Trace Logic Locking: Improving the Parametric Space of Logic Locking

To protect against an untrusted foundry, logic locking must 1) inject sufficient error to ensure critical application failures for any wrong key (error severity) and 2) resist any attack against it (attack resilient). We begin our work by deriving a fundamental tradeoff between these two goals which exists underlying all logic locking, regardless of construction. This relationship forces integrated circuit (IC) designers to sacrifice the error severity of logic locking to increase its attack resilience and vice versa. We proceed by exploring the consequences of this tradeoff through architectural simulations of ICs incorporating locking sweeping over the derived parametric space. We find that the efficacy of logic locking is severely limited by this tradeoff. In response, we propose trace logic locking (TLL), a novel enhancement of module level logic locking which enables existing art to secure arbitrary length sequences of input minterms, referred to as traces. Doing so injects an additional degree of freedom into the parametric space of locking, enabling locking techniques to overcome the limitations of our derived tradeoff. We both theoretically and empirically prove this by using TLL to enhance cutting edge locking. In ten large benchmarks, we show that TLL-enhanced logic locking provides exponentially stronger attack resilience than conventional locking with only modest additional overhead. Finally, we demonstrate the efficacy of TLL in a processor IC using architectural simulations. Despite prior art being unable to secure this IC, we find that TLL concurrently achieves strong error severity and attack resilience.