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Certificate authority (CA) software is the software and associated equipment used to operate a certificate authority within a public key infrastructure (PKI). It supports certificate requests, policy decisions, issuance, and other lifecycle tasks. The CA is the authority that approves and creates certificates; the software is how that role is implemented.
What is a certificate authority in a PKI?
A PKI sets the rules and relationships used to issue and check digital certificates. A certificate binds information—such as an organization or domain name—to a public key. A certificate authority creates and signs certificates under its operating policy. Systems and people that rely on a certificate can then check its signature and status against the trust arrangements they use.
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The distinction between the authority and its software matters. RFC 9810 distinguishes the CA as the issuer entity from the “CA equipment” used to implement it. The equipment may include online and offline components; in one common arrangement, the CA private key is available only to the offline component. That is an implementation choice, not a universal requirement. RFC 9810 (July 2025) states: “Final authority for certification creation rests with the CA.”
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Capabilities vary by product and deployment, but CA software can support a range of certificate operations:
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- Receive and process certificate requests, then apply policy and certificate profiles.
- Issue signed certificates, including deciding which requested fields and extensions to include.
- Provide enrollment interfaces and integrations for people, devices, services, or other systems.
- Support publication and status services, as well as renewal, re-key, and revocation workflows.
- Record activity for auditing and support the operational controls around certificate use.
A request is not an order the CA must fulfill exactly. Under its policy, the CA may alter requested certificate fields or extensions before issuing a certificate, or decline to issue one. The CA retains the final decision.
ACME is one standard for automating domain-control validation and certificate issuance. It also supports certificate management functions such as revocation. It is relevant to Web PKI automation, but it is not the only certificate workflow or protocol, and its existence does not mean every CA product supports it. See RFC 8555.
How the CA, RA, CA software, and HSM differ
| Term | Role |
|---|---|
| Certificate authority (CA) | The issuer that makes the final decision to create a certificate and signs it. |
| CA software and equipment | The software and associated components used to carry out the CA’s work. |
| Registration authority (RA) | A role that may perform delegated enrollment or identity-checking tasks, such as checking requests, assigning names, or reporting revocation. The exact delegation depends on the environment. |
| Hardware security module (HSM) | Separate hardware that can protect CA private keys and provide cryptographic processing. It is not CA software, and not every CA deployment requires one. |
These roles can be implemented or arranged differently across environments. An RA may perform selected checks, but that does not transfer the CA’s final authority over issuance. For a broader view of the policies and practices involved, RFC 3647 covers topics including identification, certificate lifecycle requirements, operational controls, technical security controls, and certificate and status-information profiles. It is a framework, not a mandatory product checklist or a prescribed certificate policy.
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Issuing a certificate is only one part of running a CA. A functioning operation also needs rules and processes for enrollment and validation, key generation and protection, publication, renewal or re-key, revocation, auditing, and archiving. The exact procedures depend on the certificates being issued, the organization’s policy, and the systems that depend on them.
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For example, a web service may use automated enrollment and renewal, while an organization issuing certificates to employees or internal devices may use directory integrations and different approval steps. In either case, the operation must account for what happens when a certificate expires, a key changes, or a certificate needs to be revoked.
Examples of CA software and how to evaluate them
EJBCA and Microsoft Active Directory Certificate Services (AD CS) are documented examples, but they have different deployment and licensing models. Product names alone do not establish which will fit a particular environment.
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EJBCA
EJBCA’s official material describes its Community edition as open-source PKI software and also offers a commercial Enterprise edition. The vendor describes CA, RA, and validation functions, along with enrollment protocols and integration interfaces. Its stated use cases include internal PKI, industrial environments, IoT, and DevOps. These are vendor descriptions, not an independent assessment. See EJBCA’s official site.
Microsoft AD CS
Microsoft documents AD CS as a Windows Server role service that can configure a server to act as a CA. Its installation documentation, accessed October 7, 2026, lists Windows Server 2025, 2022, 2019, and 2016. Version coverage and installation requirements can change, so confirm them in Microsoft’s current installation documentation before planning a deployment.
Questions to compare before choosing
| Area | Questions to answer |
|---|---|
| Environment | Is the PKI mainly Windows-domain, cross-platform, cloud, appliance-based, or embedded and IoT? |
| Scale and topology | How many root and issuing CAs are needed? Should online and offline responsibilities be separated? |
| Workflow and integration | Which enrollment protocols, directories, APIs, and client platforms must work together? |
| Security operations | How will CA keys be generated, protected, backed up, recovered, and used? Who approves sensitive actions? |
| Lifecycle and visibility | How will issuance, renewal, revocation, certificate-status publication, auditing, and inventory be handled? |
| Ownership and support | Can the team operate the system? Is community support adequate, or are vendor support and service commitments needed? |
| Cost and licensing | What are the current licensing, infrastructure, deployment, and staffing costs? Verify these directly; current prices are not established here. |
These questions are evaluation criteria, not a claim that every product provides every capability. Product documentation and the organization’s own operational requirements should determine the comparison.
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An HSM is dedicated hardware that can provide a protected store for CA keys and perform cryptographic operations. Microsoft’s guidance lists PCI adapters, network appliances, serial devices, and USB devices as possible form factors; it notes that a network HSM may be shared among multiple CAs. A hardware choice must be supported by the CA software and fit the organization’s security and operational requirements.
Microsoft recommends installing and configuring an HSM before setting up CAs whose keys will be stored on it. Its guidance also treats PKI planning and HSM configuration as design considerations, rather than suggesting that every CA needs an HSM. See Microsoft’s Certification Authority Guidance and PKI design considerations for AD CS. EJBCA’s official material also documents HSM support, but compatibility depends on vendor and configuration.
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