Public Key Infrastructure (PKI) Explained

Public Key Infrastructure (PKI) is the system of certificates, authorities and keys that lets you trust that the website you visit really is your bank, not an impostor. It is the trust layer of the internet — and it must survive the quantum transition.

What PKI Does

PKI solves a fundamental problem: how do you trust a public key? Anyone can generate a key pair, so receiving a public key tells you nothing about who it belongs to. PKI answers with digital certificates — signed statements that bind a public key to an identity.

The Components

• Certificate Authority (CA): a trusted organisation that verifies identity and signs certificates.
• Registration Authority: verifies the identity of certificate applicants.
• Certificates: documents binding a public key to an identity, signed by a CA.
• Certificate Revocation: CRLs and OCSP let you check if a certificate was revoked.
• Trust stores: browsers and OSes ship root CA certificates that anchor the chain of trust.

The Chain of Trust

When you visit an HTTPS site:

1. The server presents its certificate.
2. Your browser checks the signature against a CA it trusts.
3. If valid, the browser uses the certificate's public key to set up encrypted communication.

This chain — root CA → intermediate CA → end-entity certificate — is what makes the web trustworthy.

Where Quantum Threatens PKI

PKI relies on digital signatures (RSA, ECDSA) to sign certificates. A quantum computer could:

• Forge CA signatures, impersonating any website.
• Forge end-entity certificates, enabling man-in-the-middle attacks.
• Break long-lived root CA keys, collapsing trust in entire hierarchies.

Because root CA keys can last 10-25 years, they are among the most urgent things to migrate to post-quantum signatures (ML-DSA, SLH-DSA).

Migrating PKI to Post-Quantum

• New root CAs should use post-quantum or hybrid signatures from the start.
• Hybrid certificates (classical + post-quantum signatures) allow gradual migration.
• Larger signatures (ML-DSA, SLH-DSA) require updates to certificate formats and parsers.
• Shorter certificate lifetimes reduce exposure during transition.

NIST and the CA/Browser Forum are already working on post-quantum certificate standards. The migration will take years because every browser, OS and device must support the new signatures.

PKI Beyond the Web

PKI also secures:

• Code signing (software updates, app stores).
• Email (S/MIME).
• Identity (national eID, smart cards).
• IoT device authentication.

Each of these has its own migration timeline and challenges.

What You Can Do

• Use a modern CA that has a public post-quantum migration plan.
• Prefer shorter certificate lifetimes during the transition.
• For internal PKI, plan to issue post-quantum root CAs soon.
• For high-value signing (firmware, root keys), adopt ML-DSA or SLH-DSA now.

Bottom Line

PKI is the invisible trust infrastructure of the digital world. Keeping it trustworthy in the quantum era means migrating its signatures — and because roots last decades, that migration should already be underway.