Crypto-agility

Crypto-agility is a system's ability to replace cryptographic algorithms without redesign: swapping a deprecated hash, rotating to a post-quantum scheme, or running two schemes side by side during migration. It is achieved through versioned formats, algorithm identifiers in stored artifacts, and protocol negotiation, and it determines whether an algorithm break is an incident or a catastrophe.

Agility is mostly a data-format property decided years before it is needed. An artifact that records which algorithm and which key produced it can be verified after a migration; one that assumes a single algorithm has to be re-signed or trusted on faith.

The same applies to stored digests: a version stamp beside the value is what makes it possible to read old records under an old scheme while writing new ones under a new one.

Agility is decided by data formats, and data formats are decided years before the migration that tests them. An artifact that records the algorithm and key that produced it can be verified after a change; one that assumes a single algorithm has to be re-signed, and re-signing an old record proves only what it says now rather than what it said then.

The same reasoning applies to stored digests as to signatures. A version stamp beside each value is what lets a system read historical records under the old scheme while writing new ones under the new one, which is the difference between rotating a pepper and abandoning the ability to match anything written before the change.

Negotiation is the protocol half of the property, and its failure mode is downgrade. A system that will accept whichever algorithm a counterparty offers is agile in the direction that helps an attacker, so the set of acceptable algorithms belongs in policy that can be tightened centrally rather than in per-integration defaults.

Deprecation is the part most plans omit. Adding support for a new scheme is straightforward; removing the old one requires knowing which artifacts and which counterparties still depend on it, and that inventory is exactly what the algorithm identifiers in stored artifacts provide.

It is the property that decides whether adopting post-quantum cryptography is a migration or a rewrite.