Public Key Infrastructure (PKI) is one of the most important technologies in modern cybersecurity, yet 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 to make that interaction secure.

PKI is the framework of technologies, policies, and procedures that manages the creation, distribution, and revocation of digital certificates. These certificates verify identities, encrypt data, and establish the trust that digital systems depend on to function. Without PKI, 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.

How PKI Works

At its core, PKI uses asymmetric cryptography — a system built on mathematically linked pairs of cryptographic keys. Each entity (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 that 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 digital signatures that prove their identity.

The security of the entire model relies on the mathematical relationship between these two keys. 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.

When you visit a website secured with HTTPS, your browser performs a TLS handshake. During this process, the web server presents a digital certificate containing its public key. Your browser verifies that the certificate was issued by a trusted Certificate Authority, checks that it has not expired or been revoked, and 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 work together to 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 a certificate, 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 often 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, which contains 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 that an entity is who it claims to be. For a deeper explanation, see our guide to digital certificates, SSL/TLS, and X.509.

Certificate Revocation

Certificates sometimes need to be invalidated before their natural expiry — for example, if a private key is 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 certificate status checks. Effective revocation mechanisms are critical; without them, compromised certificates could continue to be trusted.

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 that private keys cannot be extracted, copied, or accessed by unauthorised parties, even by system administrators. They are standard practice in government, defence, and financial services PKI deployments.

How PKI Uses Encryption

PKI relies on two forms of encryption working together:

Asymmetric encryption uses the public/private key pair described above. It is computationally expensive, so it is typically used 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 encryption. Algorithms like AES-256 are the standard here.

In practice, PKI combines both. During a TLS connection, asymmetric encryption is used to securely exchange a symmetric session key. That session key then encrypts the actual data flowing between the two parties. This hybrid approach delivers both the trust of asymmetric cryptography and 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 this possible.

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

For organisations, maintaining digital trust means ensuring that certificates are valid, keys are protected, and PKI infrastructure is properly governed. When this breaks down — through expired certificates, misconfigured CAs, or compromised keys — the consequences can range from service outages to full security breaches. For a deeper exploration of why this matters, read our article on the importance of digital trust.

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. Here are some of the most common applications:

Web Security (HTTPS/TLS)

The padlock icon in your browser signals that the connection is encrypted and the server’s identity has been verified through a PKI-issued certificate. Without this, every online transaction — from shopping to banking — 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, and the recipient’s public key encrypts it so only they can read it. This is 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 this signature to confirm the software has not been modified since it was published and that it 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 can 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, PKI provides the cryptographic backbone for classified communications and secure access control.

Healthcare

PKI protects patient data under regulations like GDPR and HIPAA, authenticates medical devices, and secures the 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

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

Without PKI, organisations face:

No identity verification — systems have no reliable way to confirm that users, devices, or applications are who they claim to be, leaving them open to impersonation and man-in-the-middle attacks.

No data integrity — there is 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 to exchange keys securely, encrypting data between parties becomes impractical at scale.

Regulatory non-compliance — regulations including 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 attacks.

The Challenges of Managing PKI

While PKI is essential, it is not straightforward to operate. Many organisations struggle with:

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 these certificates is nearly impossible. A cryptographic bill of materials can help organisations map their full cryptographic estate.

Certificate expiry — expired certificates cause immediate, visible outages. The Microsoft Teams global outage in 2020, caused by a single expired authentication certificate, is one of the most widely cited examples. With industry moves toward shorter certificate lifetimes (potentially 90-day or even 47-day validity), the margin for error shrinks further.

Fragmented ownership — certificates are often managed by different teams across an organisation with no single point of accountability. Security, infrastructure, development, and operations teams may all issue and manage certificates independently, creating gaps in governance.

Legacy infrastructure — platforms like Active Directory Certificate Services have been the default for many organisations for years, but their limitations around cloud support, automation, and scalability are becoming increasingly problematic in modern environments.

These challenges are why certificate lifecycle management has become a critical discipline. CLM platforms automate the discovery, monitoring, renewal, and revocation of certificates across the entire estate, replacing manual spreadsheets and siloed processes with centralised, policy-driven management.

PKI and Post-Quantum Cryptography

The emergence of quantum computing presents a fundamental challenge to the cryptographic algorithms that PKI currently relies on. Algorithms like RSA and elliptic curve cryptography (ECC), which underpin the vast majority of today’s digital certificates, could be broken by a sufficiently powerful quantum computer running Shor’s algorithm.

While cryptographically relevant quantum computers do not yet exist, the threat is 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 becomes available.

NIST has already published post-quantum cryptography standards (FIPS 203, 204, and 205) and set a 2030 deadline for deprecating vulnerable algorithms. Organisations that depend on PKI need to start preparing now by assessing their cryptographic estate, building crypto agility into their architecture, and developing phased migration roadmaps. For more detail, see our guide to preparing your PKI for quantum computing.