LoPher: SAT-Hardened Logic Embedding on Block Ciphers

Block ciphers are widely regarded as concrete realizations of pseudorandom permutations with established security features. However, their applicability outside the domain of encryption has not been explored so far. In this paper, we open up, for the first time, an entirely novel application of them to logic hiding. We show that a combinational circuit can always be embedded within a block cipher having a bit-permutation based diffusion layer, preserving the cipher structure and security properties. The functionality of the embedded circuit becomes transparent only on the application of a secret key, whereas a wrong key will cause behaviour that is uncorrelated to that of the circuit. As an immediate application, we propose a combinational logic-locking scheme. The proposed locking scheme is also found to be robust against the state-of-the-art (SAT-assisted and other) attacks on logic locks.