What is "hkdf"

HKDF is an HMAC based key derivation function that transforms any weak key material (e.g. Diffie Hellman exchanged shared secrets) into a cryptographically strong key material (e.g. for subsequent use in encryption, integrity and/or authentication) and described in . It extracts a pseudo random key (PRK) using an HMAC hash function (e.g. HMAC- SHA256) on an optional salt and any potentially weak input key material (IKM). It then generates the cryptographically strong output key material (OKM) of any desired length by repeatedly generating PRK-keyed hash-blocks and then appending them into the output key material, finally truncating to the desired length. For added security, the HMAC PRK keyed hash blocks are chained during their generation by prepending the previous hash block to an incrementing 8-bit counter with an optional context string in the middle before being HMACed to generate the current hash block.