Side-Channel Leakage in Suzuki Stack Circuits

Superconducting digital electronics, particularly, a single-flux quantum (SFQ) logic, is a natural candidate for large-scale in-fridge control circuitry of superconducting quantum computers. Although the SFQ logic possesses high switching speeds and low power dissipation, the integration density is still limited as compared to the complementary metal–oxide–semiconductor (CMOS) technology. To establish a connection between the SFQ circuits with CMOS memory, which can be located at higher temperature stages, and room temperature electronics, a superconductor-semiconductor (quantum-classical) interface circuits is required. This chapter focuses on a specific type of interface circuit, namely Suzuki stack (a.k.a. Josephson latching driver) that can amplify SFQ pulses to a DC signal with an amplitude of tens of mV. With the growing interest in the security of SFQ circuits, this chapter uncovers a potential side-channel leakage mechanism from Suzuki stack circuits. This side-channel leakage can lead to serious security vulnerabilities for cryogenic electronics. By measuring the supply current of Suzuki stack circuits from corresponding room temperature electronics, the Hamming weight of the input bits can potentially be leaked to a malicious adversary. The feasibility of detecting this leakage is demonstrated both analytically and with extensive simulations. A brief introduction to superconducting electronics, quantum computing, interface circuits, hardware security, and side-channel attacks is provided in Sect. 1. The applications and operation principle of Suzuki stack circuits are described in Sect. 2. The threat model of side-channel attack is proposed in Sect. 3. The simulation results of side-channel leakage in Suzuki stack circuits are analyzed in Sect. 4. Potential side-channel attacks that utilize the proposed information leakage are discussed in Sect. 5. Finally, the conclusions are drawn in Sect. 6.