Differential Analysis and Fingerprinting of ZombieLoads on Block Ciphers

Summary

Microarchitectural Data Sampling (MDS) enables to observe in-flight data that has recently been loaded or stored in shared short-time buffers on a physical CPU core. In-flight data sampled from line-fill buffers (LFBs) are also known as "ZombieLoads". We present a new method that links the analysis of ZombieLoads to Differential Power Analysis (DPA) techniques and provides an alternative way to derive the secret key of block ciphers. This method compares observed ZombieLoads with predicted intermediate values that occur during cryptographic computations depending on a key hypothesis and known data. We validate this approach using an Advanced Encryption Standard (AES) software implementation. Further, we provide a novel technique of cache line fingerprinting that reduces the superposition of ZombieLoads from different cache lines in the data sets resulting from an MDS attack. Thereby, this technique is helpful to reveal static secret data such as AES round keys which is shown in practice on an AES implementation.