*22:17* [Pub][ePrint]
Iterated group products and leakage resilience against NC^1, by Eric Miles
We show that if NC^1 \\neq L, then for every element g of the alternating group A_t, circuits of depth O(log t) cannot distinguish between a uniform vector over (A_t)^t with product = g and one with product = identity. Combined with a recent construction by the author and Viola in the setting of leakage-resilient cryptography [STOC \'13], this gives a compiler that produces circuits withstanding leakage from NC^1 (assuming NC^1 \\neq L). For context, leakage from NC^1 breaks nearly all previous constructions, and security against leakage from P is impossible.We build on work by Cook and McKenzie [J.\\ Algorithms \'87] establishing the relationship between L = logarithmic space and the symmetric group S_t. Our techniques include a novel algorithmic use of commutators to manipulate the cycle structure of permutations in A_t.

*22:17* [Pub][ePrint]
Interactive Encryption, Message Authentication, and Anonymous Key Exchange, by Yevgeniy Dodis and Dario Fiore
Public-Key Encryption (PKE) and Message Authentication (PKMA, aka as digital signatures) are fundamental cryptographic primitives. Traditionally, both notions are defined as non-interactive (i.e., single-message). In this work, we initiate rigorous study of (possibly) {\\em interactive} PKE and PKMA schemes. We obtain the following results demonstrating the power of interaction to resolve questions which are either open or impossible in the non-interactive setting.Efficiency/Assumptions.

One of the most well known open questions in the area of PKE is to build, in a ``black-box way\'\', so called chosen ciphertext attack (CCA-) secure PKE from chosen plaintext attack (CPA-) secure PKE. In contrast, we show a simple $2$-round CCA-secure PKE from any (non-interactive) CPA-secure PKE (in fact, these primitives turn out to be equivalent). Similarly, although non-interactive PKMA schemes can be inefficiently built from any one-way function, no efficient signature schemes are known from many popular number-theoretic assumptions, such as factoring, CDH or DDH. In contrast, we show an efficient $2$-round PKMA from most popular assumptions, including factoring, CDH and DDH.

Advanced Properties.

It is well known that no non-interactive signature (resp. encryption) scheme can be {\\em deniable} (resp. {\\em forward-secure}), since the signature (resp. ciphertext) can later ``serve as an evidence of the sender\'s consent\'\' (resp. ``be decrypted if the receiver\'s key is compromised\'\'). We also formalize a related notion of {\\em replay-secure} (necessarily) interactive PKMA (resp. PKE) schemes, where the verifier (resp. encryptor) is assured that the ``current\'\' message can only be authenticated (resp. decrypted) by the secret key owner {\\em now}, as opposed to some time in the past (resp. future). We observe that our 2-round PKMA scheme is both replay-secure and (passively) deniable, and our 2-round PKE scheme is both replay- and forward-secure. We also define and construct stronger forms of necessarily interactive PKE/PKMA schemes, called {\\em confirmed encryption} and {\\em confidential authentication}.

Anonymous Key Exchange.

We extend our definitional framework for interactive PKE and PKMA schemes to give definitions and constructions of (necessarily interactive) {\\em anonymous key exchange} (1-KE), where an anonymous (unkeyed) party establishes a key with an authenticated (keyed) party. Unlike the prior work, defining 1-KE by ``downgrading\'\' the hairy and complex definition of {\\em mutually authenticated} key exchange (2-KE), our definition is very ``short\'\' and easy to understand. We also show simple and general connections between anonymous KE and (interactive) confirmed PKE/confidential PKMA schemes. As a result, we obtain old and new schemes for anonymous KE in a clean and modular manner. For example, we obtain the first $2$-round anonymous KE which is both (passively) deniable and forward-secure.

*22:17* [Pub][ePrint]
On the Relation of Random Grid, Probabilistic and Deterministic Visual Cryptography, by Roberto De Prisco and Alfredo De Santis
Visual cryptography is a special type of secret sharing. Two models of visual cryptography have been independently studied: deterministic visual cryptography, introduced by Naor and Shamir, and random grid visual cryptography, introduced by Kafri and Keren. In the context of the deterministic model, Yang has introduced a third model, the probabilistic visual cryptography model. The connection between the probabilistic and the deterministic models have been explored.In this paper we show that there is a strict relation between the random grid model and the deterministic model. More specically we show that to any random grid scheme corresponds a deterministic scheme and viceversa. This allows us to use results known in a model also in the other model. In fact, the random grid model is equivalent to the probabilistic model with no pixel expansion. Exploiting the (many) results known in the deterministic model we are able to improve several schemes and to provide many upper bounds for the random grid model. Exploiting some results known for the random grid model, we are also able to provide new schemes for the deterministic model. A side eect of this paper is that future new results for any one of the two models (random grid and deterministic) should not ignore, and in fact be compared to, the results known in the other model.

*22:17* [Pub][ePrint]
Safe enclosures: towards cryptographic techniques for server protection, by Sergiu Bursuc and Julian P. Murphy
Cryptography is generally used to protect sensitive data from an untrusted server. In this paper, we investigate the converse question: can we use cryptography to protect a trusted server from untrusted data? As a first step in this direction, we propose the notion of safe enclosures. Intuitively, a safe enclosure is a cryptographic primitive that encapsulates data in a way that allows to perform some computation on it, while at the same time protecting the server from malicious data. Furthermore, a safe enclosure should come equipped with a dedicated protocol that implements the enclosing function with unconditional integrity. Otherwise, unguarded data may reach the server. We discuss the novelty of these concepts, propose their formal definition and show several realizations.

*22:17* [Pub][ePrint]
Errorless Smooth Projective Hash Function based on LWE, by Olivier Blazy and Céline Chevalier and Léo Ducas and Jiaxin Pan
Smooth Projective Hash Functions are one of the base tools to build interactive protocols; and this notion has lead to the construction of numerous protocols enjoying strong security notions, such as the security in the Bellare-Pointcheval-Rogaway (BPR) model or even Universal Composability (UC).

Yet, the construction of SPHF has been almost limited to discrete-logarithm or pairing type assumptions up to now. This stands in contrast with domains such as homomorphic encryption or functional encryption, where Lattice Based Cryptography has already caught up and overtook discrete-log/pairing based cryptography. So far, work in the direction of UC based on lattices is almost restricted to a paper from Peikert, Vaikuntanathan, and Waters (Crypto 2008) dealing with Oblivious Transfer in the UC framework, and work in the direction of password-authenticated key exchange protocols (PAKE) to one from

Katz and Vaikuntanathan (Asiacrypt 2009) on a 3-round Password-Authenticated Key Exchange, but restraining itself to the BPR model. It seems that dealing with errors in those contexts is not as easy as it is for encryption.

In this work, we identify the problem at its source, namely, the lattice version of Diffie-Hellman key exchange protocol: the key greement is only approximate. We explicit a simple folklore trick to obtain true, errorless, one-round key exchange from LWE. We then show that this trick can be adapted to various lattice encryption schemes, leading, with some technicalities, to errorless SPHF\'s. From there, we derive three new results, namely the first lattice-based following protocols: a one-round PAKE secure in the BPR model, a 3-round PAKE secure in the UC model, and a UC commitment scheme, all of them based on SIS and LWE assumptions.

*22:17* [Pub][ePrint]
Another Look at XCB, by {Debrup Chakraborty and Vicente Hernandez-Jimenez and Palash Sarkar
XCB is a tweakable enciphering scheme (TES) which was first proposed in 2004. The scheme was modified in 2007. We call thesetwo versions of XCB as XCBv1 and XCBv2 respectively. XCBv2 was later proposed as a standard for encryption of sector oriented

storage media in IEEE-std 1619.2 2010. There is no known proof of security for XCBv1 but the authors provided a concrete security bound for XCBv2 and

a \"proof\" for justifying the bound. In this paper we show that XCBv2 is not secure as a TES by showing an easy distinguishing attack on it.

For XCBv2 to be secure, the message space should contain only messages whose lengths are multiples of the block length of the block cipher.

For such restricted message spaces also the bound that the authors claim is not justified. We show this by pointing out some errors in the proof.

We provide a new security bound for XCBv2, and this bound is much worse than that has been claimed by the authors. We also for the first time

provide a concrete security bound for XCBv1. The new bounds shows that both XCBv1 and XCBv2 are worse in terms of security compared

to all TES for which a concrete security bound is known.

*22:17* [Pub][ePrint]
EPCGen2 Pseudorandom Number Generators: Analysis of J3Gen, by Alberto Peinado and Jorge Munilla and Amparo Fúster
This paper analyzes the cryptographic security of J3Gen, apromising pseudo random number generator for low-cost passive RFID

tags. Although J3Gen has been shown to fulfill the randomness

criteria set by the EPCglobal Gen2 standard and is intended for

security applications, we describe here two cryptanalytic attacks

which question its security claims: i) a probabilistic attack

based on solving linear equation systems, and ii) a

deterministic attack based on the output sequence decimation.

Numerical results, supported by simulations, show that for the

specific recommended values of the configurable parameters, a low

number of intercepted output bits are enough to crytanalyze J3Gen.

We then make some recommendations which address these issues.