An apparatus includes integrated circuitry (IC) and a further circuit. The IC includes internal circuits having sensitive/secret data (SSD) to be maintained as confidential relative to a suspect Hardware Trojan (HT) and including access ports through which information associated with the internal circuits is accessible by external circuitry associated with the HT. The further circuit to learn behavior of the internal circuits that is unique to the integrated circuitry under different operating conditions involving the internal circuits, involving the SSD and involving other data that is functionally associated with an application of the integrated circuitry.