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Certificate Revocation Lists (CRLs)

##cybersecurity/pki ##cybersecurity/cryptography ##interview/concepts

Certificate Revocation Lists (CRLs)

One-liner: Publicly accessible list of digital certificates that have been revoked before their expiration date.

🎯 What Is It?

A Certificate Revocation List (CRL) is a signed data file published by a Certificate Authority (CA) that lists all certificates that have been revoked and should no longer be trusted. CRLs are a critical component of PKI security, allowing clients to verify that certificates haven't been compromised.

📝 CRL Structure

┌────────────────────────────────┐
│  Certificate Revocation List  │
├────────────────────────────────┤
│ Issuer: CN=DigiCert CA        │
│ This Update: 2024-12-30       │
│ Next Update: 2025-01-06       │
│ Revoked Certificates:         │
│   - Serial: 123456            │
│     Revocation Date: 2024-12-25│
│     Reason: Key Compromise     │
│   - Serial: 789012            │
│     Revocation Date: 2024-12-28│
│     Reason: Cessation of Op    │
│ Signature: <CA signature>     │
└────────────────────────────────┘

Key Fields

🚫 Revocation Reasons

Reason Code Meaning Example
Key Compromise Private key exposed Server breach, stolen key
CA Compromise CA itself compromised DigiNotar breach (2011)
Affiliation Changed Owner changed organization Employee left company
Superseded New certificate issued Renewal, key rotation
Cessation of Operation Service discontinued Domain no longer owned
Certificate Hold Temporary suspension Investigation pending
Privilege Withdrawn Access revoked Terminated employee

🔄 How CRL Checking Works

1. Client receives certificate from server
   ↓
2. Client extracts CRL Distribution Point URL from certificate
   Example: http://crl.digicert.com/ca.crl
   ↓
3. Client downloads CRL from CA
   ↓
4. Client checks if certificate serial number is in CRL
   ↓
5. If found in CRL:
      → Certificate is REVOKED (reject connection)
   If NOT found:
      → Certificate is valid (proceed)

Certificate with CRL Distribution Point

X509v3 CRL Distribution Points:
    Full Name:
      URI:http://crl.digicert.com/DigiCertGlobalRootCA.crl

⚡ CRL Types

1. Base CRL

Complete list of ALL revoked certificates.

Size: Large (can be 10MB+)
Update Frequency: Weekly or monthly

2. Delta CRL

Only contains changes since last base CRL.

Size: Small
Update Frequency: Daily or hourly

Usage:

Base CRL (issued Jan 1) + Delta CRL (Jan 15) = Complete list

❌ CRL Limitations

1. Size Problem

Large CAs:
  - Millions of certificates
  - CRL size: 10-100 MB
  - Slow download on mobile/slow connections

2. Update Latency

Certificate compromised: 12:00 PM
CA publishes CRL: Next day (24 hours later)
Client downloads CRL: 36 hours after compromise

→ Gap where revoked certificate still trusted

3. Soft-Fail Problem

If client can't download CRL:

Most browsers use soft-fail → revoked certificates may be accepted.

4. Caching Issues

Clients cache CRLs for performance → may use stale CRL.

✅ Alternative: OCSP (Online Certificate Status Protocol)

CRL vs OCSP

Feature CRL OCSP
Method Download entire list Query single certificate
Size Large (10-100 MB) Small (few KB)
Latency High (hours/days) Low (real-time)
Bandwidth High Low
Privacy Better (no per-cert query) Worse (CA sees what you're checking)
Adoption Legacy Modern (preferred)

OCSP Workflow

1. Client sends OCSP request for specific certificate
   ↓
2. OCSP responder (CA) replies:
   - Good (not revoked)
   - Revoked (with reason)
   - Unknown (no info)
   ↓
3. Client makes decision

OCSP Stapling

Server fetches OCSP response and "staples" it to certificate during TLS handshake.

Benefit:

💻 Viewing CRLs

Download CRL

# Download CRL
wget http://crl.digicert.com/DigiCertGlobalRootCA.crl

# View CRL contents
openssl crl -in DigiCertGlobalRootCA.crl -inform DER -text -noout

Example Output

Certificate Revocation List (CRL):
    Issuer: /C=US/O=DigiCert Inc/CN=DigiCert Global Root CA
    Last Update: Dec 30 12:00:00 2024 GMT
    Next Update: Jan 6 12:00:00 2025 GMT
Revoked Certificates:
    Serial Number: 0123456789ABCDEF
        Revocation Date: Dec 25 08:30:00 2024 GMT
        CRL Reason: Key Compromise

Check Certificate Revocation Status

# Check certificate against CRL
openssl verify -crl_check -CRLfile ca.crl -CAfile ca.crt certificate.crt

# Check via OCSP
openssl ocsp -issuer ca.crt -cert certificate.crt \
  -url http://ocsp.digicert.com

🔒 Security Considerations

1. CRL Integrity

CRLs are signed by CA to prevent tampering.

Attacker modifies CRL → Remove compromised cert
                     → Signature verification FAILS
                     → Client rejects CRL

2. Replay Attacks

Attack: Attacker serves old CRL (before revocation).

Defense:

3. Denial of Service

Attack: Block access to CRL distribution point.

Impact: Clients can't verify revocation (soft-fail = accept anyway).

Defense:

📊 Real-World Impact

Heartbleed (2014)

Let's Encrypt

🎤 Interview Angles

Q: What is a CRL and why is it needed?

Q: What are the limitations of CRLs?

Q: What's the difference between CRL and OCSP?

Aspect CRL OCSP
Data Entire revocation list Single certificate status
Size Large (MBs) Small (KBs)
Speed Slow, periodic updates Fast, real-time
Privacy Better (no per-cert query) Worse (CA sees queries)
Modern Use Legacy/backup Primary method

Q: How would you handle a compromised certificate?

STAR Example:
Situation: Server private key was exposed in security breach.
Task: Prevent attackers from using compromised certificate.
Action: