Rule 4 of 12 · Chapter I — One Secret, Clearly Named
Size the key space so that exhaustive search is the attacker's best remaining option and still out of reach.
Why this rule exists
Concentrating all secrecy in the key only helps if the key is actually hard to guess. Key length is the one defence that scales predictably against raw computation, and it is cheap to get right at design time and expensive to change later. A short key turns a well-designed system into a solved one.
In practice
Pick key sizes from current mainstream cryptographic guidance rather than intuition, and check the effective entropy, not the nominal length — a 256-bit key derived from a six-character password is a six-character key. Audit generation: full-entropy random source, no reused seeds, no predictable derivation inputs. Leave headroom for the deployment's lifetime, including the day the hardware budget of your adversary grows.
When it doesn't apply
Constrained devices, legacy interoperability, and regulated algorithm choices sometimes cap key size; in those cases compensate with rate limiting, short key lifetimes, and tight revocation, and record the residual risk explicitly. Note also that against some future adversaries key length alone is not the whole story — algorithm choice matters as much as size.