@Override
  public String toString() {
    return "HKDF PRF Parameters (hashType: "
        + hashType
        + ", salt: "
        + salt
        + ", and "
        + keySizeBytes
        + "-byte key)";
  }
Gets the salt value, which defaults to null if not set, as per RFC 5869. The HKDF PRF
implementation must convert a null salt to a string of zeros that is the length of the hash
function output.
java_src/src/main/java/com/google/crypto/tink/prf/HkdfPrfParameters.java