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Crypto-agility is the engineering practice of designing systems so that cryptographic algorithms can be swapped via configuration, not via code re-architecture. The PQ migration is the forcing function for crypto-agility; once implemented, future algorithm changes (next-PQ generation in 2030+, post-PQ in 2040+) become routine. This module covers the design patterns: pluggable algorithm registries, multi-algorithm certificates, hybrid signatures, key-rotation infrastructure, and the organizational habits that make agility durable.
If your PQ migration ends with “we replaced RSA with ML-DSA and called it done,” you’ve done half the work. The other half: ensure that when ML-DSA is replaced by something else in 2032, you don’t go through the same painful inventory-and-replace cycle. Build the agility into the system; the migration is the opportunity.
What crypto-agility means in practice
A crypto-agile system has these properties:
- Cryptographic algorithm choices live in configuration, not code. A YAML / TOML / DB-row says “for this purpose, use this algorithm.”
- Multiple algorithms can be supported simultaneously. Migration period: producers sign with old + new; verifiers accept either. After full migration: drop old.
- Algorithm identifiers are versioned: the wire format includes “alg=ML-DSA-65” not just “alg=signed.”
- Library abstractions hide algorithm details from application code. Application says “give me a signature”; library handles which algorithm.
- Key rotation is automated and well-rehearsed. New keys can be generated, distributed, and old keys retired without service disruption.
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