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Determining Number of CAs Required

Updated: March 28, 2003

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

After you have identified your application and user requirements, you can begin to estimate the number of CAs that you need to deploy. If your organization has limited certificate requirements, a small user base, and limited expansion goals, a single CA might be sufficient. By using a single CA, you can still meet a variety of needs by customizing and deploying certificate templates and using role separation. However, if availability or distributed functionality of Certificate Services is a priority, you must deploy multiple CAs. You also need multiple CAs if you want separate CAs to issue certificates for different purposes.

To determine the number of CAs required, answer the following questions:

  • Do you require more than one CA? If you are only supporting a single application and location, and if 100 percent availability of the CA is not critical, you might be able to use a single CA. Otherwise, you probably require at least one root and multiple subordinate CAs.

  • If you need more than one CA, how many root CAs do you require? Generally, it is recommended that you have only one root CA as a single point of trust. This is because significant cost and effort is required to protect a root CA from compromise. With multiple root CAs, root maintenance becomes much more difficult.

    However, organizations with a decentralized security administration model, such as corporations with multiple, largely independent business units and no strong central administrative body, might require more than one root CA. For more information about using more than one root CA, see "Extending Your CA Infrastructure" later in this chapter.

  • How many intermediate or policy CAs do you need?

  • How many issuing CAs or RAs do you need?

    The number of intermediate and issuing CAs that you deploy depends on the following factors:

    • Usage. Certificates can be issued for a number of purposes (for example, secure e-mail, network authentication, and so on). Each of these uses might involve different issuing policies. Using separate CAs provides a basis for administering each policy separately.

    • Organizational or geographic divisions. You must have different policies for issuing certificates, depending on the role of an entity or its physical location in the organization. You can create separate subordinate CAs to administer these policies.

    • Distribution of the certificate load. You can deploy multiple issuing CAs to distribute the certificate load to meet site, network, and server requirements. For example, if network links between sites are slow or discontinuous, you might need to place issuing CAs at each site to meet Certificate Services performance and usability requirements.

    • The need for flexible configuration. You can tailor the CA environment (key strength, physical protection, protection against network attacks, and so on) to provide a balance between security and usability. For example, you can renew keys and certificates more frequently for the intermediate and issuing CAs that are at high risk for compromise, without requiring a change to established root trust relationships. Also, when you use more than one subordinate CA, you can turn off a subsection of the CA hierarchy without affecting established root trust relationships or the rest of the hierarchy.

    • The need for redundant services. If one enterprise CA fails, redundancy makes it possible for another issuing CA to provide users with uninterrupted service.

Strive to have only as many CAs and RAs as you need to function efficiently. Deploying more CAs than you need creates an unnecessary management burden, and introduces additional areas of security vulnerability.


  • You cannot install more than one CA on a server.

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