SeqL+: Secure Scan-Obfuscation With Theoretical and Empirical Validation

Scan-obfuscation is a powerful methodology to protect Silicon-based intellectual property from theft. Prior work on scan-obfuscation in the context of logic-locking have unique limitations, which are addressed by our previous work, SeqL, which looks at functional output corruption to obfuscate scan-chains, but is unable to resist removal attacks on circuits with inadequate number of flip-flops without feedback. To address this issue, we propose to scramble flip-flops with feedback to increase key length without introducing further vulnerabilities. This study reveals the first formulation and complexity analysis of Boolean satisfiability (SAT)-based attack on scan-scrambling. We formulate the attack as a conjunctive normal form (CNF) using a worst-case O(n3) reduction in terms of scramble-graph size n. In order to defeat SAT-based attack, we propose an iterative swapping-based scan-cell scrambling algorithm that has O(n) implementation time-complexity and O(2alpha.n+1}3)) SAT-decryption time-complexity in terms of a user-configurable cost constraint α ~(0 < α ≤ 1).