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Keeping Data Secret, Even From Apps That Use It
Nerval's Lobster writes "Datacenters wanting to emulate Google by encrypting their data beyond the ability of the NSA to crack it may get some help from a new encryption technique that allows data to be stored, transported and even used by applications without giving away any secrets. In a paper to be presented at a major European security conference this week, researchers from Denmark and the U.K. collaborated on a practical way to implement a long-discussed encryption concept called Multi-Party Computation (MPC). The idea behind MPC is to allow two parties who have to collaborate on an analysis or computation to do so without revealing their own data to the other party. Though the concept was introduced in 1982, ways to accomplish it with more than two parties, or with standardized protocols and procedures, has not become practical in commercial environments. The Danish/British team revamped an MPC protocol nicknamed SPDZ (pronounced 'speeds'), which uses secret, securely generated keys to distribute a second set of keys that can be used for MPC encryptions. The big breakthrough, according to Smart, was to streamline SPDZ by reducing the number of times global MAC keys had to be calculated in order to create pairs of public and private keys for other uses. By cutting down on repetitive tasks, the whole process becomes much faster; because the new technique keeps global MAC keys secret, it should also make the faster process more secure."
The summary claims that
Though the concept was introduced in 1982, ways to accomplish it with more than two parties, or with standardized protocols and procedures, has not become practical in commercial environments.
(I presume it's quoting the article, but samzenpus has managed to make the link self-referential).
That just isn't true. I've read a very interesting paper about "massively multiplayer" commercial use of MPC back in 2008. It involved Danish researchers, so it may be the same team, and there may be improvements, but it would be good to limit the claims to the actual novelties.
I just upgraded to a smart phone and hated how every app I wanted to download wanted everything. Why should Pac Man need my contacts list and GPS information? So when I saw the submission I though ALL RIGHT!!!
The solution you are looking for is SElinux, and it is already enabled in the latest cyanogenmod nightlies.
- Nec Impar Pluribus, or so I'm told.
Two datacenters owned by the same company using MPC is a really dumb use case. That won't help at all. The point of Google encrypting cross-dc communications is a forcing manoeuvre - it forces intelligence agencies to go via Google Legal to get information where the request can be analyzed and pushed back on. Even in countries where the legal system is flimsy and corrupt, that's an issue that can be improved significantly just with a single act of Congress or Parliament, whereas undoing their wiretapping infrastructure will prove somewhat harder because there's no adversarial lawyer standing in the way.
A better example might be two datacenters owned by different companies, where they don't mutually trust each other. Or, to give an actual use case, the OTR chat encryption protocol uses MPC to authenticate connections. They call it the socialist millionaires protocol. The two parties agree on a secret word (typically by one user posing a question to the other), and then a variant of MPC is used to verify that both parties selected the same word. The word itself never transits the wire and it's only used for authentication, so it's relatively strong even if the secret word is short or predictable.
Now, for some background. The paper can be found here if you want to skip the million+1 links and registration crap.
The basic idea behind MPC is that you write your shared computation in the form of a boolean circuit, made up of logic gates as if you were making an electronic circuit. The inputs to the program are represented as if they were electronic signals (i.e. as one and zero bits on wires). Once done, there are two protocols you can follow. The original one is by a guy named Andrew Yao. Each wire in the circuit is assigned a pair of keys. The details I'll gloss over now, but basically given the circuit (program) as a template, lots of random keys are created by party A, then the entire "garbled circuit" is sent to party B who will run it. Party A also selects the keys for his input wires and sends them to party B, who doesn't know whether they represent 0 or 1, only party A knows that.
Now party B wants to run the program with his input, but he doesn't want party A to know what his input is. So they use a separate protocol called an oblivious transfer protocol to get party A to cough up the right keys for B's input wires, without A finding out what they were. Finally, party B can run the program by progressively decrypting the wires until the output is arrived at.
What I described above is Yao's protocol. There is also a slightly different protocol called BGV. In BGV you don't send the entire program all at once. Instead, as party B runs through the program, each time they encounter an AND gate they do an oblivious transfer with party A. XOR gates are "free" and don't require any interaction. I forgot what happens for other kinds of gates. Basically, BGV involves both parties interacting throughout the computation, however, it can result in much less network traffic being required if your OT protocol is cheap, because if your circuit is very wide and shallow then most of the garbled program never has to even get transferred at all.
From what I can tell, most of the best results in MPC these days are coming from BGV coupled with new, highly efficient OT protocols. SPDZ appears to work on yet another design, but the basic reliance on circuit form remains.