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Interesting paper on cross-VM attacks on AES....
http://eprint.iacr.org/2014/248.

Abstract: This work exposes further vulnerabilities in virtualized
cloud servers by mounting Cross-VM cache attacks in Xen and VMware VMs
targeting AES running in the victim VM. Even though there exists a
rich literature on cache attacks on AES, so far only a single work,
demonstrating a working attack on an ARM platform running a L4Re
virtualization layer has been published. Here we show that AES in a
number popular cryptographic libraries including OpenSSL, PolarSSL and
Libgcrypt are vulnerable to Bernstein’s correlation attack when run in
Xen and VMware (bare metal version) VMs, the most popular VMs used by
cloud service providers (CSP) such as Amazon and Rackspace. We also
show that the vulnerability persists even if the VMs are placed on
different cores in the same machine. The results of this study shows
that there is a great security risk to AES and (data encrypted under
AES) on popular cloud services.