HKDF: Turning One Secret Into Many, Correctly

A common task in applied cryptography looks deceptively simple: "I have a shared secret. I need two keys from it — one for encryption, one for authentication." The wrong way to solve this is to hash the secret and slice it in half. The right way is HKDF, and the reason it exists tells you something important about why amateur cryptography breaks. HKDF is the HMAC-based Key Derivation Function, specified in RFC 5869 by Hugo Krawczyk in 2010. It's the key derivation function used in TLS 1.3, in t...