PQC Threat Models: 'Harvest Now, Decrypt Later' in Real Systems

Monthly research note. Theme: Post-Quantum Cryptography & Migration.

TL;DR

A focused memo on PQC Threat Models: 'Harvest Now, Decrypt Later' in Real Systems: define the model, state the properties, then design the system so those properties remain true under failure and adversaries.

Key takeaways

• Migration is mixed-version for years: compatibility and rollback are security features.
• Hybrid composition must be explicit and transcript-bound to resist downgrade.
• PQC changes handshake costs; plan DoS defenses and budgets.
• Prefer protocols and APIs that make invalid states hard to express.
• Define safety properties before performance goals.

Why this matters

• PQC changes bandwidth and CPU costs; DoS surfaces move.
• Hybrid designs fail if binding is ambiguous (mix-and-match, downgrade).
• Interop is the real risk: multiple stacks, vendors, and versions.
• Operationalization (monitoring, rollback) determines success more than crypto choice.

Key questions

• Which parts must be constant-time, and how will you validate that?
• How do you bind hybrid secrets to prevent downgrade and mix-and-match attacks?
• What does interoperability testing look like across vendors and stacks?
• What telemetry proves PQC is working (not just enabled)?
• How do you handle failures: decryption failures, invalid ciphertexts, malformed keys?
• Which secrets require long-term confidentiality (HNDL) and where are they today?

Assumptions

• Vendors vary: implementations and defaults differ.
• Side channels exist: timing and cache behavior leak information.
• Bandwidth is limited in some environments; larger handshakes matter.
• Deployments are mixed; old clients must interoperate or fail safely.

Non-goals

• Treating migration as a single flag flip.
• Assuming PQC is “drop-in” without changing operational processes.

Model & invariants

A KEM gives you shared secrets without discrete-log assumptions:

Treat algorithm negotiation as adversarial: explicit downgrade resistance.

Make costs explicit: measure CPU and bandwidth, then add protections.

Security properties

• Downgrade resistance: negotiation can’t silently weaken security posture.
• Evidence: critical actions emit verifiable audit events.
• Least authority: privileges are scoped by purpose and time.
• Integrity: invalid transitions are rejected (and detectable).

Failure modes

• Resource exhaustion (CPU/bandwidth/storage) turning into correctness failures.
• Timeout ambiguity causing double-apply or partial state transitions.
• Observability gaps during incidents (missing evidence).
• Recovery paths that only work when nothing is broken.

Design sketch

sequenceDiagram
  participant A as Initiator
  participant B as Responder
  A->>B: classical_keyshare + pqc_pk
  B-->>A: classical_keyshare + pqc_ct + sig
  A-->>B: sig
  Note over A,B: ss = HKDF(ss_classical || ss_pqc, transcript)

Implementation notes

Explicit binding prevents downgrade and mix-and-match. Don’t leave it implicit.

Hybrid handshake checklist:
- Explicit negotiation (no silent downgrade)
- Transcript-bound KDF
- DoS protections (rate limits, cookies, puzzles)
- Constant-time operations
- Telemetry: which mode, which failures, which clients

Verification strategy

• Downgrade tests: active attacker manipulates negotiation.
• Interop matrices across vendors/versions and failure modes.
• Side-channel tests where tooling exists; constant-time audits.
• Chaos deploys: mixed versions + rollback during partial outages.
• DoS tests: measure CPU/bandwidth amplification and mitigation impact.

Operational notes

• Roll out with canaries and explicit rollback triggers.
• Add telemetry for negotiation outcomes, failures, and client cohorts.
• Inventory long-lived secrets and migrate the highest-risk first.
• Cap handshake cost per peer/IP; use stateless cookies when needed.
• Document supported algorithm sets and deprecation timelines.

What to monitor

• Admission-control / rate-limit rejections (by reason).
• Retry/timeout rates by endpoint and client cohort.
• Invariant violation rate (should be ~0).
• Error budget burn + tail latency under load.
• Authz failures and policy denials (unexpected spikes).

Rollback plan

• Define an explicit rollback trigger (metrics + thresholds).
• Use canaries and staged rollout; stop early when signals degrade.
• Preserve evidence (configs, artifacts, audit logs) to reconstruct what changed.
• Prefer backward-compatible changes; avoid “flag day” upgrades.
• Keep dual-write / dual-verify windows where appropriate.

Evidence

• RFC 5869: HKDF (1) — Useful when discussing hybrid binding and context separation.
  • Evidence: HKDF is the workhorse for domain separation; bind purpose/context to avoid cross-protocol key reuse.
• NIST Post-Quantum Cryptography Project (2) — Standardization process and algorithm selections.
  • Evidence: Treat PQ migration as a program (inventory, interop, rollback). Use NIST status to drive prioritization and timelines.

Open questions

• How do you rotate algorithms without introducing configuration chaos?
• Which clients will fail first, and what is the safe fallback behavior?
• What is the worst-case handshake cost under attack?
• Where would a downgrade be visible today, and how would you detect it?

Checklist

• Safety properties stated as invariants.
• Failure modes enumerated with mitigations.
• Costs bounded (CPU/memory/bandwidth) under adversarial inputs.
• Assumptions listed and reviewed.
• Rollback plan rehearsed and automated.
• Telemetry captures correctness signals.

Further reading

• CRYSTALS-Kyber — KEM design and parameters commonly referenced in deployments.
• CRYSTALS-Dilithium — Signature scheme design and deployment constraints.
• RFC 5869: HKDF — Useful when discussing hybrid binding and context separation.
• NIST Post-Quantum Cryptography Project — Standardization process and algorithm selections.
• Learn TLA+ — Practical entry point for specification and model checking.
• Site Reliability Engineering (Google) — Error budgets, incident response, and reliability as an engineering discipline.