In Spring of 2007, the California Secretary of State convened a team of
security researchers to review the electronic voting systems certified for
use in California. We were provided with the source code for the systems
as well as with access to the hardware. Serious and exploitable
vulnerabilities were found in all the systems analyzed: Diebold, Hart, and
Sequoia. I'll be discussing the effort as a whole, providing an overview
of the issues that all the teams found, and then discussing in detail the
system my team studied, Hart InterCivic.
Joint work with Srinivas Inguva, Eric Rescorla, and Dan Wallach.
In a proof-of-retrievability system, a data storage center must prove to a verifier that he is actually storing all of a client's data. The central challenge is to build systems that are both efficient and *provably* secure -- that is, it should be possible to extract the client's data from any prover that passes a verification check. All previous provably secure solutions require that a prover send O(l) authenticator values (i.e., MACs or signatures) to verify a file, for a total of O(l^2) bits of communication, where l is the security parameter. The extra cost over the ideal O(l) communication can be prohibitive in systems where a verifier needs to check many files.
We create the first compact and provably secure proof of retrievability systems. Our solutions allow for compact proofs with just one authenticator value -- in practice this can lead to proofs with as little as 40~bytes of communication. We present two solutions with similar structure. The first one is privately verifiable and builds elegantly on pseudorandom functions (PRFs); the second allows for publicly verifiable proofs and is built from the signature scheme of Boneh, Lynn, and Shacham in bilinear groups. Both solutions rely on homomorphic properties to aggregate a proof into one small authenticator value.
