code: protected void assertPRF(byte[] P)
      {
          if (prf == null)
          {
              prf = new MacBasedPRF(parameters.getHashAlgorithm());
          }
          prf.init(P);
      }
docs: Factory method. Default implementation is (H)MAC-based. To be overridden
  in derived classes.
  
  @param P
  User-supplied candidate password as array of bytes.