Logic locking has emerged as a promising solution for protecting the semiconductor intellectual Property (IP) from the untrusted entities in the design and fabrication process. Logic locking hides the functionality of the IP by embedding additional key-gates in the circuit. The correct output of the chip is produced, once the correct key value is available at the input of the key-gates. The confidentiality of the key is imperative for the security of the locked IP as it stands as the lone barrier against IP infringement. Therefore, the logic locking is considered as a broken scheme once the key value is exposed. The research community has shown the vulnerability of the logic locking techniques against different classes of attacks, such as Oracle-guided and physical attacks. Although several countermeasures have already been proposed against such attacks, none of them is simultaneously impeccable against Oracle-guided, Oracle-less, and physical attacks. Under such circumstances, a defense-in-depth approach can be considered as a practical approach in addressing the vulnerabilities of logic locking. Defense-in-depth is a multilayer defense approach where several independent countermeasures are implemented in the device to provide aggregated protection against different attack vectors. Introducing such a multilayer defense model in logic locking is the major contribution of this paper. With regard to this, we first identify the core components of logic locking schemes, which need to be protected. Afterwards, we categorize the vulnerabilities of core components according to potential threats for the locking key in logic locking schemes. Furthermore, we propose several defense layers and countermeasures to protect the device from those vulnerabilities. Finally, we turn our focus to open research questions and conclude with suggestions for future research directions.
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