Sunday, January 17, 2016

Sixth Bar-Ilan University Winter School on Cryptography (Part 1)

This two-part blog post has been collaboratively written by Eduardo, Marie-Sarah, Matthias and Ralph.

Earlier this month, from 4-7 January, a few ECRYPT-NET fellows and about a hundred others attended the Bar-Ilan University winter school on cryptography. It took place in Ramat Gan, a suburb of Tel Aviv, at the Kfar Maccabiah hotel and conference centre (named after the Maccabiah, or Jewish Olympics, that take place there every four years). The school was intense, but well organized. It was split into two parts, verifiable computation and special encryption, and so will be our coverage of it.

Part 1: Verifiable Computation

Michael Walfish, Yael Tauman Kalai, and Eran Tromer guided us through methods for verifying the outsourced computation of functions. We particularly appreciated the crystal-clear overview of all sessions, given by Michael Walfish, that emphasized how the content of the talks fit together.

Let's set the scene for verifiable computation: a client (the verifier) wants to outsource the computation of a function f to a server (the prover) who has more computing resources. But how does the verifier know that the value returned by the prover is actually the result of applying the function f to the purported inputs? A malicious or a lazy server could indeed modify the process to gain some advantages as, for instance, reducing the cost of operating a system.

Verifiable computation of f(x) comprises the following two phases:
  1. Program representation (or arithmetization): The verifier (or a party he trusts) expresses the function f as a set of arithmetic constraints over a field, in terms of the input x, output y, and intermediate variables z. Each of these x, y, and z may be vectors, e.g., x=(x1,x2,x3). Typically, the format these constraints needs to have is that of degree-2 polynomials that equal 0 when they (and the function f) are satisfied.
  2. Solving and proving: The server must prove to the client that the solution it returns, y, is correct. The landscape of proof protocols shows a trade-off between efficiency, expressiveness and additional properties like zero-knowledge or non-interactivity. The speaker himself recently wrote a survey which is a very nice introduction to the state of the art and these trade-offs.

Yael Kalai's talks took a more theoretical approach, guiding us through the evolution of Probabillistically Checkable Proofs (PCPs). She emphasized the importance of "good" security assumptions, where "good" requires at least, and according to her point of view, that the underlying assumptions can be efficiently falsified. These theoretical worries were well founded, as most of today's verifiable computation protocols rely on SNARKs (standing for Succint Non-interactive ARguments of Knowledge) which cannot be proved secure via black-box reductions from (efficiently) falsifiable assumptions. 

Yael's talks provided also very interesting and intuitive examples. To give one, suppose that Peggy and Victor are playing chess. After a number of moves, Peggy (the prover) wants to prove to Victor (the verifier) that she has a checkmate. If Victor fails to see it, it is for Peggy easier to convince him by continuing the game (an interactive proof) until he does, rather than to explain all the possible combinations of moves without moving any piece (a non-interactive proof). This intuition of the power of interaction extends to the rest of the proof systems. Finally, she even showed us how the subject fits in the quantum framework, introducing us to the notion of non-signalling adversaries.

Eran Tromer recovered the line of Michael Walfish and focused on the details of SNARKs and how they are actually constructed. Among others, he has been writing libsnark, a C++ library that is used a lot for verifiable computation systems relying on SNARKs. He also showed us a potential application for them, called Zerocash. Zerocash is a protocol that provides a privacy-preserving version of Bitcoin. In contrast to Bitcoin, where all the transactions are public in the block chain, Zerocash does not contain information about the payment’s origin, destination or amount. The correctness of the transaction is guaranteed via a zero-knowledge proof. More details can be found here.

Part 1.5: Excursion to Caesarea and Binyamina Winery

Sun starting to go down in Caesarea.

Tuesday afternoon, we made an excursion to the remains of Caesarea, a Roman city on the Mediterranean coast that was built by Herod over 2000 years ago. To say that it had a tumultuous history would be an understatement. Our tour included a walk through a "graveyard" of columns and capitals, the amphitheatre (whose first row is still intact), and the hippodrome.

Next, we took an informative tour of the Binyamina winery. We learned that grapes are crushed with a flexible rubber material to simulate the skin of feet. For red wine, the grapes are fermented (skin, seeds, and all) before being crushed. For white wine, seeds and skin are removed (by sedimentation) after pressing, then fermented. The tannin (bitter-tasting substances) in wine comes from the seeds, skin, and maybe the material of the barrel in which it is matured. Whether wine is aged or matured in an American oak (sweeter) or French oak (more tannins, adds more complex flavours) barrel affects the final product. Tannins prevent oxidation, so red wine (with more tannins) can be matured longer. Stopping fermentation early makes wine more sweet.

After learning how wine is made, we concluded the day by learning how it tastes (using our five senses!) and enjoying a generous dinner.

Stay tuned for the second and last part of the post!

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