Side Channel Attacks on STTRAM and Low-Overhead Countermeasures

Abstract: Spin Transfer Torque RAM (STTRAM) is a promising candidate for Last Level Cache (LLC) due to high endurance, high density and low leakage. One of the major disadvantages of STTRAM is high write latency and write current. Additionally, the latency and current depends on the polarity of the data being written. These features introduce major security vulnerabilities and expose the cache memory to side channel attacks. In this paper we propose a novel side channel attack model where the adversary can monitor the supply current of the memory array to partially identify the sensitive cache data that is being read or written. We propose several low cost solutions such as short retention STTRAM, 1-bit parity, multi-bit random write and constant current write driver to mitigate the attack. 1-bit parity reduces the number of distinct write current states by 30% for 32-bit word and the current signature is further obfuscated by multi-bit random writes. The constant current write makes it more challenging for the attacker to extract the entire word using a single supply current signature.