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Logic locking “hides” the functionality of a digital circuit to protect it from counterfeiting, piracy, and malicious design modifications. The original design is transformed into a “locked” design such that the circuit reveals its correct functionality only when it is “unlocked” with a secret sequence of bits—the key bit-string. However, strong attacks, especially the SAT attack that uses a SAT solver to recover the key bit-string, have been profoundly effective at breaking the locked circuit and recovering the circuit functionality.We lift logic locking to Higher Order Logic Locking (HOLL) by hiding a higher-order relation, instead of a key of independent values, challenging the attacker to discover this key relation to recreate the circuit functionality. Our technique uses program synthesis to construct the locked design and synthesize a corresponding key relation. HOLL has low overhead and existing attacks for logic locking do not apply as the entity to be recovered is no more a value. To evaluate our proposal, we propose a new attack (SynthAttack) that uses an inductive synthesis algorithm guided by an operational circuit as an input-output oracle to recover the hidden functionality. SynthAttack is inspired by the SAT attack, and similar to the SAT attack, it is verifiably correct, i.e., if the correct functionality is revealed, a verification check guarantees the same. Our empirical analysis shows that SynthAttack can break HOLL for small circuits and small key relations, but it is ineffective for real-life designs.

Logic locking "hides" the functionality of a digital circuit to protect it from counterfeiting, piracy, and malicious design modifications. The original design is transformed into a "locked" design such that the circuit reveals its correct functionality only when it is "unlocked" with a secret sequence of bits-the key bit-string. However, strong attacks, especially the SAT attack that uses a SAT solver to recover the key bitstring, have been profoundly effective at breaking the locked circuit and recovering the circuit functionality. We lift logic locking to Higher Order Logic Locking (HOLL) by hiding a higher-order relation, instead of a key of independent values, challenging the attacker to discover this key relation to recreate the circuit functionality. Our technique uses program synthesis to construct the locked design and synthesize a corresponding key relation. HOLL has low overhead and existing attacks for logic locking do not apply as the entity to be recovered is no more a value. To evaluate our proposal, we propose a new attack (SynthAttack ) that uses an inductive synthesis algorithm guided by an operational circuit as an input-output oracle to recover the hidden functionality. SynthAttack is inspired by the SAT attack, and similar to the SAT attack, it is verifiably correct, i.e., if the correct functionality is revealed, a verification check guarantees the same. Our empirical analysis shows that SynthAttack can break HOLL for small circuits and small key relations, but it is ineffective for real-life designs.

No 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.