On the Difficulty of FSM-based Hardware Obfuscation

Abstract

In today’s Integrated Circuit (IC) production chains, a designer’s valuable Intellectual Property (IP) is transparent to diverse stakeholders and thus inevitably prone to piracy. To protect against this threat, numerous defenses based on the obfuscation of a circuit’s control path, i.e. Finite State Machine (FSM), have been proposed and are commonly believed to be secure. However, the security of these sequential obfuscation schemes is doubtful since realistic capabilities of reverse engineering and subsequent manipulation are commonly neglected in the security analysis. The contribution of our work is threefold: First, we demonstrate how high-level control path information can be automatically extracted from third-party, gate-level netlists. To this end, we extend state-of-the-art reverse engineering algorithms to deal with Field Programmable Gate Array (FPGA) gate-level netlists equipped with FSM obfuscation. Second, on the basis of realistic reverse engineering capabilities we carefully review the security of state-of-the-art FSM obfuscation schemes. We reveal several generic strategies that bypass allegedly secure FSM obfuscation schemes and we practically demonstrate our attacks for a several of hardware designs, including cryptographic IP cores. Third, we present the design and implementation of Hardware Nanomites, a novel obfuscation scheme based on partial dynamic reconfiguration that generically mitigates existing algorithmic reverse engineering.

A survey of algorithmic methods in IC reverse engineering

“…3 Block diagram of a hardware FSM (dashed line in the case of a Mealy machine). Taken from [24] Identifying control signals is a challenging task on its own and is not addressed in their work. Next, registers that have the same enable signal are grouped into an individual FSM.…”

“…Fyrbiak et al [24] improve the recognition by introducing an influence/dependence metric. Furthermore, they analyze the control behavior of an FSM circuit candidate by determining how many gates are influenced by the FSM.…”

“…In contrast to the previous step, state graph extraction mostly applies functional analysis [24,40,42,43]. It starts by identifying the initial state, in which the FSM wakes up after reset.…”

“…To reveal all states that can be reached from the current state, all input combinations are applied to analyze the transition behavior. This brute-force approach has a time complexity of O(|S|×2 i ) with i being the number of external inputs to the FSM [24].…”

“…Furthermore, Fyrbiak et al [24] show that most state-ofthe-art FSM obfuscation schemes fail to provide the claimed level of security. The authors demonstrate how several wellestablished obfuscation schemes can be circumvented solely by applying the described FSM reverse engineering techniques.…”