Certificate Revocation Guide

Quick Answer: CRL vs OCSP vs Stapling

CRL (Certificate Revocation List)

Download a big list of all revoked certificates from the CA. Simple but inefficient.

OCSP (Online Certificate Status Protocol)

Query the CA in real-time for a specific certificate's status. More efficient but privacy concerns.

OCSP Stapling

Server fetches OCSP response and "staples" it to the TLS handshake. Best of both worlds!

Recommendation: Enable OCSP Stapling on your server. It's faster, more private, and more reliable than client-side revocation checking.

Why Certificate Revocation Matters

Certificates can become compromised before they expire. When private keys are leaked, certificates are mis-issued, or domain ownership changes, the certificate must be invalidated immediately.

Reasons Certificates Get Revoked

Key Compromise:Private key was stolen, leaked, or exposed

CA Compromise:The issuing Certificate Authority was hacked

Superseded:Certificate replaced with a new one

CRL (Certificate Revocation Lists)

CRLs are the original method for checking revocation. The CA publishes a signed list of all revoked certificate serial numbers, which clients download periodically.

How CRL Works

1. Certificate contains a "CRL Distribution Point" URL
2. Client downloads the CRL file from that URL
3. Client searches the list for the certificate's serial number
4. If found, the certificate is revoked

Check CRL Information

# Find CRL Distribution Point in certificate
openssl x509 -in cert.pem -noout -text | grep -A 4 "CRL Distribution"

# Download and examine a CRL
curl -O http://crl.example.com/example.crl
openssl crl -in example.crl -inform DER -text -noout

Pros

• Simple to implement
• Works offline once cached
• No privacy concerns

Cons

• Large file sizes (can be 10MB+)
• Slow to update (hours/days)
• Bandwidth intensive

OCSP (Online Certificate Status Protocol)

OCSP provides real-time revocation checking. Instead of downloading a huge list, the client asks the CA about a specific certificate.

How OCSP Works

1. Certificate contains an "Authority Information Access" OCSP URL
2. Client sends certificate serial number to OCSP responder
3. OCSP responder returns: good, revoked, or unknown
4. Response is signed by CA and includes validity period

Check OCSP Status

# Get OCSP responder URL from certificate
openssl x509 -in cert.pem -noout -ocsp_uri

# Check OCSP status
openssl ocsp -issuer issuer.pem -cert cert.pem \
  -url http://ocsp.example.com -resp_text

Pros

• Real-time status
• Small response size
• Faster than CRL

Cons

• Privacy leak (CA sees your browsing)
• OCSP server can go down
• Adds latency to connections

Privacy Concern

With standard OCSP, the CA knows every website you visit because your browser queries them for each site. This is why OCSP Stapling was created.

OCSP Stapling (The Modern Solution)

OCSP Stapling moves the OCSP check from client to server. The server periodically fetches the OCSP response and includes it in the TLS handshake.

How OCSP Stapling Works

1. Server fetches OCSP response from CA (cached for hours)
2. Server "staples" the signed response to TLS Certificate message
3. Client receives certificate + OCSP proof in one round trip
4. Client verifies the CA's signature on the OCSP response

Why Stapling is Best

Faster: No extra round trip to OCSP server
Private: CA doesn't see your browsing
Reliable: Works even if OCSP server is down
Reduces CA Load: Server caches response

Check if Stapling is Enabled

# Check for OCSP Stapling
openssl s_client -connect example.com:443 -status 2>/dev/null | \
  grep -A 20 "OCSP Response"

# If you see "OCSP Response Status: successful" - stapling is enabled
# If empty or "no response" - stapling not configured

Side-by-Side Comparison

Feature	CRL	OCSP	Stapling
Latency	High (large download)	Medium (extra request)	None (included)
Privacy	Good	Poor (CA sees sites)	Good
Freshness	Hours/Days	Real-time	Hours (cached)
Reliability	Works offline	Fails if OCSP down	Graceful fallback
Server Load	Medium	High per request	Low (cached)

Server Configuration

Enable OCSP Stapling on your web server for best performance and privacy.

Nginx

server {
    listen 443 ssl;
    
    ssl_certificate /path/to/fullchain.pem;
    ssl_certificate_key /path/to/privkey.pem;
    
    # Enable OCSP Stapling
    ssl_stapling on;
    ssl_stapling_verify on;
    
    # Trusted CA certificates for OCSP verification
    ssl_trusted_certificate /path/to/chain.pem;
    
    # DNS resolver for OCSP
    resolver 8.8.8.8 8.8.4.4 valid=300s;
    resolver_timeout 5s;
}

Apache

<VirtualHost *:443>
    SSLEngine on
    SSLCertificateFile /path/to/cert.pem
    SSLCertificateKeyFile /path/to/privkey.pem
    SSLCertificateChainFile /path/to/chain.pem
    
    # Enable OCSP Stapling
    SSLUseStapling on
    SSLStaplingCache shmcb:/tmp/stapling_cache(128000)
    SSLStaplingResponderTimeout 5
    SSLStaplingReturnResponderErrors off
</VirtualHost>

Browser Revocation Behavior

Different browsers handle revocation checking differently, often with "soft-fail" defaults.

Chrome

Uses CRLSets (Google-curated subset of revoked certs). Doesn't do live OCSP checks by default. Prioritizes performance over real-time revocation.

Firefox

Uses OCSP by default but soft-fails (allows connection if OCSP unreachable). Can enable hard-fail in settings for stricter security.

Safari

Aggressive OCSP checking with Apple's own OCSP infrastructure. May cause slowdowns but provides strong revocation checking.

The "Soft-Fail" Problem

Most browsers "soft-fail" - if they can't reach the OCSP server, they proceed anyway. An attacker who can block OCSP can use revoked certificates. This is why Certificate Transparency and OCSP Must-Staple extension were created.

Frequently Asked Questions

How long does it take for a revocation to propagate?

CRLs update every few hours to days. OCSP is near-instant, but responses are often cached for hours. With stapling, your server's cache determines freshness.

What is OCSP Must-Staple?

A certificate extension that tells browsers to require stapled OCSP. If the server doesn't provide it, the connection fails. Prevents soft-fail attacks but requires careful server configuration.

Can I revoke a Let's Encrypt certificate?

Yes! Use 'certbot revoke --cert-path /path/to/cert.pem'. You can also revoke via Let's Encrypt's website if you have the account key.

Why do some sites have slow HTTPS handshakes?

Often because of OCSP lookups. The browser is waiting for the CA's OCSP responder. OCSP Stapling eliminates this latency.

Is CRL still used?

Yes, as a fallback and for some enterprise/government scenarios. Most modern systems prefer OCSP or stapling for regular web traffic.

See Revocation in Action

Watch interactive demos of CRL, OCSP, and Stapling

Try the Demo

CRL vs OCSP vs OCSP Stapling: Complete Guide

Quick Answer: CRL vs OCSP vs Stapling

Why Certificate Revocation Matters

Reasons Certificates Get Revoked

CRL (Certificate Revocation Lists)

How CRL Works

Check CRL Information

Pros

Cons

OCSP (Online Certificate Status Protocol)

How OCSP Works

Check OCSP Status

Pros

Cons

Privacy Concern

OCSP Stapling (The Modern Solution)

How OCSP Stapling Works

Why Stapling is Best

Check if Stapling is Enabled

Side-by-Side Comparison

Server Configuration

Nginx

Apache

Browser Revocation Behavior

Chrome

Firefox

Safari

The "Soft-Fail" Problem

Frequently Asked Questions

How long does it take for a revocation to propagate?

What is OCSP Must-Staple?

Can I revoke a Let's Encrypt certificate?

Why do some sites have slow HTTPS handshakes?

Is CRL still used?

See Revocation in Action

Related Resources

Certificate Revocation Deep Dive

Certificate Transparency

Certificate Lifecycle

OCSP Stapling Demo

CA Hierarchy