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A team of researchers led by MIT grad student Albert Kwon (with help from EPFL) aims to leapfrog Tor’s anonymizing technique with a brand new platform called Riffle.

Tor — has prompted computer scientists to try to come up with more secure anonymity schemes. At the Privacy Enhancing Technologies Symposium in July, researchers at MIT’s Computer Science and Artificial Intelligence Laboratory and the École Polytechnique Fédérale de Lausanne will present a new anonymity scheme that provides strong security guarantees but uses bandwidth much more efficiently than its predecessors.

The system is devised by Albert Kwon and his coauthors — his advisor, Srini Devadas, the Edwin Sibley Webster Professor of Electrical Engineering and Computer Science at MIT; David Lazar, also a graduate student in electrical engineering and computer science; and Bryan Ford SM ’02 PhD ’08, an associate professor of computer and communication sciences at the École Polytechnique Fédérale de Lausanne — employs several existing cryptographic techniques but combines them in a novel manner.

The system is a series of servers called a mixnet. Each server permutes the order in which it receives messages before passing them on to the next. If, for instance, messages from senders Alice, Bob, and Carol reach the first server in the order A, B, C, that server would send them to the second server in a different order — say, C, B, A. The second server would permute them before sending them to the third, and so on. An adversary that had tracked the messages’ points of origin would have no idea which was which by the time they exited the last server. It’s this reshuffling of the messages that gives the new system its name: Riffle. Riffle also uses a technique known as onion encryption; “Tor,” for instance, is an acronym for “the onion router.” With onion encryption, the sending computer wraps each message in several layers of encryption, using a public-key encryption system like those that safeguard most financial transactions online. Each server in the mixnet removes only one layer of encryption, so that only the last server knows a message’s ultimate destination.

To thwart message tampering, Riffle uses a technique called a verifiable shuffle. Because of the onion encryption, the messages that each server forwards look nothing like the ones it receives; it has peeled off a layer of encryption. But the encryption can be done in such a way that the server can generate a mathematical proof that the messages it sends are valid manipulations of the ones it receives. Mixnets has been around for awhile, but unfortunately it’s always relied on public-key cryptography and on public-key techniques, and has been expensive. Whats different about this research is that it reveals how to  use more efficient symmetric-key techniques to accomplish the same thing. They do one expensive shuffle using known protocols, but then they bootstrap off of that to enable many subsequent shufflings.


There’s no plan to commercialize Riffle, either, nor will it be a replacement for Tor, even though it does some things vastly better.


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