Logic Locking of Finite-State Machines Using Transition Obfuscation

In this paper, we introduce a novel algorithm for securing sequential circuits at the Register-Transfer Level (RTL) that does not require any state augmentation. The algorithm is based on the encryption of the state encodings with a key that is known only to the IP provider. When the correct key is input at runtime, the sequential circuit will operate as designed, otherwise it will operate according to a state transition map that is defined by the wrong key. We call this mode of operation: transition obfuscation. One important advantage of the proposed method is that using the wrong key does not necessarily result in the sequential circuit getting stuck at any one state or getting trapped within any black hole. As a result, the secured sequential circuit is more immune to reverse engineering attacks, and because of the large number of wrong full-state transition maps, more immune to side-channel attacks. A full low-complexity, RTL design methodology based on the new algorithm is presented along with extensive experiments quantifying its design overhead and illustrating its advantages in terms of immunity to reverse engineering attacks.