Crypto agility
The ability to swap algorithms, key sizes or parameters without redesigning the system or breaking existing data.
Crypto agility means algorithm choice is configuration, not architecture. Ciphertexts, signatures and hashes carry an algorithm or version identifier; code calls an abstraction rather than a hardcoded primitive; and you know where every algorithm is used, often recorded in a CBOM (cryptography bill of materials, supported by CycloneDX).
It matters now because of the post-quantum migration: RSA and elliptic curve key exchange and signatures will need replacing with ML-KEM and ML-DSA, and organisations that never inventoried their crypto cannot plan that. It also matters for routine deprecations such as SHA-1 and TLS 1.0.
The trap is too much agility. Letting the attacker pick the algorithm (JWT alg header, TLS downgrade) is how agility becomes a vulnerability. Agility belongs to the defender: the verifier decides the allowed set, and versions only move forward.