extract salt ikm is prk, the pseudorandom key of hash length octets. The salt is an optional non-secret random value, ikm the input key material.
val expand : prk:string ->?info:string ->int -> string
extract prk info length is okm, the output keying material. Given the pseudorandom key of hash length (usually output of !extract step), and an optional context and application specific information info, the okm is generated.