getParams = (keyName, salt) => ({
  name: HKDF_ALGORITHM_NAME,
  info: stringToArrayBuffer(keyName),
  hash: sha256.HASH_ALGORITHM_NAME,
  salt
})
RFC 5869:

 "the use of salt adds significantly to the strength of HKDF...
 Ideally, the salt value is a random (or pseudorandom) string of the
 length HashLen"

 https://tools.ietf.org/html/rfc5869#section-3.1