What Is a Certificate Authority (CA)?

A certificate authority (CA) is a trusted organization that issues and verifies digital certificates used to prove the identity of websites, servers, individuals, or devices online.

What Is a Certificate of Authority?

A certificate authority is a trusted third-party entity within a public key infrastructure (PKI) responsible for issuing, validating, and managing digital certificates. When an organization, website, or user requests a certificate, the CA verifies their identity using predefined validation procedures, such as checking domain ownership, business records, or legal documentation.

After successful verification, the CA signs a digital certificate with its own private key, binding the subject’s identity to their public key. Because operating systems, browsers, and many applications include a set of pretrusted “root” CAs, any certificate that chains back to one of these roots is automatically treated as trustworthy. In practice, this allows users to confirm that they are communicating with the right server and to establish encrypted connections (for example, via HTTPS) without manually checking keys.

Beyond issuance, CAs also manage certificate revocation through mechanisms like certificate revocation lists (CRLs) and the online certificate status protocol (OCSP), helping ensure that compromised or invalid certificates can be flagged and no longer trusted.

What Are the Different Types of Certificate Authorities?

Different types of certificate authorities work together to build a scalable and secure public key infrastructure (PKI). Each type has a specific role in the trust chain, from anchoring global trust to issuing certificates for everyday use.

Root Certificate Authority (Root CA)

A root CA is the top-level authority in a PKI hierarchy and serves as the ultimate trust anchor. Its root certificate is self-signed and preinstalled in operating systems, browsers, and devices. Because any compromise of a root CA would undermine trust in all certificates beneath it, root CAs are typically kept offline, heavily protected, and used only to sign intermediate CA certificates rather than end-entity certificates directly.

Intermediate (Subordinate) Certificate Authority

An intermediate CA, also called a subordinate CA, sits between the root CA and the end-entity certificates used by websites, services, or users. The root CA signs the intermediate CA’s certificate, and the intermediate CA then issues certificates further down the chain. This layered design limits risk so if an intermediate CA is compromised, the root can revoke just that intermediate certificate without invalidating the entire PKI or replacing the root keys.

Issuing Certificate Authority

An issuing CA is the authority that actually signs and issues end-entity certificates for servers, applications, devices, or users. In some PKI designs, the same CA functions as both intermediate and issuing CA; in others, issuing CAs are separate from policy or offline intermediates to better isolate roles and reduce exposure. Issuing CAs handle the bulk of operational work, such as processing certificate requests, applying validation policies, and managing renewals and revocations.

Public (Commercial) Certificate Authority

A public CA is a commercial or public provider whose root certificates are trusted by major browsers, operating systems, and devices. These CAs issue certificates for domains and organizations on the public internet, following industry standards (such as the CA/Browser Forum Baseline Requirements) and undergoing regular audits. When you see a valid HTTPS padlock in your browser, it usually indicates that a public CA has validated the site’s identity and issued its TLS certificate.

Private (Enterprise or Internal) Certificate Authority

A private CA is operated by an organization for its own internal use rather than for the general public. Its root certificate is not automatically trusted by external systems but can be deployed to company devices, servers, and applications to establish an internal trust environment. Private CAs are commonly used to issue certificates for internal services, VPNs, Wi-Fi authentication, device management, and user authentication, giving organizations tighter control over policies, lifetimes, and usage while keeping certificate issuance costs and dependencies in-house.

Certificate Authority Example

One concrete example of a certificate authority is Let’s Encrypt. It is a nonprofit CA operated by Internet Security Research Group (ISRG) that provides free, automated TLS/SSL certificates for anyone who owns a domain.

Let’s Encrypt issues mostly “domain-validated” certificates, meaning it confirms only that the applicant controls the domain, not necessarily their full identity, and aims to make encrypted connections the default for the web.

What Is the Purpose of a Certificate Authority?

The purpose of a certificate authority is to act as a trusted third party that vouches for digital identities so secure communication can happen over untrusted networks like the internet. A CA verifies that a public key actually belongs to a specific domain, organization, or user and then issues a digital certificate that binds this identity to the key. Because operating systems, browsers, and applications are configured to trust certain CAs, they can automatically validate these certificates and establish encrypted connections (for example, via HTTPS) without manual checks.

In practice, this means the CA’s main role is to enable authentication (you are talking to the right party), confidentiality (data is encrypted), and integrity (data is not altered in transit) in online interactions.

How Does a Certificate Authority Work?

A certificate authority verifies identities and then uses cryptography to bind those identities to public keys so that other systems can automatically trust them. The process follows a series of steps that turn a simple key pair into a trusted digital certificate, including:

1. Key pair generation. The organization, website, or device first generates a cryptographic key pair: a private key (kept secret) and a public key (shared). This pair forms the basis for encryption and digital signatures, ensuring that only the holder of the private key can decrypt data or prove their identity.
2. Certificate signing request (CSR) creation. Next, the key pair owner creates a certificate signing request (CSR). The CSR bundles the public key with identifying information, such as the domain name and organization details. This step prepares a standardized package the CA can examine and, if approved, sign.
3. Submission to the CA. The CSR is sent to the certificate authority. At this point, the CA knows what identity is being claimed (for example, a specific domain or company) and which public key should be associated with that identity. This step formally starts the validation process.
4. Identity and domain validation. The CA then verifies the requester’s control over the domain and, depending on certificate type, may also validate organization details (e.g., legal name, address, corporate records). This step is crucial because it confirms that the entity requesting the certificate is legitimately tied to the claimed identity.
5. Certificate issuance and signing. Once validation succeeds, the CA creates a digital certificate that includes the subject’s identity information, the public key, validity period, and other metadata. The CA signs this certificate with its own private key. This step cryptographically binds the identity to the public key and makes the certificate verifiable by anyone who trusts the CA.
6. Certificate installation and use. The organization installs the issued certificate (and often any intermediate CA certificates) on its server or device. When clients connect, for example, via HTTPS, the server presents this certificate. This step enables clients to see who they are connecting to and retrieve the correct public key for secure communication.
7. Client validation and ongoing trust management. The client (browser, app, or device) checks the certificate’s signature, chain of trust, validity dates, and revocation status against its built-in list of trusted CAs. If everything checks out, it establishes an encrypted session; if not, it warns the user. Over time, the CA also maintains revocation lists and status services (CRL/OCSP) to flag compromised or expired certificates, preserving trust in the overall system.

Certificate Authority Best Practices

Certificate authorities must follow strict operational and security practices to remain trustworthy. Good CA hygiene protects private keys, reduces attack surface, and ensures that issued certificates accurately represent real identities. Here are the best practices to implement:

• Protect root CA keys offline. Keep root CA keys on highly protected, offline systems (or hardware security modules) and use them only to sign intermediate CA certificates. This minimizes exposure: if an online system is compromised, attackers cannot directly access the root key.
• Use hardware security modules (HSMs). Store and use CA private keys inside certified HSMs rather than in software or on general-purpose servers. HSMs provide tamper resistance, strong access controls, and secure key operations, greatly reducing the risk of key theft or misuse.
• Separate root, intermediate, and issuing CAs. Design a hierarchy where the root signs intermediate CAs, and intermediates/issuing CAs handle day-to-day certificate issuance. This separation lets you revoke or replace a compromised or misconfigured intermediate without destroying trust in the entire PKI.
• Enforce strong validation procedures. Apply clear, documented identity-validation policies for each certificate type (DV, OV, EV, internal). Consistent checks for domain control and organization identity prevent fraudulent certificates and keep assurance levels predictable.
• Implement strict access control and auditing. Limit who can approve, issue, or revoke certificates, and enforce multi-factor authentication for administrative access. Comprehensive logging and regular audit reviews help detect misuse, policy violations, or suspicious issuance patterns.
• Maintain timely revocation mechanisms. Publish accurate certificate revocation lists (CRLs) and support online certificate status protocol (OCSP) with good availability and performance. Rapid revocation ensures that compromised, misissued, or deprecated certificates are quickly marked untrustworthy.
• Monitor issuance and use certificate transparency (CT). Log public-facing certificates to certificate transparency logs and monitor them for anomalies (unexpected domains, typo-squats, or policy violations). This open visibility helps detect misissuance and supports rapid remediation.
• Keep software and configurations hardened. Regularly patch CA systems, disable weak cryptographic algorithms, and enforce modern key sizes and TLS configurations. Hardening reduces the chance that attackers can exploit outdated software or weak crypto to compromise the CA.
• Perform regular third-party audits and compliance checks. Subject CA operations to independent security and compliance audits (e.g., WebTrust, ETSI, CA/B Forum requirements for public CAs). External scrutiny validates that policies are followed in practice and helps maintain trust with browsers and relying parties.
• Define clear lifecycle and incident-response procedures. Document how keys and certificates are generated, rotated, renewed, and retired, and establish a playbook for handling key compromise or misissuance. A well-defined lifecycle and response plan ensures consistent behavior under normal conditions and during security incidents.

How to Find a Certificate Authority?

You can find a certificate authority based on where and how you plan to use digital certificates. Most organizations rely on public CAs that are already trusted by browsers and operating systems. These trusted providers are listed in the “root certificate store” built into major platforms, meaning certificates they issue will be automatically accepted on the public internet. Many well-known security vendors operate as public CAs and offer different validation levels depending on your needs.

In internal or private environments, an organization can set up its own private CA using tools such as Microsoft Active Directory Certificate Services or dedicated PKI platforms, then distribute the root certificate to company devices. In either case, the goal is to choose a CA you trust to securely verify identities and reliably support certificate lifecycle management.

The Benefits and Challenges of CA

Certificate authorities bring clear benefits by enabling trusted encryption, authentication, and integrity for online communication, but they also introduce dependencies and risks that must be carefully managed. Understanding both the advantages and the challenges of CAs helps organizations design a PKI that is secure, resilient, and aligned with their security and compliance needs.

What Are the Benefits of Using a Certificate Authority?

Using a certificate authority provides a structured way to establish digital trust at scale. CAs make it practical for users, systems, and organizations to authenticate each other and protect data over untrusted networks. Here are its main benefits:

• Strong authentication of identities. A CA verifies that a public key belongs to a specific domain, organization, or user, then binds that identity to the key in a certificate. This gives clients confidence that they are connecting to the right server or service, not an impostor.
• Encrypted communication over untrusted networks. Certificates issued by a CA enable protocols like HTTPS, TLS, and VPNs to set up encrypted channels. This protects data in transit from eavesdropping and interception, even when traffic crosses public networks such as the internet or shared Wi-Fi.
• Integrity and tamper detection. CA-signed certificates use digital signatures that allow clients to verify that the certificate contents have not been altered. Combined with TLS, this ensures that data exchanged during a session cannot be silently modified without detection.
• Scalable, automated trust model. Because operating systems and browsers ship with pre-trusted root CAs, certificates that chain back to those roots are accepted automatically. This enables global, large-scale deployment of secure connections without users having to manually manage keys.
• Support for compliance and regulatory requirements. Many regulations and security frameworks (such as PCI DSS, HIPAA, and ISO 27001) expect or require encrypted communications and strong authentication. Using a reputable CA helps organizations meet these requirements and demonstrate due diligence.
• Centralized certificate lifecycle management. CAs provide tools and processes for issuing, renewing, and revoking certificates. This centralized lifecycle management makes it easier to maintain up-to-date encryption, rotate keys regularly, and respond quickly if a certificate or key is compromised.
• Interoperability across platforms and ecosystems. Certificates from well-known CAs work across different operating systems, browsers, devices, and applications. This interoperability makes it possible to build secure services that can be accessed by diverse users and clients without custom trust setups.

What Are the Challenges of Using a Certificate Authority?

Using certificate authorities also comes with challenges that organizations must understand and manage. These issues mostly revolve around operational complexity, security risk, and reliance on external trust anchors:

• Single points of trust and failure. A CA becomes a central trust anchor: if it is compromised, misconfigured, or behaves improperly, many certificates and systems can be affected at once. Public CA incidents can force emergency revocations and mass certificate replacements across the internet.
• Operational complexity and overhead. Running or integrating with a CA involves managing key pairs, certificate requests, renewals, revocations, and policy enforcement. Without good tooling and processes, teams struggle with expired certificates, inconsistent configurations, and manual “fire drills.”
• Risk of misissuance and human error. Weak validation procedures or mistakes in identity checks can result in certificates being issued to the wrong party. Misissued certificates can enable impersonation (e.g., fraudulent HTTPS sites) and often require rapid revocation and public incident handling.
• Key management and protection challenges. CA private keys must be safeguarded using HSMs, strict access controls, and strong internal security. Any leakage or misuse of these keys undermines the entire PKI but maintaining that level of protection over time is demanding and expensive.
• Revocation effectiveness and performance. Revocation mechanisms like CRLs and OCSP are not always reliable or fast. Clients may ignore revocation checks, fall back to “soft fail,” or experience latency and availability issues, leaving revoked certificates still effectively trusted in practice.
• Dependency on external policies and audits. Organizations relying on public CAs must trust that those CAs follow strong security practices, adhere to industry standards, and pass regular audits. Policy changes, distrust events, or ecosystem rules (e.g., browser requirements) can force unexpected migrations.
• Scalability and automation at a large scale. In environments with thousands of services and short certificate lifetimes, keeping certificates renewed and correctly deployed requires extensive automation (e.g., ACME, DevOps integration). Without it, outages from expired certificates become a constant risk.
• Interoperability and legacy constraints. Not all clients support the same algorithms, key sizes, or modern PKI features. Balancing compatibility with older systems against security best practices (e.g., phasing out weak ciphers or SHA-1) can complicate CA configuration and migration planning.

Certificate Authority FAQ

Here are the answers to the most commonly asked questions about certificate authorities.

Are Free Certificate Authorities Safe?

Free certificate authorities can be safe as long as they are reputable, widely trusted by major browsers and operating systems, and follow strong security and validation standards. The “free” aspect usually reflects their business model or mission (for example, automating and democratizing HTTPS), not weaker security by default. However, you still need to follow best practices on your side: protect your private keys, use HTTPS correctly, keep software up to date, and ensure you obtain certificates only from CAs included in standard trust stores and with a solid track record of audits and compliance.

What to Do if a Certificate Authority Is Hacked?

If a certificate authority is hacked, the immediate priority is to revoke any potentially compromised certificates and keys, then replace them with new ones issued from a trusted CA or a newly secured infrastructure. Organizations should quickly update affected systems, redeploy certificates, rotate keys, and verify that clients no longer trust the compromised CA. Communication and coordination are essential, as relying parties must be informed so they can remove the breached CA from their trust stores and prevent attackers from using forged or misissued certificates to impersonate legitimate services.

How Long Should a Certificate Authority Be Valid?

How long a certificate authority should be valid depends on its role in the PKI hierarchy and how you balance long-term stability with security and flexibility. Here are typical guidelines.

A root CA certificate often has a long validity, commonly 10–25 years, because it serves as the ultimate trust anchor and replacing it frequently would require re-deploying trust on all clients. For a subordinate (intermediate or issuing) CA, a shorter validity is wiser, often about half the root CA’s lifetime (e.g. if root is 20 years, intermediate might be 10 years), to limit exposure and make planned renewals manageable.

Shorter validity for CA certificates helps mitigate future cryptographic risks or changes to security standards, while still giving enough time to maintain trust without frequent renewals.