Non-Deterministic Timers for Hardware Trojan Activation (or How a Little Randomness Can Go the Wrong Way).

The security of digital Integrated Circuits (ICs) is essential to the security of a computer system that comprises them. A particularly pernicious attack is the insertion of a hardware backdoor, that is triggered in the field using a timer that is also inserted in the hardware. Prior work has addressed deterministic timer-based triggers--those that are designed to trigger at a specific time with probability 1. We address open questions related to the feasibility of realizing non-deterministic timer-based triggers in hardware -- those that are designed with a random component. We show that such timers can be realized in hardware in a manner that is impractical to detect or disable using existing countermeasures of which are aware. We discuss our design, implementation and analysis of such a timer. We show that the attacker can have surprisingly fine-grained control over the time-window within which the timer triggers. Our timer has several other appealing features as well, from the attacker's standpoint. For example, it is practical and effective with only a few bits of Non-Volatile (NV) memory and a small time-window within which volatile state needs to be maintained. Our work raises the bar considerably for defense mechanisms for hardware security.