A Combined Logical and Physical Attack on Logic Obfuscation

Logic obfuscation protects integrated circuits from an untrusted foundry attacker during manufacturing. To counter obfuscation, a number of logical (e.g. Boolean satisfiability) and physical (e.g. electro-optical probing) attacks have been proposed. By definition, these attacks use only a subset of the information leaked by a circuit to unlock it. Countermeasures often exploit the resulting blind-spots to thwart these attacks, limiting their scalability and generalizability. To overcome this, we propose a combined logical and physical attack against obfuscation called the CLAP attack. The CLAP attack leverages both the logical and physical properties of a locked circuit to prune the keyspace in a unified and theoretically-rigorous fashion, resulting in a more versatile and potent attack. To formulate the physical portion of the CLAP attack, we derive a logical formulation that provably identifies input sequences capable of sensitizing logically expressive regions in a circuit. We prove that electro-optically probing these regions infers portions of the key. For the logical portion of the attack, we integrate the physical attack results into a Boolean satisfiability attack to find the correct key. We evaluate the CLAP attack by launching it against four obfuscation schemes in benchmark circuits. The physical portion of the attack fully specified 60.6% of key bits and partially specified another 10.3%. The logical portion of the attack found the correct key in the physical-attack-limited keyspace in under 30 minutes. Thus, the CLAP attack unlocked each circuit despite obfuscation.