  In this paper, we introduce DFSSD, a novel logic locking solution for
sequential and FSM circuits with a restricted (locked) access to the scan
chain. DFSSD combines two techniques for obfuscation: (1) Deep Faults, and (2)
Shallow State Duality. Both techniques are specifically designed to resist
against sequential SAT attacks based on bounded model checking. The shallow
state duality prevents a sequential SAT attack from taking a shortcut for early
termination without running an exhaustive unbounded model checker to assess if
the attack could be terminated. The deep fault, on the other hand, provides a
designer with a technique for building deep, yet key recoverable faults that
could not be discovered by sequential SAT (and bounded model checker based)
attacks in a reasonable time.
