Abstract
The CPU cache is a hardware element that leaks significant information about the software running on the CPU. Particularly, any application performing sequences of memory access that depend on sensitive information, such as private keys, is susceptible to suffer a cache attack, which would reveal this information. 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, has made these attacks quite popular. Traditionally, cache attacks against AES use the information about the victim to access an address. In contrast, we show that using non-access provides much more information and demonstrate that the power of cache attacks has been underestimated during these last years. This novel approach is applicable to existing attacks: Prime+Probe, Flush+Reload, Flush+Flush and Prime+Abort. In all cases, using cache misses as source of information, we could retrieve the 128-bit AES key with a reduction in the number of samples of between 93% and 98% compared to the traditional approach. Further, this attack was adapted and extended in what we call the encryption-by-decryption cache attack (EBD), to obtain a 256-bit AES key. In the best scenario, our approach obtained the 256 bits of the key of the OpenSSL AES T-table-based implementation using fewer than 10,000 samples, i.e., 135 milliseconds, proving that AES-256 is only about three times more complex to attack than AES-128 via cache attacks. Additionally, the proposed approach was successfully tested in a cross-VM scenario.
Highlights
Cloud computing aims to provide its users with compute resources as they are required, eliminating the need of acquiring and maintaining expensive computing infrastructure
Cloud providers offer computing capabilities at low prices because of economies of scale: by achieving high utilization of their servers, they can divide costs between more customers. This means that multiple virtual machines (VMs) are co-hosted on a single physical host relying on a virtual machine manager (VMM) to provide logical isolation between them
We show that our approach improved the performance of previous cache attacks and demonstrate its effectiveness for Flush+Reload, Flush+Flush Prime+Probe and Prime+Abort
Summary
Cloud computing aims to provide its users with compute resources as they are required, eliminating the need of acquiring and maintaining expensive computing infrastructure. We consider the T-table-based implementation of AES This implementation is known to be vulnerable to cache attacks. It is commonly used for comparison, and to demonstrate different attack techniques In this work, it serves to our purpose of showing the accuracy of the information gained from the non-access to memory and to quantify the improvement that this approach represents compared to the traditionally used approach based on access. We show that our approach improved the performance of previous cache attacks and demonstrate its effectiveness for Flush+Reload, Flush+Flush Prime+Probe and Prime+Abort. We extend the non-access attack to gain information from more rounds of the algorithm, introducing EBD, a practical attack implementation that provides the full 256-bit AES encryption key using cache side channel attacks.
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