Cache Misses and the Recovery of the Full AES 256 Key

In recent years, CPU caches have revealed themselves as one of the most powerful sources of information leakage. This information leakage affects any implementation whose memory accesses, to data or instructions, depend on sensitive information such as private keys. In most cases, side-channel cache attacks do not require any specific permission and just need access to a shared cache. This fact, combined with the spread of cloud computing, where the infrastructure is shared between different customers, have made these attacks quite popular. In this paper, we present a novel approach to exploit the information obtained from the CPU cache. First, we introduce a non-access attack that provides a 97\% reduction in the number of encryptions required to obtain a 128-bit AES key. Next, this attack is adapted and extended in what we call the encryption-by-decryption cache attack or EBD, to obtain a 256-bit AES key. When EBD is applied to AES-256, we are able to obtain the 256 bits of the key with less than 10000 encryptions. These results make EBD, to the best of our knowledge, the first practical attack on AES-256 and also demonstrate that AES-256 is only about 3 times more complex to attack than AES-128 via cache attacks. In both cases the target is the AES T-table-based implementation, and we also demonstrate that our approach works in a cross-VM scenario.