Chapter 35 — Design studio II

Lesson map

Lesson 35 published. Map, example, EX103.

flowchart TD
  N0["Learning outcomes"]
  N1["35.1 The new requirement"]
  N2["35.2 Evaluating options"]
  N3["35.3 Critique and plan"]
  N4["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4
  N4 --> EX["Worked example"]
  EX --> Q["Exercise EX103"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["35.1 The new requirement"]
  N2["35.2 Evaluating options"]
  N3["35.3 Critique and plan"]
  N4["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> Q["EX103"]

[Learning outcomes]
    |
    v
[35.1 The new requirement]
    |
    v
[35.2 Evaluating options]
    |
    v
[35.3 Critique and plan]
    |
    v
[Synthesis]
    |
    v
[Worked example] --> [Exercise EX103]

Learning outcomes

You will (1) redesign the system for a new requirement; (2) evaluate post-quantum readiness; (3) present a migration plan for critique.

35.1 The new requirement

In the second design studio, the regulator announces that utilities must plan migration of long-lived signatures to post-quantum algorithms. Meters last fifteen years, so firmware signed today must remain verifiable and secure until about 2041. Aiyana's team must redesign firmware signing so meters can accept post-quantum signatures without hardware replacement. They read FIPS 204 for ML-DSA and FIPS 205 for SLH-DSA, noting that both produce much larger signatures and keys than Ed25519 or ECDSA. The team also reads NIST's transition guidance so its plan matches expected timelines, and records which meter generations can be updated in the field.

35.2 Evaluating options

The team compares options. ML-DSA offers moderate signature sizes of a few kilobytes and fast verification; SLH-DSA relies only on hash functions, offering conservative security assumptions, with larger signatures. A hybrid scheme requires both a classical and a post-quantum signature to verify, protecting against failure of either. For firmware, size matters less than for readings because updates are infrequent. The team proposes hybrid ECDSA plus ML-DSA signatures on manifests, with meters accepting firmware only when both verify, and a later phase dropping the classical signature.

35.3 Critique and plan

Reviewers question whether meters have enough memory and flash to verify post-quantum signatures. The team measures with the vendor's library on a test meter and finds verification fits but needs a firmware update to the bootloader, which itself must be signed classically. They write a migration plan: update bootloaders with classical signatures now, introduce hybrid manifests next, and retire classical-only verification after confidence grows. Each step has a rollback. Reviewers accept the plan with a request to track NIST guidance on transition timelines.

Worked example

The team proposes hybrid ECDSA plus ML-DSA manifest signatures, verifies feasibility on a test meter, and stages migration through a classically signed bootloader update.

Synthetic case. The regulator's rule and measurements are invented; FIPS 204 and FIPS 205 are real.

Errors and limits

Hybrid schemes add complexity and size; they must be designed by experts and implemented with vetted libraries. Timelines change, so plans need review.

Exercises

• EX103 — Why plan now. Why must Ridgeway plan post-quantum firmware signing now?
  Show marking criteria

  Rubric: 3 = meters last fifteen years, so signatures must stay secure until about 2041;; 2 = long life;; 1 = no need;; 0 = none.
• EX104 — Hybrid rule. How does the proposed hybrid scheme accept firmware?
  Show marking criteria

  Rubric: 3 = only when both ECDSA and ML-DSA signatures verify;; 2 = both;; 1 = either;; 0 = none.
• EX105 — Bootloader step. Why does the plan begin with a bootloader update?
  Show marking criteria

  Rubric: 3 = verification of PQ signatures needs updated bootloader, itself signed classically;; 2 = bootloader;; 1 = skip;; 0 = none.

Synthesis

Post-quantum readiness for long-lived devices needs early, staged, hybrid migration grounded in measurement and standards.

Continue studying — chapter 35 of 50

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