Chapter 31 — Quantum and post-quantum security: advanced cases
Lesson map
Lesson 31 published. Map, example, EX061.
flowchart TD
N0["Learning outcomes"]
N1["31.1 Constrained devices"]
N2["31.2 Stateful signatures in practice"]
N3["31.3 Long-term signatures and archives"]
N4["31.4 Partners and suppliers"]
N5["31.6 Practical activity"]
N6["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6
N6 --> EX["Worked example"]
EX --> Q["Exercise EX061"]
Logical drawing
flowchart LR
N0["Learning outcomes"]
N1["31.1 Constrained devices"]
N2["31.2 Stateful signatures in practice"]
N3["31.3 Long-term signatures and archives"]
N4["31.4 Partners and suppliers"]
N5["31.6 Practical activity"]
N6["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6 --> Q["EX061"]
[Learning outcomes]
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[31.1 Constrained devices]
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[31.2 Stateful signatures in practice]
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v
[31.3 Long-term signatures and archives]
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v
[31.4 Partners and suppliers]
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v
[31.6 Practical activity]
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v
[Synthesis]
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v
[Worked example] --> [Exercise EX061]
Learning outcomes
You will (1) analyse migrations with constrained devices, long-term signatures and partners; (2) apply stateful signature management; (3) plan long-term archive protection; and (4) reason about third-party dependencies.
31.1 Constrained devices
Card terminals, ATMs and building controllers have limited memory, slow processors and long lifetimes. ML-KEM is fast enough for most, but key and signature sizes may exceed buffers, and boot ROMs that verify only RSA cannot change. The realistic plan combines firmware upgrades where possible, gateways that terminate post-quantum protocols in front of devices that cannot, and procurement requirements so that replacements are ready.
For HCU, the 3,000 card terminals are the clearest case. Each model needs a record of what its firmware can verify and how much memory it has, so that the plan can say which terminals will be upgraded and which will sit behind a gateway until they are replaced.
31.2 Stateful signatures in practice
LMS and XMSS (SP 800-208) require that each one-time key is used once. State must survive crashes, backups and HSM failover without rollback. SP 800-208 requires these schemes to be generated and used within hardware cryptographic modules and forbids exporting private keys, which reflects the danger of state duplication. Organisations using them plan the total number of signatures in advance and monitor remaining capacity.
Running out of one-time keys stops signing completely, so capacity planning is part of the design.
31.3 Long-term signatures and archives
A mortgage signed with ECDSA today must still be verifiable in 30 years. If a quantum computer can later forge ECDSA, a forger could backdate documents. Archive systems address this with timestamps and evidence records renewed with stronger algorithms before the old ones weaken; the renewal proves the document existed while its original signature was still trustworthy. Encrypted archives need similar care: re-encrypting with new keys does not help if old ciphertexts or wrapped keys were already copied.
For HCU this means the archive plan covers signatures, timestamps and keys together.
31.4 Partners and suppliers
Payment networks, shared service providers and regulators set requirements HCU cannot change alone. Migration plans list partner dependencies, ask for roadmaps, and include contract clauses for algorithm support and inventory disclosure.
Where a partner cannot yet say when it will support post-quantum algorithms, the plan records that as an open dependency with an owner and a date to ask again, rather than assuming the partner will be ready in time. HCU tracks its payment network and core banking provider this way, since both control flows it cannot change on its own.
31.5 Worked example
HCU's ATM vendor offers a firmware update supporting ML-KEM for the ATM-to-switch link but not post-quantum firmware signing, because the boot ROM verifies only RSA. Decision: deploy the key-exchange update to protect PIN-related traffic (short-lived but sensitive), accept forge-later residual risk for firmware until replacement in five years, and require LMS or ML-DSA boot verification in the replacement tender.
The residual risk is written into the risk register with an owner, so that it is reviewed each year rather than forgotten until the replacement arrives.
31.6 Practical activity
Write a one-page plan for HCU's 30-year mortgage archive covering signatures, timestamps, encryption and key wrapping.
Say which documents need protection for the full period, when evidence records will be renewed, how the renewal is triggered, and who owns it. Explain how the plan deals with ciphertexts and wrapped keys that may already have been copied, since re-encryption alone cannot help them. Finish with the first three actions HCU should take this year and the evidence each will produce. Keep it to one page.
31.7 Common errors and limits
- Using stateful signatures in software without protected state.
- Assuming re-encryption fixes past exposure.
- Leaving partner dependencies out of the plan.
- Treating a gateway in front of old devices as permanent rather than as a bridge to replacement.
- Renewing archive timestamps only after the old algorithm is already doubted, when the renewal can no longer prove anything.
- Planning the total number of stateful signatures without allowing for test, backup and failover use.
- Buying replacement devices without post-quantum boot verification in the tender.
Exercises
- EX061 — State rollback. Describe what happens if an LMS signer is restored from yesterday's backup and signs again. Rubric: 3 = one-time key reuse, forgery risk, prevention.
- EX062 — Archive renewal. Explain why timestamp renewal must happen before the original algorithm is weakened. Rubric: 3 = proof of existence while trust held.
Synthesis
Constrained devices, stateful signatures, long-term archives and partner dependencies demand plans that combine upgrades, gateways, procurement, renewal and contracts.
Continue studying — chapter 31 of 40
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