  Existing logic-locking attacks are known to successfully decrypt functionally
correct key of a locked combinational circuit. It is possible to extend these
attacks to real-world Silicon-based Intellectual Properties (IPs, which are
sequential circuits) through scan-chains by selectively initializing the
combinational logic and analyzing the responses. In this paper, we propose
SeqL, which achieves functional isolation and locks selective flip-flop
functional-input/scan-output pairs, thus rendering the decrypted key
functionally incorrect. We conduct a formal study of the scan-locking problem
and demonstrate automating our proposed defense on any given IP. We show that
SeqL hides functionally correct keys from the attacker, thereby increasing the
likelihood of the decrypted key being functionally incorrect. When tested on
pipelined combinational benchmarks (ISCAS,MCNC), sequential benchmarks (ITC)
and a fully-fledged RISC-V CPU, SeqL gave 100% resilience to a broad range of
state-of-the-art attacks including SAT[1], Double-DIP[2], HackTest[3], SMT[4],
FALL[5], Shift-and-Leak[6] and Multi-cycle attacks[7].
