## CryptoDB

### Akshay Degwekar

#### Publications

**Year**

**Venue**

**Title**

2019

TCC

On the Complexity of Collision Resistant Hash Functions: New and Old Black-Box Separations
Abstract

The complexity of collision-resistant hash functions has been long studied in the theory of cryptography. While we often think about them as a Minicrypt primitive, black-box separations demonstrate that constructions from one-way functions are unlikely. Indeed, theoretical constructions of collision-resistant hash functions are based on rather structured assumptions.We make two contributions to this study: 1.A New Separation: We show that collision-resistant hashing does not imply hard problems in the class Statistical Zero Knowledge in a black-box way.2.New Proofs: We show new proofs for the results of Simon, ruling out black-box reductions of collision-resistant hashing to one-way permutations, and of Asharov and Segev, ruling out black-box reductions to indistinguishability obfuscation. The new proofs are quite different from the previous ones and are based on simple coupling arguments.

2019

TCC

Statistical Difference Beyond the Polarizing Regime
Abstract

The polarization lemma for statistical distance (
$${\text {SD}}$$
), due to Sahai and Vadhan (JACM, 2003), is an efficient transformation taking as input a pair of circuits
$$(C_0,C_1)$$
and an integer k and outputting a new pair of circuits
$$(D_0,D_1)$$
such that if
$${\text {SD}}(C_0,C_1) \ge \alpha $$
then
$${\text {SD}}(D_0,D_1) \ge 1-2^{-k}$$
and if
$${\text {SD}}(C_0,C_1) \le \beta $$
then
$${\text {SD}}(D_0,D_1) \le 2^{-k}$$
. The polarization lemma is known to hold for any constant values
$$\beta < \alpha ^2$$
, but extending the lemma to the regime in which
$$\alpha ^2 \le \beta < \alpha $$
has remained elusive. The focus of this work is in studying the latter regime of parameters. Our main results are: 1.Polarization lemmas for different notions of distance, such as Triangular Discrimination (
$${{\,\mathrm{TD}\,}}$$
) and Jensen-Shannon Divergence (
$${{\,\mathrm{JS}\,}}$$
), which enable polarization for some problems where the statistical distance satisfies
$$ \alpha ^2< \beta < \alpha $$
. We also derive a polarization lemma for statistical distance with any inverse-polynomially small gap between
$$ \alpha ^2 $$
and
$$ \beta $$
(rather than a constant).2.The average-case hardness of the statistical difference problem (i.e., determining whether the statistical distance between two given circuits is at least
$$\alpha $$
or at most
$$\beta $$
), for any values of
$$\beta < \alpha $$
, implies the existence of one-way functions. Such a result was previously only known for
$$\beta < \alpha ^2$$
.3.A (direct) constant-round interactive proof for estimating the statistical distance between any two distributions (up to any inverse polynomial error) given circuits that generate them. Proofs of closely related statements have appeared in the literature but we give a new proof which we find to be cleaner and more direct.

2018

CRYPTO

From Laconic Zero-Knowledge to Public-Key Cryptography
📺
Abstract

Since its inception, public-key encryption (
$$\mathsf {PKE}$$
PKE) has been one of the main cornerstones of cryptography. A central goal in cryptographic research is to understand the foundations of public-key encryption and in particular, base its existence on a natural and generic complexity-theoretic assumption. An intriguing candidate for such an assumption is the existence of a cryptographically hard language .In this work we prove that public-key encryption can be based on the foregoing assumption, as long as the (honest) prover in the zero-knowledge protocol is efficient and laconic. That is, messages that the prover sends should be efficiently computable (given the witness) and short (i.e., of sufficiently sub-logarithmic length). Actually, our result is stronger and only requires the protocol to be zero-knowledge for an honest-verifier and sound against computationally bounded cheating provers.Languages in with such laconic zero-knowledge protocols are known from a variety of computational assumptions (e.g., Quadratic Residuocity, Decisional Diffie-Hellman, Learning with Errors, etc.). Thus, our main result can also be viewed as giving a unifying framework for constructing
$$\mathsf {PKE}$$
PKE which, in particular, captures many of the assumptions that were already known to yield
$$\mathsf {PKE}$$
PKE.We also show several extensions of our result. First, that a certain weakening of our assumption on laconic zero-knowledge is actually equivalent to
$$\mathsf {PKE}$$
PKE, thereby giving a complexity-theoretic characterization of
$$\mathsf {PKE}$$
PKE. Second, a mild strengthening of our assumption also yields a (2-message) oblivious transfer protocol.

2018

CRYPTO

On the Local Leakage Resilience of Linear Secret Sharing Schemes
📺
Abstract

We consider the following basic question: to what extent are standard secret sharing schemes and protocols for secure multiparty computation that build on them resilient to leakage? We focus on a simple local leakage model, where the adversary can apply an arbitrary function of a bounded output length to the secret state of each party, but cannot otherwise learn joint information about the states.We show that additive secret sharing schemes and high-threshold instances of Shamir’s secret sharing scheme are secure under local leakage attacks when the underlying field is of a large prime order and the number of parties is sufficiently large. This should be contrasted with the fact that any linear secret sharing scheme over a small characteristic field is clearly insecure under local leakage attacks, regardless of the number of parties. Our results are obtained via tools from Fourier analysis and additive combinatorics.We present two types of applications of the above results and techniques. As a positive application, we show that the “GMW protocol” for honest-but-curious parties, when implemented using shared products of random field elements (so-called “Beaver Triples”), is resilient in the local leakage model for sufficiently many parties and over certain fields. This holds even when the adversary has full access to a constant fraction of the views. As a negative application, we rule out multi-party variants of the share conversion scheme used in the 2-party homomorphic secret sharing scheme of Boyle et al. (Crypto 2016).

#### Coauthors

- Fabrice Benhamouda (1)
- Itay Berman (3)
- Nir Bitansky (2)
- Yuval Ishai (1)
- Tal Rabin (1)
- Ron D. Rothblum (3)
- Vinod Vaikuntanathan (2)
- Prashant Nalini Vasudevan (4)