Paper 2019/656

SeqL: SAT-attack Resilient Sequential Locking

Seetal Potluri and Akash Kumar and Aydin Aysu

Abstract

SAT-attack is known to successfully decrypt a functionally correct key of a locked combinational circuit. It is possible to extend the SAT-attack to sequential circuits through the scan-chain by selectively initializing the combinational logic and analyzing the responses. Recently, sequential locking was proposed as a defense to SAT-attack, which works by locking the scan-chains of flip-flops. ScanSAT [1], however, showed that it is possible to convert the sequentially locked instance to a locked combinational instance, and thereby decrypt the entire sequential key using SAT-attack. In this paper, we propose SeqL, a secure sequential lock defense against ScanSAT. SeqL provides functional isolation, and also encrypts selective flip-flop inputs, thereby mitigating ScanSAT and other related SAT-attacks. We conduct a formal study of the sequential locking problem and demonstrate automating our proposed defense on any given sequential circuit. We show that SeqL hides functionally correct keys from the attacker, thereby increasing the likelihood of functional output corruption. When tested on sequential benchmarks (ITC’99) and pipelined combinational benchmarks (ISCAS’85, MCNC), SeqL gave 100% resilience to ScanSAT.