Public key infrastructure (PKI)

A PKI has more moving parts than the certificate authority. The usual components are a root CA (offline, in an HSM), one or more issuing CAs, registration authorities that verify applicants, a repository for certificates and CRLs, validation services (CRL distribution points, OCSP responders), relying-party software that performs path validation per RFC 5280 §6, and the trust stores that define which roots are anchors. The governing documents are a Certificate Policy (what the certificates may be relied on for) and a Certification Practice Statement (how the CA actually operates), conventionally structured according to the RFC 3647 framework. X.509 itself dates from 1988 as part of the ITU-T directory standards; RFC 5280 is the internet profile.

Trust models vary. The Web PKI is a set of independent hierarchies selected by browser and OS vendors through their root programs and governed by the CA/Browser Forum Baseline Requirements, with Certificate Transparency (RFC 6962; RFC 9162 defines version 2) as a public audit layer. Enterprise PKIs are private hierarchies, most commonly Active Directory Certificate Services, used for 802.1X network access, VPN, smart-card logon, S/MIME and device identity through Intune or other MDM. Bridge and cross-certified structures connect separate hierarchies, as in the US Federal PKI, and name constraints (RFC 5280 §4.2.1.10) limit a subordinate CA to specific namespaces. OpenPGP's web of trust is the main non-hierarchical alternative and has seen little uptake outside niche communities.

Enrolment and life-cycle management protocols are where most operational work lies: ACME (RFC 8555) for automated domain-validated certificates, EST (RFC 7030), SCEP (RFC 8894) and CMP (RFC 4210, updated by RFC 9480) for devices and enterprise clients. With publicly trusted TLS certificates limited to 200 days since March 2026 and heading for 47 days by 2029, manual renewal is no longer viable, and certificate inventory becomes a core control.

Typical failure modes are organisational rather than mathematical: an expired root or intermediate that takes down every service at once, an internal root key kept on an online domain-joined server, revocation infrastructure that nobody monitors so CRLs silently expire and clients fail closed, and certificate templates in AD CS that let any authenticated user request a certificate for a domain administrator (the ESC1 class of misconfigurations described by SpecterOps in 2021). PKI also has to plan for algorithm transitions; the move from SHA-1 to SHA-256 took most of a decade, and migrating hierarchies to post-quantum signatures such as ML-DSA (FIPS 204) raises the same problem with much larger keys and signatures. In Denmark, MitID Erhverv and the national OCES certificates are examples of PKI operated as public infrastructure, and eIDAS qualified trust service providers are audited under ETSI EN 319 411 and supervised nationally.