Glossary · Cryptography & authentication
HKDF
HKDF (HMAC-based key derivation function, RFC 5869) expands one strong secret into any number of independent, purpose-bound keys via extract-then-expand — the standard way to turn a shared secret into separate encryption, signing, and session keys.
Reusing one key for multiple purposes is a classic protocol flaw. HKDF solves it in two steps: extract compresses input keying material into a uniform pseudorandom key, and expand derives per-purpose keys from it using distinct info labels — so a compromise or misuse of one derived key tells an attacker nothing about its siblings.
HKDF is not for passwords — it assumes its input already has full entropy (a Diffie-Hellman result, an export key, a random master key). Passwords go through key stretching first; HKDF then fans the result out. Protocols like TLS 1.3 and OPAQUE use HKDF internally, and Ciphera uses it to derive purpose-separated keys from vault master secrets.