International Association for Cryptologic Research

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2015-07-20
09:17 [Pub][ePrint]

Cryptographic accumulators are a tool for compact set representation and secure set membership proofs. When an element is added to a set by means of an accumulator, a membership witness is generated. This witness can later be used to prove the membership of the element. Typically, the membership witness has to be synchronized with the accumulator value, and to be updated every time another element is added to the accumulator. In this work we propose an accumulator that, unlike any prior scheme, does not require strict synchronization.

In our construction a membership witness needs to be updated only a logarithmic number times in the number of subsequent element additions. Thus, an out-of-date witness can be easily made current. Vice versa, a verifier with an out-of-date accumulator value can still verify a current membership witness. These properties make our accumulator construction uniquely suited for use in distributed applications, such as blockchain-based public key infrastructures.

09:17 [Pub][ePrint]

In this paper we investigate relations between several masking schemes. We show that the Ishai--Sahai--Wagner private circuits construction is closely related to Threshold Implementations and the Trichina gate. The implications of this observation are manifold. We point out a higher-order weakness in higher-order Threshold Implementations, suggest a mitigation and provide new sharings that use a lower number of input shares.

09:17 [Pub][ePrint]

We consider randomized encodings (RE) that enable encoding a Turing machine Pi and input x into its randomized encoding\'\' \\hat{Pi}(x) in sublinear, or even polylogarithmic, time in the running-time of Pi(x), independent of its output length. We refer to the former as sublinear RE and the latter as compact RE. For such efficient RE, the standard simulation-based notion of security is impossible, and we thus consider a weaker (distributional) indistinguishability-based notion of security: Roughly speaking, we require indistinguishability of \\hat{Pi}_0(x_0) and \\hat{Pi}_0(x_1) as long as Pi_0,x_0 and Pi_1,x_1 are sampled from some distributions such that Pi_0(x_0),Time(Pi_0(x_0)) and Pi_1(x_1),Time(Pi_1(x_1)) are indistinguishable.

We first observe that compact RE is equivalent to a variant of the notion of indistinguishability obfuscation (iO)---which we refer to as puncturable iO---for the class of Turing machines without inputs. For the case of circuits, puncturable iO and iO are equivalent (and this fact is implicitly used in the powerful punctured program\'\' paradigm by Sahai and Waters [SW13]).

We next show the following:

- Impossibility in the Plain Model: Assuming the existence of subexponentially secure one-way functions, subexponentially-secure sublinear RE does not exists. (If additionally assuming subexponentially-secure iO for circuits we can also rule out polynomially-secure sublinear RE.) As a consequence, we rule out also puncturable iO for Turing machines (even those without inputs).

- Feasibility in the CRS model and Applications to iO for circuits: Subexponentially-secure sublinear RE in the CRS model and one-way functions imply iO for circuits through a simple construction generalizing GGM\'s PRF construction. Additionally, any compact (even with sublinear compactness) functional encryption essentially directly yields a sublinear RE in the CRS model, and as such we get an alternative, modular, and simpler proof of the results of [AJ15,BV15] showing that subexponentially-secure sublinearly compact FE implies iO.

- Applications to iO for Unbounded-input Turing machines: Subexponentially-secure compact RE for natural restricted classes of distributions over programs and inputs (which are not ruled out by our impossibility result, and for which we can give candidate constructions) imply iO for unbounded-input Turing machines. This yields the first construction of iO for unbounded-input Turing machines that does not rely on (public-coin) differing-input obfuscation.

2015-07-18
15:17 [Pub][ePrint]

Simeck is a new lightweight block cipher design based on

combining the Simon and Speck block cipher. While the design allows a

smaller and more efficient hardware implementation, its security margins are not well understood. The lack of design rationals of its predecessors further leaves some uncertainty on the security of Simeck.

In this work we give a short analysis of the impact of the design changes by comparing the lower bounds for differential and linear characteristics with Simon. We also give a comparison of the effort of finding those bounds, which surprisingly is significant less for Simeck while covering a larger number of rounds.

Furthermore, we provide new differentials for Simeck which can cover

more rounds compared to previous results on Simon. Based on this we

mount key recovery attacks on 19/26/33 rounds of Simeck32/48/64,

which also give insights on the reduced key guessing effort due to the

different set of rotation constants.

15:17 [Pub][ePrint]

In an implicit authentication system, a user profile is used as an additional factor to strengthen the authentication of mobile users. The profile consists of features that are constructed using the history of user actions on her mobile device over time. The profile is stored on the server and is used to authenticate an access request originated from the device at a later time. An access request will include a vector of recent measurements of the features on the device, that will be subsequently matched against the features stored at the server, to accept or reject the request. The features however include private information such as user location or web sites that have been visited. We propose a privacy-preserving implicit authentication system that achieves implicit authentication without revealing information about the usage profiles of the users to the server. We propose an architecture, give a formal security model and a construction with provable security in two settings where: (i) the device follows the protocol, and (ii) the device is captured and behaves maliciously.

15:17 [Pub][ePrint]

We describe a methods for generating parameter sets and calculating security estimates for NTRUEncrypt. Analyses are provided for the standardized product-form parameter sets from IEEE 1363.1-2008 and for the NTRU Challenge parameter sets.

15:17 [Pub][ePrint]

Mobile application spoofing is an attack where a malicious mobile application

mimics the visual appearance of another one. If such an attack is successful,

the integrity of what the user sees as well as the confidentiality of what she

inputs into the system can be violated by the adversary. A common example of

mobile application spoofing is a phishing attack where the adversary tricks the

user into revealing her password to a malicious application that resembles the

legitimate one.

In this work, we propose a novel approach for addressing mobile application

spoofing attacks by leveraging the visual similarity of application screens. We

use deception rate as a novel metric for measuring how many users would confuse

a spoofing application for the genuine one. We conducted a large-scale online

study where participants evaluated spoofing samples of popular mobile

applications. We used the study results to design and implement a prototype

spoofing detection system, tailored to the estimation of deception rate for

15:17 [Pub][ePrint]

Storage requirements for visual data have been increasing in recent years, following the emergence of many new highly interactive multimedia services and applications for both personal and corporate use. This has been a key driving factor for the adoption of cloud-based data outsourcing solutions. However, outsourcing data storage to the Cloud also leads to new challenges that must be carefully addressed, especially regarding privacy. In this paper we propose a secure framework for outsourced privacy-preserving storage and retrieval in large image repositories. Our proposal is based on IES-CBIR, a novel Image Encryption Scheme that displays Content-Based Image Retrieval properties. Our solution enables both encrypted storage and searching using CBIR queries while preserving privacy. We have built a prototype of the proposed framework, formally analyzed and proven its security properties, and experimentally evaluated its performance and precision. Our results show that IES-CBIR is provably secure, allows more efficient operations than existing proposals, both in terms of time and space complexity, and enables more reliable practical application scenarios.

15:17 [Pub][ePrint]

The aim of this work is to find large S-Boxes, typically operating on 8

bits, having both good cryptographic properties and a low implementation

cost. Such S-Boxes are suitable building-blocks in many lightweight

block ciphers since they may achieve a better security level than

designs based directly on smaller S-Boxes. We focus on S-Boxes

corresponding to three rounds of a balanced Feistel and of a balanced

MISTY structure, and generalize the recent results by Li and Wang on the

best differential uniformity and linearity offered by such a

construction. Most notably, we prove that Feistel networks supersede

MISTY networks for the construction of 8-bit permutations. Based on

these results, we also provide a particular instantiation of an 8-bit

permutation with better properties than the S-Boxes used in several

ciphers, including Robin, Fantomas or CRYPTON.

15:17 [Pub][ePrint]

It has long been known (Shoup and Gennaro 1998) that non-interactive proofs in the Random Oracle model that rely on rewinding extractors can be problematic.

Recent results by Seurin and Treger and Bernhard et al. formally confirmed such limitations for proofs derived from the Schnorr protocol via the Fiat-Shamir transform.

The limitations relate to the concept of adaptive proofs where an extractor needs to recover witnesses from proofs selected adaptively, as opposed to the standard setting where the extractor needs to work just for one proof.

Their main result is a separation between these two settings: under the one-more discrete log assumption, no efficient adaptive extractor can recover all witnesses from non-interactive Schnorr proofs (selected adaptively).

In this paper we generalize, strengthen and extend these results.

First we show that the above separation result holds for generic Sigma-protocols under the natural generalization of the one-more dlog assumption.

Next, we strengthen the theorem by weakening the hypothesis.

Our new assumption, which we call Sigma-one-wayness, says that a dishonest verifier in a single execution of an interactive Sigma protocol cannot recover the witness.

This assumption is incomparable to zero-knowledge, as we will explain.

The main result of this paper clarifies the relation between adaptive proofs of knowledge (with rewinding) and other existing notions.

Bernhard et al. introduced adaptive proofs as a new concept lying

between proofs of knowledge (PoKs, with a rewinding extractor) and

straight-line extractable proofs. They showed a separation between PoKs and

adaptive proofs but left open the question whether adaptive proofs are always

straight-line.

against the honest prover. This means that adaptive proofs are not a new class

of proofs after all but simply another way to describe proofs with

straight-line extractors.

Finally, we ask ourselves whether the result could be extended to a

reduction to one-wayness of the function concerned -- for Schnorr, this would

mean solving the discrete logarithm (DLOG) problem. Our answer is negative: if

there is any generic metareduction from adaptivity of Fiat-Shamir-Schnorr to

DLOG then there is also a meta-metareduction breaking DLOG directly.

15:17 [Pub][ePrint]

This paper offers a new version of the hHB protocol denoted Light-hHB. This proposal uses the same framework as hHB, that is a two stages protocol: the first one for the establishment of a session key between the reader and the tag and the second one similar to HB+. We also introduce in this paper a novel and lightweight key exchange protocol inspired by the BB84 protocol named the non-quantum key exchange protocol. With the use of a practical implementation of the latter protocol in the first stage of Light-hHB, the transmission cost is drastically reduced compared to the one of hHB, which is its main drawback. In the context of RFID tags, Light-hHB is significantly more practical than hHB and achieves the same security goals.