KeyDerivation

Key derivation refers to the process of producing cryptographic keys from a secret material, such as a password or a master key, using a key derivation function (KDF). The goal is to obtain keys suitable for encryption, authentication, or other cryptographic operations while preserving security properties of the input secret.

A KDF takes input material, a salt, parameters such as iteration count or memory-hardness, and a desired key length, and outputs keying material. It is deterministic: the same inputs always yield the same output. The salt ensures the output is unique per derivation and protects against precomputation attacks. KDFs are designed to be computationally expensive or memory-hard to slow brute-force attempts.

Main families include password-based KDFs such as PBKDF2, scrypt, and Argon2, and extract-then-expand constructions like HKDF used to derive keys from an initial keying material. Argon2 has memory-hard variants; scrypt combines CPU/memory hardness; PBKDF2 uses HMAC and is widely supported but may be less resistant to specialized hardware.

Typical uses include deriving encryption keys from a user password for storage or transport, generating per-session or per-file keys, and deriving subkeys for different cryptographic primitives from a single master key. In protocols, KDFs can separate key material from secret values, reducing exposure risk.

Security considerations include using a unique random salt per derivation, choosing appropriate iteration counts or memory parameters, and selecting an algorithm suited to the threat model. It is also important to match key length to the requirements of the target algorithm and to update KDF choices as hardware capabilities evolve.