by 
David Close, Chief Solutions Architect
Understanding Certificate Authorities (CA) and Their Role in Digital Trust
Overview
A certificate authority (CA) is a trusted entity in cryptography that issues digital certificates to verify identities in network communications. Within a public key infrastructure (PKI), a CA authenticates users, devices, and websites by binding their identity to an asymmetric key pair stored in a certificate. So, if you see HTTPS and a padlock in your browser, a certificate has been obtained from a certificate authority to certify that the site exists and can be trusted.
Table of Contents
- What Is a Certificate Authority (CA)?
- How Certificate Authorities Work
- Certificate Authority Hierarchy
- Asymmetric Encryption and Digital Certificates
- Certificate Lifecycle and Revocation
- Benefits of Certificate Authority
- Securing CAs with HSMs: Futurex Solutions
- Next steps
- FAQ
What Is a Certificate Authority?
A certificate authority (CA) is a trusted organization that issues digital certificates to bind public keys with identities. Each certificate contains the entity's identifying information (such as a domain name or username) and the CA's digital signature.
When a certificate authority issues a certificate, it signs the certificate with its private key so that anyone can verify the signature to confirm the certificate's authenticity.
In practice, when a browser connects to an HTTPS site, the site presents an X.509 certificate issued by a certificate authority. The browser checks the certificate using the certificate authority's public key to ensure the signature is valid.
If the signature checks out, the browser displays the padlock icon and establishes an encrypted connection. Thus, a padlock indicates a trusted certificate authority has verified the site's identity.
How Certificate Authorities Work
Certificate authorities operate as part of a public key infrastructure (PKI). PKI is "the set of roles, policies, hardware, software and procedures needed to create, manage, distribute, use, store and revoke digital certificates," with CAs at its core.
A PKI binds public keys with identities through CA-issued certificates. Below is a simplified view of how a CA typically works:
Certificate Signing Request (CSR)
An entity (server, user, or device) generates an asymmetric key pair (public and private). It then generates a certificate signing request (CSR) containing the public key and some identifying information, which it sends to a certificate authority.
Identity Verification
The certificate authority (or a delegated Registration Authority) verifies the requester's identity. For a website certificate, the CA verifies ownership of the domain. For an organization certificate, it may check business credentials. Public CAs may use email validation, DNS challenges, or documentation.
Validation Levels
Public CAs issue certificates at varying levels of validation. A Domain Validation (DV) certificate requires only proof of domain control (for example, by email or DNS challenge), while an Organization Validation (OV) certificate also verifies the organization's identity. Extended Validation (EV) certificates require the most thorough checks (legal documents, etc.). Regardless of the validation level, once approved, the CA issues the certificate using its standard process.
Certificate Issuance
Once the CA validates the identity, it uses its private key to sign a new certificate (creating a digital signature over the certificate contents). This signed certificate includes the requester's public key, identity info, validity period, and the CA's signature.
Certificate Distribution
The CA provides the signed certificate to the requester. The requester installs or publishes this certificate so others can retrieve it (for example, a web server serves its certificate during a TLS handshake).
Trust Establishment
Other parties (browsers, email clients) use the certificate authority's public key (from the CA's certificate) to verify the signature. If the signature is valid, the certificate and, by extension, the claimed identity is also considered valid.
A certificate authority's private key is crucial because it is used to stamp every certificate the CA issues. Organizations typically store the CA's private key in a hardware security module (HSM) to protect it. By contrast, the CA's public key is widely distributed (for example, as a self-signed root certificate in trust stores).
Certificate Authority Hierarchy
To scale and secure a PKI, organizations often use a multi-tier CA hierarchy:
Root CA
The top-level certificate authority. Its self-signed certificate serves as the ultimate trust anchor. The root CA is used only to sign intermediate CA certificates and is never involved in routine operations.
The best practice is to store this root CA offline, not powered on, and secured in a locked environment. This way, the root key only briefly comes online for tasks like signing an intermediate CA certificate and then is taken offline again.
Intermediate CAs
These certificate authorities are subordinate to the root CA. Each intermediate CA is trusted because it holds a certificate that the root CA has signed. Intermediate CAs may also be authorized to issue certificates to end-users or other subordinate CAs.
Using intermediate CAs helps protect the root; if an intermediate's private key is compromised, you can revoke that intermediate and issue a new one without re-establishing the root CA.
Issuing CAs
These issuing CAs provide end-entity certificates to users, servers, and devices. An organization may operate multiple issuing CAs under different intermediate CAs to separate workloads (for example, one for SSL/TLS and another for email). Each issuing CA's certificate links back up to the root CA through its chain of parent CAs.
Registration Authority (RA)
A registration authority (RA) is a non-mandatory entity validating certificate requests. It verifies the identities and credentials within CSRs and forwards approved requests to the certificate authority for signing, thereby streamlining the process.
Asymmetric Encryption and Digital Certificates
Certificate authorities rely on asymmetric encryption to secure the PKI process. Each participant (CAs and end-entities) uses a public/private key pair. The private key of the CA is used to sign certificates, and clients use the public key of the CA to authenticate signatures.
When a CA gives you a certificate, it signs it using the CA's private key. Any recipient of that certificate can then use the CA's public key (included in the CA's certificate) to check the signature.
If the decrypted signature is equal to the certificate's contents, we know the certificate is authentic.
This asymmetric signing process prevents tampering.
Asymmetric encryption also underlies data encryption in TLS/SSL. During a TLS handshake, the server presents its certificate (which contains a public key). After verifying the certificate, the client uses that public key to encrypt a symmetric session key.
Only the server can decrypt this session key using its private key. Because trusted certificate authorities issue certificates, the client can trust that it's using the genuine public key.
In short, a trusted CA vouches for an entity's public key, enabling secure and confidential communication.
Certificate Lifecycle and Revocation
Certificate authorities manage the entire certificate lifecycle: creation, issuance, renewal, and revocation of certificates. Key stages include:
Issuance
After validating the CSR, the CA creates and signs a new certificate and provides it to the requester.
Distribution
The certificate is deployed where needed. For example, a web server installs its certificate, or an email client receives its user certificate.
Expiration and Renewal
Certificates include an expiration date. Before expiry, the owner typically requests a renewal from the CA (often by submitting a new CSR or using an automated protocol).
For security reasons, many public SSL certificates now expire after 90 days. Some systems offer automatic renewal using protocols such as ACME.
If a certificate is renewed with a new key, it means a new certificate is issued; administrators should plan renewals ahead of time to avoid service interruption.
Revocation
A CA performs certificate revocation when a certificate's private key is compromised or the certificate is no longer valid. The CA then publishes the revoked certificate's serial number in a signed Certificate Revocation List (CRL) or includes it in responses to the Online Certificate Status Protocol (OCSP).
CRLs are signed lists of serial numbers that clients check; OCSP allows clients to query a CA/OCSP server about a specific certificate's status.
In summary, a certificate authority issues certificates and tracks and revokes them as needed. This complete lifecycle management, from creation to destruction, makes a PKI robust and secure.
Benefits of Certificate Authority
Using a certificate authority provides many benefits for secure networks:
Trust and Identity Verification
Certificate authorities verify the identity of the requesting party before issuing a certificate. This means that the certificates issued by the CA can be trusted to belong to the legitimate entity. The CA acts as a trusted third party that vouches for identities.
Strong Encryption
By issuing digital certificates containing public keys, certificate authorities enable end-to-end encryption. When two parties exchange certificates from trusted CAs, they can securely encrypt data, knowing the keys are authentic.
Scalability
Certificate authorities and PKI can scale to support millions of certificates. CAs often automate the processes of issuance, renewal, and certificate revocation, allowing organizations to manage large deployments of secure devices or services efficiently.
For example, a large enterprise might automatically issue and renew certificates for all servers and users via scripts or management tools.
Centralized Management
A CA offers a single point to manage certificates and keys. Administrators can configure policies (such as expiration periods and allowed uses) and use CRLs or OCSP to control trust. This centralized control simplifies auditing and compliance and reduces human error in certificate handling.
Protection Against Attacks
Because CAs validate identities and sign certificates, they help protect against man-in-the-middle and impersonation attacks. An attacker cannot impersonate a legitimate site or user without obtaining a valid certificate from a trusted CA for that identity. If someone attempts this, browsers warn users if an untrusted or unknown CA signs the certificate.
Non-Repudiation
Certificates used for digital signatures ensure that the signer cannot later deny having made the signature. Since only the holder of the private key corresponding to the certificate can create the signature, a CA-backed certificate establishes a verifiable link between the signer's identity and the signed content.
Regulatory Compliance
Many regulations and standards (PCI DSS, HIPAA, eIDAS, etc.) require strong authentication and encryption. Using a CA-backed PKI helps organizations meet these requirements since it provides an auditable framework for key management and certificate usage.
Versatility of Use Cases
Certificate authorities aren't just for websites. They also issue certificates for email encryption (S/MIME), code signing, VPN access, IoT devices, and more. Each certificate extends trust to a specific purpose within the digital ecosystem.
For example, CAs issue special code-signing certificates that allow software developers to sign executables, assuring users that the code hasn't been tampered with.
In essence, a certificate authority establishes digital trust within complex networks. One industry source notes that CAs "verify the authenticity and legitimacy of entities, ensuring secure communication" and "enable encryption" by providing the necessary keys.
These capabilities make CAs fundamental to any secure network.
Securing CAs with HSMs: Futurex Solutions
Given the critical nature of a CA's private keys, they should be protected in hardware security modules (HSMs). HSMs are hardened devices (or services) that securely generate, store and use cryptographic keys.
Futurex offers several HSM-backed solutions for building and protecting CAs:
Excrypt Series HSMs
A high-performance hardware security module ideal for on-premises PKI. The Excrypt Series HSMs can securely generate and store a CA's key pairs and perform certificate signings inside its secure boundary. It meets FIPS 140-3 Level 3 and Common Criteria standards, ensuring compliance for sensitive key operations.
KMES Series 3 Key Management Server
A comprehensive on-premises PKI and key management server, KMES Series 3 includes an embedded FIPS 140-3 Level 3 HSM and can establish an offline root certificate authority, create issuing CAs, and automate certificate lifecycles. It provides a unified interface (console and APIs) for managing all CA functions.
KMES also supports multi-tenant and role-based operations, so large organizations can securely partition CA operations (for example, separating issuing CAs for different business units) under the same root trust.
VirtuCrypt Cloud HSM
Futurex's cloud-based HSM service allows you to deploy HSMs and key management servers on demand in data centers worldwide. This is ideal for cloud or hybrid PKI. For example, a virtual issuing CA running in AWS or Azure can use a VirtuCrypt HSM for key operations.
VirtuCrypt offers compliance and full redundancy across global regions, reducing the cost and maintenance of on-premises HSMs while delivering the same cryptographic protections.
Futurex's HSM-backed CA solutions ensure that your root and issuing CAs operate under the highest security standards. Combining robust hardware protection with streamlined CA management, they help organizations confidently deploy trusted PKI infrastructure.
Next Steps
A well-architected certificate authority transforms your security posture by embedding trust at every network layer. It starts with a hardened offline root and continues with segmented intermediate and issuing CAs.
Protecting private keys and eliminating single points of failure ensures that every digital interaction, from user logins to device authentication, is built on a verifiable foundation.
Continuous lifecycle management keeps certificates current, while real-time status checks allow you to detect and revoke compromised keys the moment an issue arises.
The result is a resilient PKI that meets stringent compliance requirements and adapts seamlessly as you deploy new services, onboard IoT devices, or scale globally.
Schedule a demo today to learn how a certificate authority can integrate with your environment and deliver uninterrupted enterprise-grade trust.
Frequently Asked Questions (FAQs)
How is a trust established in a certificate authority hierarchy?
Trust is based on the chain of trust. The root certificate authority's self-signed certificate is the trust anchor that must be pre-trusted (for example, built into a browser or OS). Each CA certificate is signed by its parent CA, forming a verifiable chain. A client verifies a certificate by following this chain of signatures up to the root. The leaf certificate is considered trustworthy if every step in the chain is valid and leads to the trusted root CA certificate.
What is an intermediate certificate authority, and why is it used?
An intermediate CA is a subordinate certificate authority under the root CA. It issues certificates to end-entities or additional subordinate CAs. Using intermediate CAs enables the root CA to remain offline, limiting exposure to the most sensitive key. If an intermediate CA is ever compromised, it can be revoked (via CRL) without affecting the root CA or other intermediates. It also lets organizations segment trust domains; for example, one intermediate can handle SSL certificates while another handles internal device certificates.
Why is the root CA kept offline in a certificate hierarchy?
The root CA's private key is the most sensitive part of the system. Keeping it offline (in secure, unconnected storage) prevents network attacks or malware from stealing it. An offline root CA is only brought online for rare tasks (like signing an intermediate CA certificate) and then turned off. This maximizes security: if the root key never touches a network, the risk of compromise is minimized.
What happens if a root CA is compromised?
If a root certificate authority (CA) is compromised, it undermines the overall trust model. All certificates that chain to that root must be considered untrustworthy. The organization would need to generate a new root CA, issue new certificates for all intermediates and end entities, and distribute the latest root certificate to all clients. In public CA scenarios, browsers and systems must remove the compromised root from trust stores. This is a catastrophic event and very expensive to fix, so protecting the root CA is paramount.
Can an organization have multiple issuing CAs?
Yes. Organizations often deploy multiple issuing certificate authorities to manage different scopes, locations, or loads. For example, a company could run separate issuing CAs for various geographic regions, business units, or device types. Each issuing CA would inherit trust from the same root (or intermediate) CA. This setup improves performance, manageability, and scalability. For instance, a multinational enterprise might issue CAs in each data center, ultimately trusting the same corporate root CA.
What is the difference between a root CA and an issuing CA?
A root CA is a certificate authority at the top of the hierarchy that is not subordinate to any other authority. It serves as the ultimate trust anchor and is implicitly trusted by systems without requiring additional validation. In contrast, an issuing CA, a subordinate CA, is lower in the hierarchy and is responsible for issuing end-entity certificates to users, servers, or devices.
The root CA typically only issues certificates to intermediate CAs (and is kept offline), while issuing CAs handle the day-to-day operations of signing certificates.