SR-SFLL: Structurally Robust Stripped Functionality Logic Locking

Abstract

Logic locking was designed to be a formidable barrier to IP piracy: given a logic design, logic locking modifies the logic design such that the circuit operates correctly only if operated with the “correct” secret key. However, strong attacks (like SAT-based attacks) soon exposed the weakness of this defense. Stripped functionality logic locking (SFLL) was recently proposed as a strong variant of logic locking. SFLL was designed to be resilient against SAT attacks, which was the bane of conventional logic locking techniques. However, all SFLL-protected designs share certain “circuit patterns” that expose them to new attacks that employ structural analysis of the locked circuits.In this work, we propose a new methodology—Structurally Robust SFLL ($$\mathcal{S}\mathcal{R}$$ S R -SFLL)—that uses the power of modern satisfiability and synthesis engines to produce semantically equivalent circuits that are resilient against such structural attacks. On our benchmarks, $$\mathcal{S}\mathcal{R}$$ S R -SFLL was able to defend all circuit instances against both structural and SAT attacks, while all of them were broken when defended using SFLL. Further, we show that designing such defenses is challenging: we design a variant of our proposal, $$\mathcal{S}\mathcal{R}$$ S R -SFLL(0), that is also robust against existing structural attacks but succumbs to a new attack, SyntAk (also proposed in this work). SyntAk uses synthesis technology to compile $$\mathcal{S}\mathcal{R}$$ S R -SFLL(0) locked circuits into semantically equivalent variants that have structural vulnerabilities. $$\mathcal{S}\mathcal{R}$$ S R -SFLL, however, remains resilient to SyntAk.