What Is PKI? Public Key Infrastructure Explained (2026)

Public Key Infrastructure (PKI) is the framework of technologies, policies and procedures that manages the creation, distribution and revocation of digital certificates. Those certificates verify identities, encrypt data and establish the trust that digital systems depend on to function.

It is one of the most important technologies in modern cybersecurity, and most people have never heard of it. Every time you log into online banking, send a signed email, connect to a corporate VPN or see the padlock icon in your browser, PKI is working behind the scenes.

Without it, there would be no reliable way to confirm that the website you are visiting is genuine, that the software update you are installing has not been tampered with, or that the person sending you an encrypted message is who they claim to be.

This guide explains how PKI works, what it protects, why organisations need it, and what happens when it is poorly managed.

Key points

• PKI uses pairs of mathematically linked keys — one public, one private — to prove identity and protect data.
• It is an ecosystem, not a product: certificate authorities, registration authorities, certificates, revocation infrastructure and key protection.
• Most organisations run PKI successfully for years, then discover they have no complete picture of what they have.
• Shortening certificate lifetimes and post-quantum migration are both raising the operational bar within this decade.

How PKI works

At its core, PKI uses asymmetric cryptography — a system built on mathematically linked pairs of cryptographic keys. Each entity, whether a person, device, application or server, is issued a key pair.

• Public key — shared openly. Used by others to encrypt data sent to the key's owner, or to verify a digital signature the owner has created.
• Private key — kept secret and never shared. Used by the owner to decrypt data encrypted with their public key, or to create signatures proving their identity.

The security of the whole model rests on the mathematical relationship between the two. Data encrypted with a public key can only be decrypted with the corresponding private key, and vice versa. Even with access to the public key, deriving the private key is computationally infeasible with current technology. For more on the underlying mechanics, see our guide to encryption keys.

When you visit a website secured with HTTPS, your browser performs a TLS handshake. The server presents a digital certificate containing its public key. Your browser verifies that the certificate was issued by a trusted Certificate Authority, checks it has not expired or been revoked, then uses the public key to help establish an encrypted session. All of this happens in milliseconds, entirely invisibly.

The core components of PKI

PKI is not a single product or tool. It is an ecosystem of interconnected components that together create and maintain digital trust.

Certificate Authorities (CAs)

A Certificate Authority is the trusted entity that issues digital certificates. CAs verify the identity of the requesting party before issuing, acting as the foundation of the trust chain.

Organisations may use public CAs such as DigiCert, Sectigo or Let's Encrypt for externally facing services, or operate private CAs using platforms like Microsoft AD CS or EJBCA for internal systems. Large enterprises usually run both.

Registration Authorities (RAs)

Registration Authorities handle identity verification on behalf of the CA. They validate certificate requests, confirm the identity of the applicant, and pass approved requests to the CA for issuance. In many deployments the RA function is integrated into the CA platform itself.

Digital certificates

A digital certificate binds a public key to a verified identity. The most common standard is X.509, containing the subject's name, the public key, the issuing CA's signature, a validity period and information about permitted uses.

Certificates act as digital passports — they prove an entity is what it claims to be. For a deeper explanation, see our guide to digital certificates, SSL/TLS and X.509.

Certificate revocation

Certificates sometimes need invalidating before their natural expiry — most often because a private key has been compromised. PKI supports this through Certificate Revocation Lists (CRLs), which are published lists of revoked certificates, and the Online Certificate Status Protocol (OCSP), which provides real-time status checks.

Revocation is the component most often assumed to work rather than tested. In segmented or air-gapped networks, systems frequently have no route to a public OCSP responder or CRL distribution point, and validation behaves very differently in production than it did in the lab.

Hardware Security Modules (HSMs)

In high-assurance environments, private keys are stored in hardware security modules — dedicated physical devices engineered to protect cryptographic material. HSMs ensure private keys cannot be extracted, copied or accessed by unauthorised parties, including system administrators. They are standard practice in government, defence and financial services deployments.

How PKI uses encryption

PKI relies on two forms of encryption working together.

Asymmetric encryption uses the public and private key pair described above. It is computationally expensive, so it is typically reserved for short operations: exchanging session keys, creating digital signatures and authenticating identities.

Symmetric encryption uses a single shared key for both encryption and decryption. It is fast and efficient, making it suitable for bulk data. AES-256 is the standard.

In practice PKI combines both. During a TLS connection, asymmetric encryption securely exchanges a symmetric session key. That session key then encrypts the actual data flowing between the two parties. The hybrid approach delivers the trust of asymmetric cryptography with the performance of symmetric encryption.

PKI and digital trust

Digital trust is the confidence that users, customers and systems place in an organisation's ability to protect data, verify identities and deliver reliable services. PKI is the technical foundation that makes it possible.

Every time a certificate is presented and validated — a browser checking a website, an application authenticating to an API, a device proving its identity on a network — PKI is establishing trust between parties that may never have interacted before.

The model scales from a single website to entire national identity schemes. Countries including Estonia, the Netherlands and Spain issue citizens digital identity cards with embedded PKI certificates, enabling legally binding electronic signatures and secure access to government services. We have looked at what those national programmes get right separately.

For organisations, maintaining digital trust means ensuring certificates are valid, keys are protected and the infrastructure is properly governed. When that breaks down — expired certificates, misconfigured CAs, compromised keys — consequences range from service outages to full security breaches. Our article on the importance of digital trust explores this in more depth.

Where PKI is used

PKI is not confined to IT departments or server rooms. It is embedded across virtually every sector and touches almost every digital interaction.

Web security (HTTPS/TLS)

The padlock icon signals that the connection is encrypted and the server's identity has been verified through a PKI-issued certificate. Without it, every online transaction would be vulnerable to interception and impersonation.

Email security (S/MIME)

PKI enables digitally signed and encrypted email. The sender's private key signs the message to prove authenticity; the recipient's public key encrypts it so only they can read it. Standard practice in government, legal and financial communications.

Code signing

Software publishers use PKI certificates to sign their code. When you install an application, your operating system checks the signature to confirm the software has not been modified since publication and comes from a verified source.

Device authentication and IoT

In transport, manufacturing and critical infrastructure, PKI authenticates connected devices from roadside sensors to aircraft systems, ensuring only trusted devices communicate on the network. As IoT deployments scale, PKI becomes essential for maintaining device trust at volume.

Government and national identity

PKI underpins digital identity programmes in central government, enabling secure citizen services, electronic document signing and remote identity verification. In defence environments it provides the cryptographic backbone for classified communications and secure access control.

Healthcare

PKI protects patient data under regulations including GDPR, authenticates medical devices and secures data flows between clinical systems. Our work in healthcare PKI addresses the specific challenges of this highly regulated sector.

Financial services

Banks and financial services firms use PKI to secure transactions, authenticate customers, protect API communications between platforms and meet regulatory requirements around data protection and access control.

For more real-world examples, see our article on everyday examples of PKI in action.

Why organisations need PKI

PKI solves a fundamental problem: how do you establish trust between digital entities that may never have interacted before?

Without it, organisations face:

• No identity verification. Systems have no reliable way to confirm that users, devices or applications are what they claim to be, leaving them open to impersonation and man-in-the-middle attacks.
• No data integrity. No mechanism to detect whether data has been altered in transit, meaning intercepted communications could be modified without detection.
• No encryption framework. Without a trust model for exchanging keys securely, encrypting data between parties becomes impractical at scale.
• Regulatory non-compliance. GDPR, eIDAS, HIPAA and PCI DSS either explicitly require or implicitly depend on PKI capabilities. Organisations without effective PKI face audit findings, fines and reputational damage.

PKI also enables zero trust security strategies, where every access request is authenticated and authorised regardless of network location. In zero trust architectures, digital certificates replace passwords as the primary authentication mechanism, providing stronger security and eliminating entire categories of credential-based attack.

Where PKI goes wrong in practice

Most organisations run PKI successfully for years. Problems surface not because the cryptography fails, but because the operational picture drifts away from reality.

The patterns we encounter most often when assessing PKI estates:

Scale and visibility

A typical enterprise may have tens of thousands of certificates issued across multiple CAs, cloud environments and business units. Without centralised visibility, tracking them is effectively impossible. In most assessments, the number of certificates found exceeds the number the organisation had on record. A cryptographic bill of materials is how organisations map the full estate.

Fragmented ownership

Certificates are often managed by different teams with no single point of accountability. Security, infrastructure, development and operations may all issue and manage certificates independently. When something expires, the first question is usually whose certificate it was.

Certificate expiry

Expired certificates cause immediate, visible outages. The Microsoft Teams global outage in 2020, caused by a single expired authentication certificate, remains the most cited example — but smaller versions happen constantly and never make the news. With public TLS lifetimes falling to 47 days, the margin for error narrows considerably.

Trust hierarchies built for a different estate

PKI hierarchies are frequently designed for one purpose then extended well beyond it. Root CAs end up on hardware nobody wants to touch, key ceremonies go undocumented and recovery procedures are never tested. The infrastructure works — right up until somebody needs to change it.

Legacy platforms

Active Directory Certificate Services has been the default for many organisations for years, but its limitations around cloud support, automation and scale are increasingly problematic. AD CS issues certificates well. It provides no discovery, no central inventory and no automated renewal beyond auto-enrolment for domain-joined Windows machines.

These challenges are why certificate lifecycle management has become a discipline in its own right. CLM platforms automate discovery, monitoring, renewal and revocation across the estate, replacing manual spreadsheets and siloed processes with centralised, policy-driven management. Our article on the four pillars of CLM sets out what good looks like.

PKI and post-quantum cryptography

Quantum computing presents a fundamental challenge to the algorithms PKI currently relies on. RSA and elliptic curve cryptography, which underpin the vast majority of today's certificates, could be broken by a sufficiently powerful quantum computer running Shor's algorithm.

Cryptographically relevant quantum computers do not yet exist. The threat is still not theoretical. Nation-state adversaries are believed to be executing harvest now, decrypt later strategies — intercepting and storing encrypted data today with the intention of decrypting it once quantum capability arrives. Any data with a confidentiality requirement extending beyond a decade is already exposed.

NIST published its first post-quantum cryptography standards in 2024 — FIPS 203, 204 and 205 — and its transition guidance deprecates RSA and ECC after 2030, with the intention of disallowing them entirely by 2035.

Organisations that depend on PKI should start now by assessing the cryptographic estate, building crypto agility into the architecture and developing a phased migration roadmap. Our guides to the NIST PQC roadmap and preparing your PKI for quantum computing cover the detail.