International Association for Cryptologic Research

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2015-03-12
09:17 [Pub][ePrint]

Definitions of election verifiability in the computational model of cryptography are proposed. The definitions formalize notions of voters verifying their own votes, auditors verifying the tally of votes, and auditors verifying that only eligible voters vote. The Helios (Adida et al., 2009) and JCJ (Juels et al., 2010) election schemes are shown to satisfy these definitions. Two previous definitions (Juels et al., 2010; Cortier et al., 2014) are shown to permit election schemes vulnerable to attacks, whereas the new definitions prohibit those schemes.

2015-03-11
16:45 [Event][New]

Submission: 24 May 2015
From October 3 to October 7
Location: Jaipur, India

12:17 [Pub][ePrint]

In a secure physical computation, a set of parties each have physical inputs and jointly compute a function of their inputs in a way that reveals no information to any party except for the output of the function. Recent work in CRYPTO\'14 presented examples of physical zero-knowledge proofs of physical properties, a special case of secure physical two-party computation in which one party has a physical input and the second party verifies a boolean function of that input. While the work suggested a general framework for modeling and analyzing physical zero-knowledge protocols, it did not provide a general theory of how to prove any physical property with zero-knowledge. This paper takes an orthogonal approach using disposable circuits (DC)--cheap hardware tokens that can be completely destroyed after a computation--an extension of the familiar tamper-proof token model. In the DC model, we demonstrate that two parties can compute any function of their physical inputs in a way that leaks at most 1 bit of additional information to either party. Moreover, our result generalizes to any multi-party physical computation. Formally, our protocols achieve unconditional UC-security with input-dependent abort.

12:17 [Pub][ePrint]

We explore time-memory and other tradeoffs for memory-hard functions, which are supposed to impose significant computational and time penalties if less memory is used than intended. We analyze two schemes: Catena, which has been presented at Asiacrypt 2014, and Lyra2, the fastest finalist of the Password Hashing Competition (PHC).

We demonstrate that Catena\'s proof of tradeoff resilience is flawed, and attack it with a novel \\emph{precomputation tradeoff}. We show that using $M^{2/3}$ memory instead of $M$ we may have no time penalties. We further generalize our method for a wide class of schemes with predictable memory access.

For Lyra2, which addresses memory unpredictability (depending on the input), we develop a novel \\emph{ranking tradeoff} and show how to decrease the time-memory and the time-area product by significant factors. We also generalize the ranking method for a wide class of schemes with unpredictable memory access.

12:17 [Pub][ePrint]

In this paper we address the problem of large space consumption for protocols in the Bounded Retrieval Model (BRM), which require users to store large secret keys subject to adversarial leakage.

We propose a method to derive keys for such protocols on-the-fly from weakly random private data (like text documents or photos, users keep on their disks anyway for non-cryptographic purposes) in such a way that no extra storage is needed. We prove that any leakage-resilient protocol (belonging to a certain, arguably quite broad class) when run with a key obtained this way retains a similar level of security as the original protocol had. Additionally, we guarantee privacy of the data the actual keys are derived from. That is, an adversary can hardly gain any knowledge about the private data except that he could otherwise obtain via leakage. Our reduction works in the Random Oracle model.

As an important tool in the proof we use a newly established bound for min-entropy, which can be of independent interest. It may be viewed as an analogue of the chain rule -- a weaker form of the well-known formula $\\mathbf{H}(X \\vert Y) = \\mathbf{H}(X, Y) - \\mathbf{H}(Y)$ for random variables $X$, $Y$, and Shannon entropy, which our result originates from. For min-entropy only a much more limited version of this relation is known to hold. Namely, the min-entropy of $X$ may decrease by up to the bitlength of $Y$ when $X$ is conditioned on $Y$, in short: $\\widetilde{\\mathbf{H}(X \\vert Y) \\geq \\mathbf{H}_\\infty(X) - \\lvert Y\\rvert$.

In many cases this inequality does not offer tight bounds, and such significant entropy loss makes it inadequate for our particular application. In the quasi chain rule we propose, we inject some carefully crafted side information (spoiling knowledge) to show that with large probability the average min-entropy of $X$ conditioned on both: $Y$ and this side information can be almost lower bounded by the min-entropy of $(X, Y)$ decreased by the min-entropy of $Y$ conditioned on the side information.

2015-03-09
22:31 [Job][New]

The main objective of the research in the group Applied Cryptography & Telecom is to propose efficient and robust hardware architectures aimed at applied cryptography and telecom that are resistant to passive and active cryptographic attacks. Currently, the central theme of this research consists in designing architectures for Secure Embedded Systems implemented in logic devices such as FPGAs and ASICs. More information on http://laboratoirehubertcurien.fr/spip.php?rubrique29

The PhD thesis will take part of the European H2020 Project HECTOR (Hardware Enabled CrypTOgraphy and Randomness) aims at bridging the gap between the mathematics of cryptography with the reality of hardware implementations. HECTOR consortium: Technickon (Austria), KU Leuven (Belgium), Univ. Jean Monnet St-Etienne (France), TU Graz (Austria), THALES Comm. & Security SAS (France), STMicroelectronics Rousset SAS (France), STMicroelectronics SRL (Italia), Brighstsight (Netherlands), Micronic (Slovakia).

Objective of this thesis is to propose a set of tools that would allow simplifying physical unclonable functions (PUFs) design and security assessment.

We are looking for candidates with an outstanding Master in electrical engineering. Strong knowledge in VLSI design, FPGA & ASIC design are required. A first experience with PUF or TRNG design would be also appreciated. Notions of stochastic modelling will be a bonus. Skills in oral presentation and scientific writing in English are required (French is not required but recommended).

The PhD position will start before October 2015, it is funded for 36 months.

21:17 [Pub][ePrint]

A pseudorandom function F : K x X -> Y is said to be key homomorphic if given F(k1, x) and F(k2, x) there is an efficient algorithm to compute F(k1 xor k2, x), where xor denotes a group

operation on k1 and k2 such as xor. Key homomorphic PRFs are natural objects to study and have a number of interesting applications: they can simplify the process of rotating encryption keys for encrypted data stored in the cloud, they give one round distributed PRFs, and they can be the basis of a symmetric-key proxy re-encryption scheme. Until now all known constructions

for key homomorphic PRFs were only proven secure in the random oracle model. We construct the first provably secure key homomorphic PRFs in the standard model. Our main construction

is based on the learning with errors (LWE) problem. In the proof of security we need a variant of LWE where query points are non-uniform and we show that this variant is as hard as the standard LWE. We also construct key homomorphic PRFs based on the decision linear assumption in groups with an l-linear map. We leave as an open problem the question of constructing standard model key homomorphic PRFs from more general assumptions.

21:17 [Pub][ePrint]

A Eurocrypt 2013 paper \"Security evaluations beyond computing power: How to analyze side-channel attacks you cannot mount?\" by Veyrat-Charvillon, Gérard, and Standaert proposed a \"Rank Estimation Algorithm\" (REA) to estimate the difficulty of finding a secret key given side-channel information from independent subkeys, such as the 16 key bytes in AES-128 or the 32 key bytes in AES-256. The lower and upper bounds produced by the algorithm are far apart for most key ranks. The algorithm can produce tighter bounds but then becomes exponentially slower; it also becomes exponentially slower as the number of subkeys increases.

This paper introduces two better algorithms for the same problem. The first, the \"Extended Rank Estimation Algorithm\" (EREA), is an extension of REA using statistical sampling as a second step to increase the speed of tightening the bounds on the rank. The second, the \"Polynomial Rank Outlining Algorithm\" (PRO), is a new approach to computing the rank. PRO can handle a much larger number of subkeys efficiently, is easy to implement in a computer-algebra system such as Sage, and produces much tighter bounds than REA in less time.

21:17 [Pub][ePrint]

Known-key distinguishers for block ciphers were proposed by Knudsen and Rijmen at ASIACRYPT 2007 and have been a major research topic in cryptanalysis since then. A formalization of known-key attacks in general is known to be difficult. In this paper, we tackle this problem for the case of block ciphers based on ideal components such as random permutations and random functions as well as propose new generic known-key attacks on generalized Feistel ciphers. We introduce the notion of known-key indifferentiability to capture the security of such block ciphers under a known key. To show its meaningfulness, we prove that the known-key attacks on block ciphers with ideal primitives to date violate security under known-key indifferentiability. On the other hand, to demonstrate its constructiveness, we prove the balanced Feistel cipher with random functions and the multiple Even-Mansour cipher with random permutations known-key indifferentiable for a sufficient number of rounds. We note that known-key indifferentiability is more quickly and tightly attained by multiple Even-Mansour which puts it forward as a construction provably secure against known-key attacks.

21:17 [Pub][ePrint]

In this paper we experiment with cube testers on reduced round Trivium that can act as a distinguisher. Using heuristics, we obtain several distinguishers for Trivium running more than 800 rounds (maximum 829) with cube sizes not exceeding 27. In the process, we also exploit state biases that has not been explored before. Further, we apply our techniques to analyse Trivia-SC, a stream cipher proposed by modifying the parameters of Trivium and used as a building block for TriviA-ck (an AEAD scheme, which is submitted to the ongoing CAESAR

competition). We obtain distinguishers till 900 rounds of Trivia-SC with a cube size of 21 only and our results refute certain claims made by the designers. These are the best results reported so far, though our work does not affect the security claims for the ciphers with full initialization rounds, namely 1152.

21:17 [Pub][ePrint]

Cloud storage has rapidly become a cornerstone of many IT infrastructures, constituting a seamless solution for the backup, synchronization, and sharing of large amounts of data. Putting user data in the direct control of cloud service providers, however, raises security and privacy concerns related to the integrity of outsourced data, the accidental or intentional leakage of sensitive information, the profiling of user activities and so on. Furthermore, even if the cloud provider is trusted, users having access to outsourced files might be malicious and misbehave. These concerns are particularly serious in sensitive applications like personal health records and credit score systems.

To tackle this problem, we present GORAM, a cryptographic system that protects the secrecy and integrity of outsourced data with respect to both an untrusted server and malicious clients, guarantees the anonymity and unlinkability of accesses to such data, and allows the data owner to share outsourced data with other clients, selectively granting them read and write permissions. GORAM is the first system to achieve such a wide range of security and privacy properties for outsourced storage. In the process of designing an efficient construction, we developed two new, generally applicable cryptographic schemes, namely, batched zero-knowledge proofs of shuffle and an accountability technique based on chameleon signatures, which we consider of independent interest. We implemented GORAM in Amazon Elastic Compute Cloud (EC2) and ran a performance evaluation demonstrating the scalability and efficiency of our construction.