# Friday October 23 at MIT

Location:
32 Vassar St (Stata Center)
G-449 (Patil/Kiva)
Cambridge MA 02139.

Thanks to NSF MACS Project for their generous support.

### Program:

 9:30 – 10:00. Introduction/Coffee 10:00 – 11:00. Eshan Chattopadhyay, University of Texas Austin Non-Malleable Extractors and Codes, with their Many Tampered Extensions 11:30 – 12:30. Shai Halevi, IBM Research The State of Multi-linear Maps 12:30 – 1:30. Lunch (provided) 1:30 – 2:00. Rump Session 2:00 – 3:00. Ron Rothblum, MIT Proofs and Arguments of Proximity: Verifying Computations in Sub-Linear Time 3:30 – 4:30. abhi shelat, University of Virginia Impossibility and Difficulty in Constructing Obfuscation Schemes

### Rump program:

 Aanchal Malhotra, BU Attacking the Network Time Protocol Adam Sealfon, MIT Shortest Paths and Distances with Differential Privacy Yilei Chen, BU On the correlation intractability of obfuscated pseudorandom functions (a.k.a. the foundation of bitcoin hash functions) Zahra Jafargholi, NEU The New Realization of Adaptively Secure Garbled Circuits

### Abstracts:

Title: Non-Malleable Extractors and Codes, with their Many Tampered Extensions

Randomness extractors and error correcting codes are fundamental objects in computer science. Recently, there have been several natural generalizations of these objects, in the context and study of tamper resilient cryptography. These are seeded non-malleable extractors, introduced by Dodis and Wichs; seedless non-malleable extractors, introduced by Cheraghchi and Guruswami; and non-malleable codes, introduced by Dziembowski, Pietrzak and Wichs. Besides being interesting on their own, they also have important applications in cryptography. For example, seeded non-malleable extractors are closely related to privacy amplification with an active adversary, non-malleable codes are related to non-malleable secret sharing, and seedless non-malleable extractors provide a universal way to construct explicit non-malleable codes.

However, explicit constructions of non-malleable extractors appear to be hard, and the known constructions are far behind their non-tampered counterparts. Indeed, the best known seeded non-malleable extractor requires min-entropy rate at least 0.49; while explicit constructions of non-malleable two-source extractors were not known even if both sources have full min-entropy, and was left as an open problem in the work of Cheraghchi-Guruswami. In addition, current constructions of non-malleable codes in the information theoretic setting only deal with the situation where the codeword is tampered once, and may not be enough for certain applications.

In this paper we make progress towards solving the above problems. Our contributions are as follows.

(1) We construct an explicit seeded non-malleable extractor for min-entropy $k > \log^2 n$. This dramatically improves all previous results and gives a simpler 2-round privacy amplification protocol with optimal entropy loss, matching the best known result by Li.

(2) We construct the first explicit non-malleable two-source extractor for min-entropy $k > n-n^{\Omega(1)}$, with output size $n^{\Omega(1)}$ and error $2^{-n^{\Omega(1)}}$.

(3) We motivate and initiate the study of two natural generalizations of seedless non-malleable extractors and non-malleable codes, where the sources or the codeword may be tampered many times. For this, we construct the first explicit non-malleable two-source extractor with tampering degree t up to $n^{\Omega(1)}$, which works for min-entropy $k \geq n-n^{\Omega(1)}$, with output size $n^{\Omega(1)}$ and error $2^{-n^{\Omega(1)}}$.

We further show that we can efficiently sample uniformly from any pre-image. By the connection in [CG14b], we also obtain the first explicit non-malleable codes with tampering degree t up to $n^{\Omega(1)}$, relative rate $n^{\Omega(1)}/n$, and error $2^{-n^{\Omega(1)}}$.

Speaker: Shai Halevi

Title: The State of Cryptographic Multilinear Maps

This talk will give an overview of current state of the constructions of and attacks against cryptographic multilinear maps.

Speaker: Ron Rothblum

Title: Proofs and Arguments of Proximity: Verifying Computations in Sub-Linear Time

An interactive proof of proximity (IPP) is an interactive protocol in which a prover tries to convince a sublinear-time verifier that x \in L. Since the verifier runs in sublinear-time, following the property testing literature, the verifier is only required to reject inputs that are far from L. In a recent work, (Guy) Rothblum, Vadhan and Wigderson (STOC, 2013) constructed an IPP for every language computable by a low depth circuit.

In this work we consider the computational analogue, where soundness is required to hold only against a computationally bounded cheating prover. We call such protocols interactive arguments of proximity.

Assuming the existence of a sub-exponentially secure FHE scheme, we construct a one-round argument of proximity for every language computable in time t, where the running time of the verifier is o(n) + polylog(t) and the running time of the prover is poly(t).

As our second result, assuming sufficiently hard cryptographic PRGs, we give a lower bound, showing that the parameters obtained both in the IPPs of Rothblum et-al, and in our arguments of
proximity, are close to optimal.

Based on joint work with Yael Kalai.

Speaker: abhi shelat

Title: Impossibility and Difficulty in Constructing Obfuscation Schemes

The golden standard for obfuscation, Virtual blackbox obfuscation, was shown to be impossible to achieve for general circuits in the standard model by the celebrated work of Barak et al (CRYPTO 2001). Recently, Brakerski and Rothblum (TCC’15), and Barak et al (Eurocrypt’14) overcome the impossibility and show how to achieve general-purpose VBB obfuscation by using an idealized-graded encoding scheme that enables performing \emph{high-degree} “zero-tests” on encodings.

Building on a result of Canetti, Kalai and Paneth (TCC’15), we first show the impossibility of VBB obfuscation when the idealized-graded encoding scheme only allows evaluating constant-degree zero-tests on encodings. The main technique is to show how constant-degree zero-tests used in an obfuscation scheme can be “removed” by learning what the zero-tests would have answered, resulting in approximately-correct VBB obfuscation. This main technique also rules out sub-exponential secure VBB for general circuits when the idealized graded encoding scheme only allows evaluating degree n^\alpha zero-tests.

We then apply the technique to indistinguishability obfuscation schemes and combine with well-known complexity results to show that constructing iO schemes from constant-degree graded encoding schemes in a blackbox way is as hard as basing public-key cryptography on one-way functions.

This is joint work with Rafael Pass, and with Mohammad Mahmoody, Ameer Mohammed, and Soheil Nematihaji.