Along with issuing certificates, a Certificate Manager can also revoke certificates. One question for administrators is how to handle certificates if they are lost, compromised, or if the person or equipment for which they are issued leaves the company. Revoking a certificate invalidates it before its expiration date, and a list of revoked certificates is compiled and published by the issuing CA so that clients can verify the certificate status.
5.1.2. Planning for Lost Keys: Key Archival and Recovery
One operation the CA cannot perform, though, is key archival and recovery. A very real scenario is that a user is going to lose his private key — for instance, the keys could be deleted from a browser database or a smart card could be left at home. Many common business operations use encrypted data, like encrypted email, and losing the keys which unlock that data means the data itself is lost. Depending on the policies in the company, there probably has to be a way to recover the keys in order to regenerate or reimport a replacement certificate, and both operations require the private key.
That requires a KRA, the subsystem which specially archives and retrieves keys.
Figure 5.2. CA and KRA
The Key Recovery Authority stores encryption keys (key archival) and can retrieve those keys so that the CA can reissue a certificate (key recovery). A KRA can store keys for any certificate generated by Certificate System, whether it is done for a regular certificate or when enrolling a smart card.
The key archival and recovery process is explained in more detail in Section 2.4.5, “Archiving, Recovering, and Rotating Keys”.
The KRA is intended for archival and recovery of private encryption keys only. Therefore, end users must use a browser that supports dual-key generation to store their public-private key pairs.
5.1.3. Balancing Certificate Request Processing
Another aspect of how the subsystems work together is load balancing. If a site has high traffic, then it is possible to simply install a lot of CAs, as clones of each other or in a flat hierarchy (where each CA is independent) or in a tree hierarchy (where some CAs are subordinate to other CAs); this is covered more in Section 5.2, “Defining the Certificate Authority Hierarchy”.
5.1.4. Balancing Client OCSP Requests
If a certificate is within its validity period but needs be invalidated, it can be revoked. A Certificate Manager can publish lists of revoked certificates, so that when a client needs to verify that a certificate is still valid, it can check the list. These requests are online certificate status protocol requests, meaning that they have a specific request and response format. The Certificate Manager has a built-in OCSP responder so that it can verify OCSP requests by itself.
However, as with certificate request traffic, a site may have a significant number of client requests to verify certificate status. Example Corp. has a large web store, and each customer's browser tries to verify the validity of their SSL/TLS certificates. Again, the CA can handle issuing the number of certificates, but the high request traffic affects its performance. In this case, Example Corp. uses the external OCSP Manager subsystem to verify certificate statuses, and the Certificate Manager only has to publish updated CRLs every so often.
Figure 5.3. CA and OCSP
5.1.5. Using Smart Cards
Smart cards usually require in-person enrollment and approval processes, since they use a physical medium to store key and certificate data. That means that multiple agents need to have access to the TPS and, possibly, multiple TPS subsystems in different offices or geographic locations.
The token management system is very scalable. Multiple TPSs can be configured to work with a single CA, TKS, or KRA instance, while multiple Enterprise Security Clients can communicate with a single TPS. As additional clients are installed, they can point back to the TPS instance without having to reconfigure the TPS; likewise, as TPSs are added, they can point to the same CA, TKS, and KRA instances without having to reconfigure those subsystems.
After installation, the TPS configuration can be edited to use additional CA, KRA, and TKS instances for failover support, so if the primary subsystem is unavailable, the TPS can switch to the next available system without interrupting its token services.