What Is PKI? The Ultimate Guide to Public Key Infrastructure

Public key infrastructure (PKI) is critical for modern network security. PKI enables passwordless authentication and encrypted communication. Organizations such as CISA and NSA recommend PKI-based certificates in their joint hardening guidance.

PKI certificates are complicated, and even very senior security admins may find them difficult to understand.

“Cloud PKI services exist because PKI is just really hard to do in a safe, secure way. There are hundreds of pages of documentation showing how you might set up a legacy PKI like Active Directory Certificate Services in an insecure way just by using the default settings. There are tons of vulnerabilities that can happen if you don’t set it up right.”

Micah Spady, Director of Product Marketing at SecureW2

What Is PKI?

Public key infrastructure (PKI) is the set of processes and technologies that manage digital certificates and public-key encryption. A digital certificate, or PKI certificate, enables secure digital communication by acting as a digital passport to bind a public key to an entity such as a device, person, or server.

Info: At a very high level, the purpose of PKI is to allow organizations to use encryption for their various security needs.

It provides phishing-resistant authentication for applications or Wi-Fi and enables your web servers to communicate in encrypted tunnels, as shown in the graphic below.

PKI and digital certificates are fundamental and technologically advanced components of cybersecurity. We’ll break down how they work in the following sections.

You can also learn about managed PKI in this video.

How PKI Interacts With Digital Certificates

A PKI certificate is entirely dependent on the underlying PKI to give it authority. Without the system behind it to issue, validate, and track it, a digital credential cannot be trusted by a network.

This interaction forms the core of PKI security. The architecture securely binds a specific network identity to a cryptographic PKI key pair. When an enterprise user authenticates against an Identity Provider like Okta or Azure, the system evaluates the certificate to verify that both the user and the device are authorized.

To run this process smoothly at scale, modern PKI solutions, often delivered as PKI-as-a-Service (PKIaaS), leverage automated PKI software to manage the operational workload across three areas:

• PKI Encryption: The architecture leverages the relationship between a public and private PKI key to protect data transit lanes.
• PKI Management: Automated PKI certificate management monitors active lifecycles to swap out expiring credentials before they trigger a system outage.
• Unified Governance: This robust PKI cybersecurity model gives infrastructure teams full control over trust chains and revocation lists.

The video below explains how PKI works at a high level:

What Are the Components of Public Key Infrastructure?

PKI infrastructure is made up of several components that work together to manage the entire certificate lifecycle. While the field continues to evolve, these are the core components every modern PKI needs:

1. Certificate Authority (CA)
2. Certificate Templates
3. Certificate Lifecycle Management
4. Hardware Security Module (HSM)

Here is what each component does and why it matters, beginning with a graphic illustration of how a certificate chain is structured.

Certificate Authority (CA) 

A PKI Certificate Authority (CA) is the backbone of PKI management. It is the trusted entity responsible for issuing and signing digital certificates that bind a public key to a verified identity.

Here is how the CA process works:

• The client generates a public-private key pair.
• The public key and identity information are sent to the CA.
• The CA validates the request and creates a signed digital certificate.
• That certificate is then used to authenticate the user or device during access attempts.

Every certificate can be traced back through a chain of trust to a single root CA, which is the ultimate source of trust in any PKI infrastructure.

Root Certificate Authority 

The root CA sits at the top of the PKI trust chain and is the most sensitive component in the entire infrastructure.

• It signs and authorizes intermediate CAs, which then issue certificates to end users and devices.
• It never issues certificates directly to end users.
• If the root CA is compromised, every certificate in the PKI is at risk.
• For this reason, root CAs are kept offline and isolated from the public internet.

Intermediate Certificate Authority 

The intermediate CA acts as a layer between the root CA and the end user certificates.

• It inherits trust from the root CA, so any certificate it issues is treated as trusted.
• Keeping the root CA offline is possible because the intermediate CA handles day-to-day certificate issuance.
• The best practice is to always issue PKI certificates from an intermediate CA, never directly from the root CA.

Certificate Templates

Certificate templates are used to determine how a certificate is structured and what it will be used for.

For public key infrastructures such as Active Directory Certificate Services (AD CS), certificate templates are enormously important and dictate many of the permissions required for receiving a certificate and what it will be used for. Exploiting a misconfigured certificate template in AD CS is one of the most common compromises.

Below is an example of a Default Certificate Template in the SecureW2 PKI. You can see attributes such as Validity Period, Signature Algorithm, Extended Key Usage (EKU, or what the certificate will be used for), and much more.

Certificate Lifecycle Management

Certificate lifecycle management covers the processes used to manage certificates from enrollment to expiration or revocation. The key stages are:

1. Enrollment: A user, device, or application requests a certificate from a Certificate Authority (CA).
2. Issuance: The CA verifies the requester’s identity and issues the certificate if it meets the required policies.
3. Validation: During authentication, the certificate is checked for a valid trust chain, expiration, and revocation status using mechanisms such as CRL or OCSP.
4. Renewal: Certificates are renewed before expiration to maintain continuous authentication. Renewal may involve identity verification and generating a new key pair.
5. Revocation: A certificate is revoked before expiration if its private key is compromised, the identity is no longer trusted, or other security risks arise.

For a detailed overview, see SecureW2’s certificate lifecycle management guide.

Hardware Security Module

A Hardware Security Module is an optional PKI component that significantly strengthens PKI security by protecting and managing cryptographic keys throughout their entire lifecycle, from creation and rotation to deletion and auditing.

Do You Need a Two-Tier or Three-Tier PKI?

A two-tier PKI uses a root CA and one intermediate CA. A three-tier PKI adds a second layer of intermediate CAs beneath the first, usually split by department, region, or certificate type.

Most small and mid-sized organizations only need two tiers. A three-tier setup makes sense when you need to isolate issuance across business units, or when a single compromised Intermediate CA should not put every certificate in the organization at risk.

The other trade-off is whether the issuing CA stays online.

An issuing CA has to remain online to sign certificates on demand, which makes it the most exposed part of the hierarchy. The root CA should never be online, regardless of how many tiers you run. Keeping it offline and airgapped, and only bringing it online for scheduled signing events, is what limits the blast radius if an issuing CA is ever compromised.

 As a best practice, always issue certificates from an intermediate CA.

How Does Public Key Infrastructure Work?

At its core, PKI turns random mathematical keys into trusted digital identities. It works like a digital passport system for your network. A central authority checks a user's identity, confirms it is real, and issues a digital certificate. This certificate proves to the rest of the network that the user or device can be trusted.

To understand how this infrastructure scales securely without performance lag, we must look at how the underlying encryption styles work together:

1. Asymmetric Cryptography (Proving Identity)

This layer uses two different keys that work together as a pair:

• The Public Key: Shared openly with everyone.
• The Private Key: Kept strictly secret on the device.

When a device wants to log in, it uses its secret private key to sign a request. The server uses the open public key to verify that signature. Because the private key never leaves the device, hackers cannot steal it in transit like a traditional password.

Example of Asymmetric Encryption

The classic way of explaining asymmetric encryption is the Alice and Bob diagram, below:

The PKI system achieves strong asymmetric encryption using clever mathematics. With RSA, for example, the process involves multiplying two huge prime numbers (each about 1,024 bits long for a 2,048-bit key) and relying on how hard it is to factor that product back into the original primes, though the full explanation is much more complicated. 

These two videos help explain the math behind PKI in more detail:

• Public Key Cryptography ← how public key infrastructure works at a high level
• Diffie-Hellman – The Mathematics ← the math behind asymmetric encryption

2. Symmetric Cryptography (Moving Data Fast)

Asymmetric encryption is great for proving identity, but it takes too much computing power to encrypt large amounts of live network traffic.

Symmetric encryption fixes this. It uses a single shared key to scramble and unscramble data at lightning-fast speeds.

3. The Hybrid Handshake (The Solution)

PKI combines both types of encryption during each connection:

• Step 1: The device and server use the secure asymmetric keys to verify each other's identity.
• Step 2: During that conversation, they use a key exchange such as ECDHE to derive a shared, temporary symmetric session key.
• Step 3: The asymmetric keys step aside, and the fast symmetric key encrypts the rest of the active session.

The 4-Step Process in Action

The actual day-to-day operation of PKI follows a straightforward lifecycle, as mapped out in the table:

What Is PKI Used For?

PKI is important for cybersecurity because it binds identities to public keys through certificates issued by a trusted certificate authority. This gives every user and device a verified source of truth on the network. When paired with systems like 802.1X, VPN gateways, or mutual TLS, PKI confirms identity before granting any access.

Validates Trustworthiness of Users and Devices

Each user or device gets a PKI certificate from a trusted CA that confirms its identity. To connect, it proves possession of the private key without ever exposing it, which makes impersonation and stolen credentials far harder to use. Many enterprises pair this with PKI smart card authentication for high-assurance, hardware-bound login, including hardware tokens like YubiKeys.

This process makes it much harder for attackers to impersonate someone or use stolen credentials.

Leverages EAP-TLS in a WPA2-Enterprise Network

Digital certificates power mutual EAP-TLS authentication on WPA2-Enterprise networks. This gives the EAP-TLS protocol a framework for faster, more secure mutual authentication between clients and servers.

Protects Remote Cloud Environments

Certificate-based 802.1X authentication lets users and devices connect to a remote server without the risk of stolen or leaked passwords. A certificate management system tied to a PKI automates the certificate lifecycle for managed, BYOD, and guest devices, so only authorized personnel can access specific data and applications.

The following table explains the differences between a password-based network and a PKI-based network.

Wi-Fi Authentication

PKI enables secure, certificate-based Wi-Fi authentication using EAP-TLS, which verifies both the client device and the RADIUS server. This replaces weaker password-based methods with asymmetric cryptography and a trusted CA. For setup steps, see our guide to configuring PKI for Wi-Fi.

Wired Security PKI

This secures wired connections through 802.1X port-based authentication, where a switch blocks access until the device presents a valid certificate. It prevents unauthorized devices from joining the LAN and blocks rogue network access.

Web Application Authentication

A user connecting to a web application has their identity confirmed by the application server, much like Wi-Fi authentication. Since the certificate is signed by a trusted CA, the user gains access.

VPN Authentication

Certificates are preferred over passwords for VPN access since VPNs reach critical information. The root or intermediate CA is usually stored on the firewall, and once authenticated, a secure tunnel opens to the network.

Email Security

Email encryption uses the S/MIME protocol, which requires both sender and receiver to hold a CA-signed certificate. This proves the message origin and encrypts the content so only the recipient’s private key can decrypt it.

Mutual TLS (mTLS) for Service-to-Service Communication

Mutual TLS uses PKI certificates to verify both sides of a connection, not just the server. This makes it a common choice for service-to-service communication in microservices, Kubernetes clusters, and AI agent workflows, where each service or workload needs to prove its identity before another will talk to it. Because both parties authenticate with certificates instead of shared secrets, mTLS removes the need for API keys or tokens that can be leaked or reused.

See PKI in action: Read our case study describing how a community bank deployed PKI across five branches with SecureW2.

Improve End-User Experience

Passwords require repeated input and frequent resets, and adding MFA on top often frustrates users.

Digital certificates with a PKI are becoming the preferred choice as more organizations move to passwordless authentication, a shift reflected in NIST's specific guidelines on digital identity and phishing-resistant authentication.

Info: The National Institute of Standards and Technology (NIST) prescribes specific frameworks to secure critical data in an enterprise network and the organization strongly recommends using digital certificates with a PKI for Wi-Fi and VPN authentication.

Prevent Breaches Due to Stolen Credentials

A device must prove possession of its private key to connect, and that key can't be phished or reused like a password. Even if an attacker intercepts network traffic, they can't authenticate without the key itself.

Reduce Password-Related Support Tickets

Since certificates replace passwords, users no longer need to remember, reset, or change credentials. Automated issuance and renewal also cut down manual work and support costs.

Increase Visibility Over Network Access

Unlike passwords, certificates are device-bound and traceable. Detailed RADIUS logs show exactly who and what connected or accessed the network at authentication time.

What Is the Difference Between PKI and SSL/TLS?

PKI is the broader framework that manages digital trust, while SSL/TLS (Secure Sockets Layer/Transport Layer Security) is a protocol that uses certificates to secure network communication.

The following table explains the key differences between PKI and SSL/TLS.

The Challenges of Managing PKI at Scale

There are a number of challenges for teams managing PKI at scale, as the following table explains.

Why Post-Quantum Cryptography Matters for PKI

Public key infrastructure relies entirely on mathematical algorithms like RSA and ECC to secure connections and authenticate entities. However, future quantum computers will easily solve these math problems, rendering today’s standard digital certificates ineffective.

Post-Quantum Cryptography (PQC) matters for PKI because certificates protect long-term infrastructure, trusted identities, and historical data. Upgrading a PKI isn't an overnight task; it requires inventorying legacy certificates, upgrading protocols, and adopting quantum-resistant algorithms.

Preparing for PQC ensures your PKI retains data confidentiality and device trust as cryptographic standards evolve, preventing massive network disruption before traditional algorithms become obsolete.

PKI Implementation Best Practices for Your Network

Using PKI is a strong step toward a secure network, but a PKI only works if it is implemented the right way. A vetted strategy and a forward-looking rollout plan turn PKI into real identity and context-based device trust across your network.

Note: A PKI alone won’t address security needs unless properly implemented.

• Keep the root CA separate with an intermediate CA: Set up an intermediate CA under the root, then split issuance by department or location, so the root CA stays offline and well protected.
• Automate certificate lifecycle management: A managed PKI automates enrollment, issuance, renewal, and revocation from one platform, so nothing gets missed.
• Run regular audits and compliance checks: Keep an audit trail of every certificate issued, renewed, or revoked, and set up automated alerts so security teams catch unusual activity fast.
• Integrate PKI with IAM, IdP, and MDM tools: Connecting PKI to your identity provider and MDM lets you use EAP-TLS to tie certificate-based authentication directly to user and device identity.
• Protect private keys with an HSM: Root and intermediate keys should never leave the hardware security module, and access to them should be tightly limited and logged.
• Automate onboarding for managed and BYOD devices: Onboarding software issues certificates automatically using protocols like SCEP and ACME, letting BYOD and guest devices self-enroll safely.
• Automate short-lived certificates for CI/CD and Kubernetes: Protocols like ACME, EST, and SCEP can issue and rotate short-lived certificates automatically for workloads in CI/CD pipelines and Kubernetes clusters, so ephemeral infrastructure gets authenticated without manual certificate handling.
• Plan for shorter certificate lifespans: Major browsers and CAs are moving toward much shorter TLS validity periods, so building automation now keeps your PKI ready ahead of time.
• Build in crypto agility: Post-quantum algorithms are already being standardized and piloted, so choose a PKI that can swap algorithms and key types without a full rebuild.

Automate Your PKI Without the Overhead of On-Premises Infrastructure

Running your own PKI means managing certificate lifecycle, intermediate CAs, revocation infrastructure, and renewal workflows, often with tools that weren’t built for the speed modern environments demand.

The SecureW2 JoinNow Dynamic PKI continuously evaluates device posture using real-time API integrations during certificate issuance, renewal, authentication, and whenever security signals change. That ensures certificate trust reflects the device’s current security state, not a point-in-time assessment made months earlier.

Eliminate the need for an on-premises CA, revoke certificates in seconds, and deploy a full-stack platform designed to scale across diverse device ecosystems, not just a single MDM vendor.

Our PKI handles the infrastructure so you can focus on strategy. Reach out to us for a free demo of Dynamic PKI today.