extract
The "extract" part of HKDF.
If null is passed for the salt parameter, an array the same length as the hash function for algorithm is used, with all entries set to zero.
Return
a pseudorandom key, the same length as the hash function for algorithm.
Parameters
algorithm
the KDF algorithm to use e.g. Algorithm.HMAC_SHA256.
ikm
input key material.
salt
optional salt value (a non-secret random value).