Abstract
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(2α.n+13) SAT-decryption time-complexity in terms of a user-configurable cost constraint α (0<α≤1).
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