Lab-05: PQC OCSP

Real-Time Certificate Verification with OCSP

Key Message: Real-time certificate verification with OCSP works exactly the same with PQC. Same HTTP protocol, same tools.

Important distinction: OCSP does not revoke certificates. It only reports revocation status. Revocation is a CA operation (see Revocation). OCSP is a distribution mechanism — like asking “is this certificate still valid?” rather than “revoke this certificate.”

The Scenario

You have a CRL. But it’s updated every hour. A certificate was revoked 30 seconds ago. Clients don’t know yet.

TIMELINE: The CRL Staleness Problem
───────────────────────────────────

  03:00    03:30    04:00    04:30    05:00
    │        │        │        │        │
    ▼        ▼        ▼        ▼        ▼
  CRL      Cert     CRL      CRL      CRL
  published REVOKED published published published

              ↑
              │
              For 30 min, clients
              still trust the
              revoked certificate!

“I need real-time certificate status. CRLs are too slow. Does OCSP work with PQC?”

Yes. Same HTTP protocol, same request/response format. Only signature sizes change.

What We’ll Do

Create a PQC CA 1b. Issue OCSP responder certificate
Start OCSP responder 2b. Issue TLS certificate
Query certificate status (GOOD)
Revoke the certificate 3b. Query again (REVOKED) 2c. Stop OCSP responder

Run the Demo

./journey/05-ocsp/demo.sh

The Commands

Step 1: Create CA

# Create PQC CA with ML-DSA-65
qpki ca init --profile profiles/pqc-ca.yaml \
    --var cn="PQC CA" \
    --ca-dir output/pqc-ca

qpki ca export --ca-dir output/pqc-ca --out output/pqc-ca/ca.crt

Step 1b: Issue OCSP Responder Certificate

# Generate OCSP responder key and CSR
qpki csr gen --algorithm ml-dsa-65 \
    --keyout output/ocsp-responder.key \
    --cn "OCSP Responder" \
    --out output/ocsp-responder.csr

# Best practice: CA key stays offline
qpki cert issue --ca-dir output/pqc-ca \
    --profile profiles/pqc-ocsp-responder.yaml \
    --csr output/ocsp-responder.csr \
    --out output/ocsp-responder.crt

Step 2: Start OCSP Responder

# Start with delegated certificate (recommended)
qpki ocsp serve --port 8888 --ca-dir output/pqc-ca \
    --cert output/ocsp-responder.crt \
    --key output/ocsp-responder.key

Step 2b: Issue TLS Certificate

# Generate TLS server key and CSR
qpki csr gen --algorithm ml-dsa-65 \
    --keyout output/server.key \
    --cn server.example.com \
    --out output/server.csr

qpki cert issue --ca-dir output/pqc-ca \
    --profile profiles/pqc-tls-server.yaml \
    --csr output/server.csr \
    --out output/server.crt

Step 3: Query Certificate Status (GOOD)

# Generate OCSP request
qpki ocsp request --issuer output/pqc-ca/ca.crt \
    --cert output/server.crt \
    --out output/request.ocsp

# Send OCSP request (RFC 6960) - response is immediate, unlike CRL
curl -s -X POST \
    -H "Content-Type: application/ocsp-request" \
    --data-binary @output/request.ocsp \
    http://localhost:8888/ \
    -o output/response.ocsp

qpki ocsp info output/response.ocsp

Step 4: Revoke Certificate

# Revoke certificate
qpki cert revoke <serial> --ca-dir output/pqc-ca --reason keyCompromise

Step 3b: Query Again (REVOKED)

# Query again - status changes immediately! (CRL would take hours)
curl -s -X POST \
    -H "Content-Type: application/ocsp-request" \
    --data-binary @output/request.ocsp \
    http://localhost:8888/ \
    -o output/response2.ocsp

qpki ocsp info output/response2.ocsp
# Status: revoked

Step 2c: Stop OCSP Responder

# Stop the OCSP responder
qpki ocsp stop --port 8888

CRL vs OCSP

Criteria	CRL	OCSP
Update	Periodic (hourly/daily)	Real-time
Size	Can be large (full list)	Small (one response)
Availability	Works offline	Requires network
Latency	Local read	Network request
Vuln. window	Until next CRL	Nearly zero

In practice: Use BOTH

OCSP for real-time checks
CRL as offline fallback

OCSP Architecture

CA-Signed Mode (Simple)

┌─────────────┐                      ┌──────────────────┐
│   Client    │ ─── OCSP Request ──► │  OCSP Responder  │
│ (curl/app)  │ ◄── OCSP Response ── │ (qpki ocsp serve) │
└─────────────┘                      └────────┬─────────┘
                                              │
                                     Signs with CA key
                                     (CA key online - risk!)

Delegated Responder Mode (Recommended)

┌─────────────┐                      ┌──────────────────┐
│   Client    │ ─── OCSP Request ──► │  OCSP Responder  │
│ (curl/app)  │ ◄── OCSP Response ── │ (qpki ocsp serve) │
└─────────────┘                      └────────┬─────────┘
                                              │
                                     Signs with responder key
                                     (CA key stays offline!)
                                              │
                                     ┌────────▼─────────┐
                                     │ OCSP Responder   │
                                     │   Certificate    │
                                     │ (id-kp-OCSPSign) │
                                     └──────────────────┘

The OCSP responder certificate has:

Extended Key Usage: id-kp-OCSPSigning (1.3.6.1.5.5.7.3.9)
OCSP No Check extension (prevents infinite verification loop)

Size Comparison

Component	Classical (ECDSA)	Post-Quantum (ML-DSA)	Notes
OCSP Request	~100 bytes	~100 bytes	Same format
OCSP Response	~300 bytes	~3,500 bytes	PQC signature larger

Responses are larger due to ML-DSA signatures, but the protocol is unchanged.

Response Times

Operation	Classical	PQC	Notes
Request generation	<1ms	<1ms	Same
Network round-trip	~Xms	~Xms	Same protocol
Signature verification	<1ms	~2-5ms	ML-DSA slightly slower

When to Use OCSP

Industry	Use Case	Why Real-Time Matters
Banking/Finance	Transaction signing	Reject compromised certs instantly
E-commerce	Payment processing	Prevent fraud during checkout
Healthcare	EHR access	Immediate access revocation
Government	Citizen portals	Real-time credential validation
Cloud/SaaS	API authentication	Block compromised service accounts

CRL alone is insufficient when:

Transactions are high-value (financial, legal)
Compliance requires real-time status (PCI-DSS, HIPAA)
Attack window must be minimized (< 1 hour tolerance)

What You Learned

Same HTTP protocol: RFC 6960 works unchanged with PQC
Delegated responders: Best practice keeps CA keys offline
Real-time status: Revocation changes are immediate
Size tradeoff: PQC responses are larger but acceptable
Drop-in replacement: Existing OCSP clients work with PQC responders

References

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