docs: public void ensureMutualAuth(String returnChallenge) throws AuthenticationException {
          try {
              processChallenge(returnChallenge);
          } catch (MalformedChallengeException mce) {
              throw new AuthenticationException("Received invalid response header for mutual authentication", mce);
          }
          try {
              String token = getNegotiateToken();
              if (!spnegoNegotiator.established() || token != null) {
                  throw new AuthenticationException("Could not complete SPNEGO Authentication, Mutual Authentication Failed");
              }
          } catch (GSSException gsse) {
              throw new AuthenticationException("Could not complete SPNEGO Authentication", gsse);
          }
      }