Tree-Based Logic Encryption for Resisting SAT Attack

Abstract

Logic encryption is an IC protection technique which inserts key gates or logic blocks controlled by key inputs to hide a circuit's functionality. An encrypted circuit needs to be activated with a secret key for being functional. Recently, a powerful attack method based on SAT solving was proposed. It successfully breaks most of the existing logic encryption algorithms within a few hours. In this paper, we propose a new logic encryption method for resisting the SAT attack by inserting XOR/XNOR key gates. The method consists of two stages. The first stage encrypts AND trees and OR trees for maximizing SAT solving iterations. The second stage carefully inserts key gates for increasing the computational effort of SAT solving. Furthermore, we propose an obfuscation method to protect the encrypted trees from being identified and removed. The experimental results show that the proposed method uses less key inputs while achieving better resilience against the SAT attack for each benchmark circuit, compared to a prior logic encryption method which encrypts a circuit by inserting XOR/XNOR key gates as well. A total of 22 benchmark circuits that can be successfully decrypted within 10 minutes become uncrackable within 1 hour by encrypting them with the proposed method.