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21:17 [Pub][ePrint] Publicly Verifiable Ciphertexts, by Juan Manuel Gonz{\\\'a}lez Nieto and Mark Manulis and Bertram Poettering and Jothi Rangasamy and Douglas Stebila

  In many applications, where encrypted traffic flows from an open (public) domain to a protected (private) domain, there exists a gateway that bridges the two domains and faithfully forwards the incoming traffic to the receiver. We observe that indistinguishability against (adaptive) chosen-ciphertext attacks (IND-CCA), which is a mandatory goal in face of active attacks in a public domain, can be essentially relaxed to indistinguishability against chosen-plaintext attacks (IND-CPA) for ciphertexts once they pass the gateway that acts as an IND-CCA/CPA filter by first checking the validity of an incoming IND-CCA ciphertext, then transforming it (if valid) into an IND-CPA ciphertext, and forwarding the latter to the recipient in the private domain. ``Non-trivial filtering\'\' can result in reduced decryption costs on the receivers\' side.

We identify a class of encryption schemes with \\emph{publicly verifiable ciphertexts} that admit generic constructions of (non-trivial) IND-CCA/CPA filters. These schemes are characterized by existence of public algorithms that can distinguish between valid and invalid ciphertexts. To this end, we formally define (non-trivial) public verifiability of ciphertexts for general encryption schemes, key encapsulation mechanisms, and hybrid encryption schemes, encompassing public-key, identity-based, and tag-based encryption flavours. We further analyze the security impact of public verifiability and discuss generic transformations and concrete constructions that enjoy this property.

21:17 [Pub][ePrint] PICARO - A Block Cipher Allowing Efficient Higher-Order Side-Channel Resistance -- Extended Version --, by Gilles Piret and Thomas Roche and Claude Carlet

  Many papers deal with the problem of constructing an efficient masking scheme for existing block ciphers. We take the reverse approach: that is, given a proven masking scheme (Rivain and Prouff, CHES 2010) we design a block cipher that fits well the masking constraints. The difficulty of implementing efficient masking for a block cipher comes mainly from the S-boxes. Therefore the choice of an adequate S-box is the first and most critical step of our work. The S-box we selected is non-bijective; we discuss the resulting design and security problems. A complete design of the cipher is given, as well as some implementation results.

21:17 [Pub][ePrint] Another look at non-uniformity, by Neal Koblitz and Alfred Menezes

  We argue that it is unnatural and undesirable to use the non-uniform model of complexity for practice-oriented security reductions in cryptography.

21:17 [Pub][ePrint] Multiple Differential Cryptanalysis using \\LLR and $\\chi^2$ Statistics, by Céline Blondeau and Benoît Gérard and Kaisa Nyberg

  Recent block ciphers have been designed to be resistant against differential

cryptanalysis. Nevertheless it has been shown that such resistance claims

may not be as tight as wished due to recent advances in this field.

One of the main improvements to differential cryptanalysis is the use of many differentials to reduce the data complexity. In this paper we propose a general model for understanding multiple differential cryptanalysis and propose new attacks based on tools used in multidimensional linear cryptanalysis (namely \\LLR and $\\CHI$ statistical tests). Practical cases are considered on a reduced version of the cipher PRESENT to evaluate different approaches for selecting and combining the differentials considered. We also consider the tightness of the theoretical estimates corresponding to these attacks.

21:17 [Pub][ePrint] Quantum Key Distribution in the Classical Authenticated Key Exchange Framework, by Michele Mosca and Douglas Stebila and Berkant Ustaoglu

  Key establishment is a crucial primitive for building secure channels: in a multi-party setting, it allows two parties using only public authenticated communication to establish a secret session key which can be used to encrypt messages. But if the session key is compromised, the confidentiality of encrypted messages is typically compromised as well. Without quantum mechanics, key establishment can only be done under the assumption that some computational problem is hard. Since digital communication can be easily eavesdropped and recorded, it is important to consider the secrecy of information anticipating future algorithmic and computational discoveries which could break the secrecy of past keys, violating the secrecy of the confidential channel.

Quantum key distribution (QKD) can be used generate secret keys that are secure against any future algorithmic or computational improvements. QKD protocols still require authentication of classical communication, however, which is most easily achieved using computationally secure digital signature schemes. It is generally considered folklore that QKD when used with computationally secure authentication is still secure against an unbounded adversary, provided the adversary did not break the authentication during the run of the protocol.

We describe a security model for quantum key distribution based on traditional classical authenticated key exchange (AKE) security models. Using our model, we characterize the long-term security of the BB84 QKD protocol with computationally secure authentication against an eventually unbounded adversary. By basing our model on traditional AKE models, we can more readily compare the relative merits of various forms of QKD and existing classical AKE protocols. This comparison illustrates in which types of adversarial environments different quantum and classical key agreement protocols can be secure.

21:17 [Pub][ePrint] Achieving Constant Round Leakage-Resilient Zero-Knowledge, by Omkant Pandey

  Recently there has been a huge emphasis on constructing cryptographic protocols that maintain their security guarantees even in the presence of side channel attacks. Such attacks exploit the physical characteristics of a cryptographic device to learn useful information about the internal state of the device. Designing protocols that deliver meaningful security even in the presence of such leakage attacks is a challenging task.

The recent work of Garg, Jain, and Sahai formulates a meaningful notion of zero-knowledge in presence of leakage; and provides a construction which satisfies a weaker variant of this notion called (1+e)-leakage-resilient-zero-knowledge, for every constant e>0. In this weaker variant, roughly speaking, if the verifier learns L bits of leakage during the interaction, then the simulator is allowed to access (1+e).L bits of leakage. The round complexity of their protocol is n/e.

In this work, we present the first construction of leakage-resilient zero-knowledge satisfying the ideal requirement of e=0. While our focus is on a feasibility result for e=0, our construction also enjoys a constant number of rounds. At the heart of our construction is a new ``public-coin preamble\'\' which allows the simulator to recover arbitrary information from a (cheating) verifier in a ``straight line.\'\' We use non-black-box simulation techniques to accomplish this goal.

21:17 [Pub][ePrint] A Unified Indifferentiability Proof for Permutation- or Block Cipher-Based Hash Functions, by Anne Canteaut and Thomas Fuhr and Mar\\\'{i}a Naya-Plasencia and Pascal Paillier and Jean-Ren\\\'{e} Reinh

  In the recent years, several hash constructions have been

introduced that aim at achieving enhanced security margins by strengthening the Merkle-Damg{\\aa}rd mode. However, their security analysis have been conducted independently and using a variety of proof methodologies. This paper unifies these results by proposing a unique indifferentiability proof that considers a broadened form of the general compression function introduced by Stam at FSE09. This general definition enables us to capture in a realistic model most of the features of the mode of operation ({\\em e.g.}, message encoding, blank rounds, message insertion,...) within the pre-processing and post-processing functions. Furthermore, it relies on an

inner primitive which can be instantiated either by an ideal block cipher, or by an ideal permutation. Then, most existing hash functions can be seen as the Chop-MD construction applied to some compression function which fits the broadened Stam model. Our result then gives the tightest known indifferentiability bounds for several general modes of operations, including Chop-MD, Haifa or sponges. Moreover, we show that it applies in a quite automatic way, by providing the security bounds for 7 out of the 14 second round SHA-3 candidates, which are in some cases improved over previously known ones.

21:17 [Pub][ePrint] Zero-Knowledge Proofs with Low Amortized Communication from Lattice Assumptions, by Ivan Damgard and Adriana Lopez-Alt

  We construct zero-knowledge proofs of plaintext knowledge (PoPK) and correct multiplication (PoPC) for the Regev encryption scheme with low amortized communication complexity. Previous constructions of both PoPK and PoPC had communication cost linear in the size of the public key (roughly quadratic in the lattice dimension, ignoring logarithmic factors). Furthermore, previous constructions of PoPK suffered from one of the following weaknesses: either the message and randomness space were restricted, or there was a super-polynomial gap between the size of the message and randomness that an honest prover chose and the size of which an accepting verifier would be convinced. The latter weakness was also present in the existent PoPC protocols.

In contrast, O(n) proofs (for lattice dimension n) in our PoPK and PoPC protocols have communication cost linear in the public key. Thus, we improve the amortized communication cost of each proof by a factor linear in the security parameter. Furthermore, we allow the message space to be \\Z_p and the randomness distribution to be the discrete Gaussian, both of which are natural choices for the Regev encryption scheme. Finally, in our schemes there is no gap between the the size of the message and randomness that an honest prover chooses and the size of which an accepting verifier is convinced.

Our constructions use the ``MPC-in-the-head\'\' technique of Ishai et al. (STOC 2007). At the heart of our constructions is a protocol for proving that a value is bounded by some publicly known bound. This uses Lagrange\'s Theorem that states that any positive integer can be expressed as the sum of four squares (an idea previously used by Boudot (EUROCRYPT 2000)), as well as techniques from Cramer and Damg{\\aa}rd (CRYPTO 2009).

21:17 [Pub][ePrint] Public Auditing for Ensuring Cloud Data Storage Security With Zero Knowledge Privacy, by Wang Shao-hui, Chen Dan-wei, Wang Zhi-wei, Chang Su-qin

  In cloud storage service, clients upload their data together with authentication information to cloud storage server. To ensure the availability and integrity of clients\' stored data, cloud server(CS) must prove to a verifier that he is actually storing all of the client\'s data unchanged. And, enabling public auditability for cloud storage is of critical importance to users with constrained computing resources, who can resort to a third party auditor (TPA) to check the integrity of outsourced data. However, most of the existing proofs of retrievability schemes or proof of data possession schemes do not consider data privacy problem. Zero knowledge privacy requires TPA or the adversary can not deduce any information of the file data from auditing system. In this paper, We point out the second scheme in [16] can not provide zero knowledge privacy as they claimed. After giving a new construction of a recently proposed cryptographic primitive named aggregatable signature based broadcast ($ASBB$) encryption scheme, we present an efficient public auditing scheme with zero knowledge privacy. The new scheme is as efficient as the scheme presented by Shacham and Waters without considering privacy and is secure in the random oracle model.

21:17 [Pub][ePrint] Securing Circuits Against Constant-Rate Tampering, by Dana Dachman-Soled and Yael Tauman Kalai

  We present a compiler that converts any circuit into one that remains secure even if a constant fraction of its wires are tampered with. Following the seminal work of Ishai et al. (Eurocrypt 2006), we consider adversaries who may choose an arbitrary set of wires to corrupt, and may set each such wire to 0 or to 1, or may toggle with the wire. We prove that such adversaries, who continuously tamper with the circuit, can learn at most logarithmically many bits of secret information (in addition to black-box access to the circuit). Our results are information theoretic.

21:17 [Pub][ePrint] On Continual Leakage of Discrete Log Representations, by Shweta Agrawal and Yevgeniy Dodis and Vinod Vaikuntanathan and Daniel Wichs

  Let $\\G$ be a group of prime order $q$, and let $g_1,\\ldots,g_n$ be

random elements of $\\G$. We say that a vector $\\vecx =

(x_1,\\ldots,x_n)\\in \\Z_q^n$ is a {\\em discrete log representation} of

some some element $y\\in\\G$ (with respect to $g_1,\\ldots,g_n$) if

$g_1^{x_1}\\cdots g_n^{x_n} = y$. Any element $y$ has many discrete log

representations, forming an affine subspace of $\\Z_q^n$. We show

that these representations have a nice {\\em continuous leakage-resilience} property as follows. Assume some attacker

$\\A(g_1,\\ldots,g_n,y)$ can repeatedly learn $L$ bits of information on arbitrarily many random representations of $y$.

That is, $\\A$ adaptively chooses polynomially many leakage functions

$f_i:\\Z_q^n\\rightarrow \\{0,1\\}^L$, and learns the value

$f_i(\\vecx_i)$, where $\\vecx_i$ is a {\\em fresh and random}

discrete log representation of $y$. $\\A$ wins the game if it eventually outputs a

valid discrete log representation $\\vecx^*$ of $y$. We show that if

the discrete log assumption holds in $\\G$, then no polynomially

bounded $\\A$ can win this game with non-negligible probability, as

long as the leakage on each representation is bounded by $L\\approx (n-2)\\log q = (1-\\frac{2}{n})\\cdot |\\vecx|$.

As direct extensions of this property, we design very simple continuous leakage-resilient (CLR) one-way function (OWF) and public-key encryption (PKE) schemes in the so called ``invisible key update\'\' model introduced by Alwen et al. at CRYPTO\'09. Our CLR-OWF is based on the standard Discrete Log assumption and our CLR-PKE is based on the standard Decisional Diffie-Hellman assumption. Prior to our work, such schemes could only be constructed in groups with a bilinear pairing.

As another surprising application, we show how to design the first leakage-resilient {\\em traitor tracing} scheme, where no attacker, getting the secret keys of a small subset of decoders (called ``traitors\'\') {\\em and} bounded leakage on the secret keys of all other decoders, can create a valid decryption key which will not be traced back to at least one of the traitors.