Cryptology Academy · Lesson

Certificate Revocation: CRL & OCSP

Understand how compromised certificates are revoked in practice.

Lesson 3 of 413 steps

Certificate Revocation: CRL & OCSP is a free Cryptology Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Cryptology Academy learning path, and your progress syncs across the web and the CoddyKit app. The Cryptology Academy course includes 4 lessons in total.

Welcome

Certificates expire, but sometimes a private key is compromised before expiry. Revocation mechanisms let CAs announce that a certificate is no longer trustworthy.

Why Revocation Matters

If an attacker steals your TLS private key, they can impersonate your server until the certificate expires. Revocation lets the CA invalidate the certificate immediately.

Certificate Revocation List (CRL)

A CRL is a signed list of revoked certificate serial numbers published by the CA. Clients download the CRL and check if the target cert's serial is in the list.

CRL Problems

CRLs can be large (millions of revoked certs for major CAs). They are downloaded periodically — a freshly revoked cert might not appear in the CRL for hours. Latency window = security gap.

OCSP: Online Certificate Status Protocol

OCSP sends a real-time query to a CA-operated responder: Request: 'Is serial 1234 still valid?' Response: 'Good' / 'Revoked' / 'Unknown' Signed by the CA's OCSP signing key.

OCSP Privacy Problem

Every time you visit a website, your browser tells the CA's OCSP responder which site you're visiting. This leaks browsing history to CAs. Firefox disables OCSP for most connections.

OCSP Stapling

OCSP stapling solves the privacy problem: the server periodically fetches its own OCSP response and 'staples' it to the TLS handshake. Client gets the response without contacting the CA.

OCSP Must-Staple Extension

The OCSP Must-Staple X.509 extension tells browsers to reject the certificate if no stapled OCSP response is provided. Prevents soft-fail attacks where OCSP check is skipped.

Soft-Fail vs Hard-Fail

Soft-fail: if OCSP/CRL is unavailable, continue anyway (security risk). Hard-fail: if revocation status cannot be confirmed, reject the connection. Chrome uses soft-fail by default.

CRLite & OneCRL

Firefox uses CRLite: a compact probabilistic data structure containing all revoked certificates, updated daily. OneCRL is a smaller list of revoked intermediate certs. Both avoid OCSP queries.

How to Revoke Your Own Certificate

Let's Encrypt ACME: certbot revoke --cert-path /etc/letsencrypt/live/domain/cert.pem You need either the private key or your ACME account credentials. Revocation takes effect immediately in OCSP responses.

Quick Check

What problem does OCSP Stapling solve compared to direct OCSP queries?

Answer it in the interactive lesson

Recap

Certificate revocation is a complex but vital topic. Next we create our own certificate authority with OpenSSL.

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Frequently Asked Questions

Is the “Certificate Revocation: CRL & OCSP” lesson free?

Yes — the full text of “Certificate Revocation: CRL & OCSP” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Cryptology Academy course, upgrade to CoddyKit PRO. The Cryptology Academy course includes 4 lessons in total.

What will I learn in “Certificate Revocation: CRL & OCSP”?

Understand how compromised certificates are revoked in practice. You practise Cryptology Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Cryptology Academy?

No prior experience is required. Cryptology Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Certificate Revocation: CRL & OCSP” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Cryptology Academy lesson?

Yes. Every Cryptology Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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