International Association for Cryptologic Research

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2013-12-02
13:28 [Job][New]

The Computer Science Department at University College London has an open postdoctoral research position under the supervision of Jens Groth. The Research Associate is funded by an ERC Starting Grant on Efficient Cryptographic Arguments and Proofs with a flexible starting date and a duration of up to 2 years.

Candidates must have a PhD with a strong publication record in cryptography or theoretical computer science. Research experience in zero-knowledge proofs, probabilistically checkable proofs or lattice-based cryptography will be considered a plus.

University College London is one of Europe\\\'s highest ranked universities and has recently been recognized by the EPSRC and GCHQ as one of UK\\\'s Academic Centres of Excellence in Cyber Security Research. The Computer Science Department is one of the largest in the UK and is located at UCL\\\'s main campus in the centre of London.

2013-12-01
19:17 [Pub][ePrint]

Since the introduction of side channel attacks in the nineties, a large amount of work has been devoted to their effectiveness and efficiency improvements. On the one side, general results and conclusions are drawn in theoretical frameworks, but the latter ones are often set in a too ideal context to capture the full complexity of an attack performed in real conditions. On the other side, practical improvements are proposed for specific contexts but the big picture is often put aside, which makes them difficult to adapt to different contexts. This paper tries to bridge the gap between both worlds. We specifically investigate which kind of issues is faced by a security evaluator when performing a state of the art attack. This analysis leads us to focus on the very common situation where the exact time of the sensitive processing is drown in a large number of leakage points. In this context we propose new ideas to improve the effectiveness and/or efficiency of the three considered attacks. In the particular case of stochastic attacks, we show that the existing literature, essentially developed under the assumption that the exact sensitive time is known, cannot be directly applied when the latter assumption is relaxed. To deal with this issue, we propose an improvement which makes stochastic attack a real alternative to the classical correlation power analysis. Our study is illustrated by various attack experiments performed on several copies of three micro-controllers with different CMOS technologies (respectively 350, 130 and 90 nanometers).

19:17 [Pub][ePrint]

The Internet of Things (IoT) will be formed by smart objects and services interacting autonomously and in real-time. Recently, Alcaide et al. proposed a fully decentralized anonymous authentication protocol for privacy-preserving IoT target-driven applications. Their system is set up by an ad-hoc community of decentralized founding nodes. Nodes can interact, being participants of cyberphysical systems, preserving full anonymity. In this study, we point out that their protocol is insecure. The adversary can cheat the data collectors by impersonating a legitimate user.

19:17 [Pub][ePrint]

Proofs of work (PoW) have been suggested by Dwork and Naor (Crypto\'92) as protection to a shared resource. The basic idea is to ask the service requestor to dedicate some non-trivial amount of computational work to every request. The original applications included prevention of spam and protection against denial of service attacks. More recently, PoWs have been used to prevent double spending in the Bitcoin digital currency system.

In this work, we put forward an alternative concept for PoWs -- so-called proofs of space (PoS), where a service requestor must dedicate a significant amount of disk space as opposed to computation. We construct secure PoS schemes in the random oracle model, using graphs with high \"pebbling complexity\" and Merkle hash-trees.

19:17 [Pub][ePrint]

We initiate the investigation of {\\em gate}-tampering attacks against

cryptographic circuits. Our model is motivated by the plausibility of

tampering directly with circuit gates and by the increasing use of {\\em tamper

resilient gates} among the known constructions that are shown to be resilient

against {\\em wire-tampering} adversaries. We prove that gate-tampering is {\\em

strictly} stronger than wire-tampering. On the one hand, we show that there is

a gate-tampering strategy that perfectly simulates any given wire-tampering

strategy. On the other, we construct families of circuits over which it is

impossible for any wire-tampering attacker to simulate a certain gate-tampering

attack (that we explicitly construct). We also provide a tamper resilience

impossibility result that applies to both gate and wire tampering adversaries

and relates the amount of tampering to the depth of the circuit. Finally, we

show that defending against gate-tampering attacks is feasible by appropriately

abstracting and analyzing the circuit compiler of Ishai et al.

\\cite{Ishai:2006a} in a manner which may be of independent interest.

Specifically, we first introduce a class of compilers that, assuming certain

well defined tamper resilience characteristics against a specific class of

attackers, can be shown to produce tamper resilient circuits against that

same class of attackers. Then, we describe a compiler in this class for which

we prove that it possesses the necessary tamper-resilience characteristics

against gate-tampering attackers.

19:17 [Pub][ePrint]

We present a new generic construction of public key encryption (PKE) scheme that is secure against a-posteriori chosen-ciphertext λ-key-leakage attacks (LR-CCA-2 secure) from any universal hash proof system (HPS). Our construction relies only on the existence of universal hash proof systems, which makes our scheme simple, clean and efficient. Furthermore, our construction is a potential way to construct LR-CCA-2 secure PKE scheme from minimal assumption.

19:17 [Pub][ePrint]

This paper investigates the mathematical structure of the Isomorphism of Polynomial with One Secret\'\' problem (IP1S). Our purpose is to understand why for practical parameter values of IP1S most random instances are easily solvable (as first observed by Bouillaguet et al.).

We show that the structure of the problem is directly linked to the

structure of quadratic forms in odd and even characteristic. We describe a completely new method allowing to efficiently solve most instances. Unlike previous solving techniques, this is not based upon Gröbner basis computations.

19:17 [Pub][ePrint]

Cube attacks can be used to analyse and break cryptographic primitives that have an easy algebraic description. One example for such a primitive is the stream cipher /Trivium.

In this article we give a new framework for cubes that are useful in the cryptanalytic context. In addition, we show how algebraic modelling of a cipher can greatly be improved when taking both cubes and linear equivalences between variables into account. When taking many instances of Trivium, we empirically show a saturation effect, i.e., the number of variables to model an attack will become constant for a given number of rounds. Moreover, we show how to systematically find cubes both for general primitives and also specifically for Trivium. For the latter, we have found all cubes up to round 446 and draw some conclusions on their evolution between rounds. All techniques in this article are general and can be applied to any cipher.

19:17 [Pub][ePrint]

At Crypto 2013 Libert, Peters, Joye and Yung introduced the notion of Linearly Homomorphic Structure Preserving Signatures (LHSPS) as a tool to perform verifiable computation on encrypted data and to create constant-size non malleable commitments to group elements. In this paper we improve our understanding of LHSPS by putting forward new methodologies and applications. First, we present a generic transform that converts LHSPS which are secure against weak random message attack (RMA) into ones that achieve full security guarantees. Next we give evidence that RMA secure linearly homomorphic structure preserving signatures are interesting in their own right by showing applications in the context of on-line/off-line homomorphic and network coding signatures. This notably provides what seems to be the first instantiations of homomorphic signatures achieving on-line/off-line efficiency trade-offs.

19:17 [Pub][ePrint]

Yoneyama et al. introduced Leaky Random Oracle Model (LROM for short) at ProvSec2008 in order to discuss security (or insecurity) of cryptographic schemes which use hash functions as building blocks when leakages from pairs of input and output of hash functions occur in the experiment of security definition. This kind of leakages occurs due to various attacks caused by sloppy usages or implementations. Their results showed that this kind of leakages may threaten the security of some cryptographic scheme. However, an important fact is that such attacks would leak not only pairs of input and output of hash functions, but also the secret key. Therefore, LROM is very limited in the sense that it considers leakages from pairs of input and output of hash functions alone, instead of taking into consideration other possible leakages from the secret key simultaneously. On the other hand, many recent leakage models mainly concentrate on leakages from the secret key and ignore leakages from hash functions for a cryptographic scheme exploiting hash functions. This is a weakness of these leakage models and there exist some schemes in these leakage models but is not secure any more when leakages from hash functions occur.

In this paper, we present an augmented model of both LROM and some leakage models. In our new model, both the secret key and pairs of input and output of hash functions can be leaked. Furthermore, the secret key can be leaked continually during the whole lifecycle of a cryptographic scheme. Hence, our new model is more universal and stronger than LROM and some leakage models (e.g. only computation leaks model and bounded memory leakage model). As an application example, we also present a public key encryption scheme which is provably IND-CCA secure in our new model.

19:17 [Pub][ePrint]

We present the first fully secure Identity-Based Encryption scheme

(IBE) from the standard assumptions where the security loss depends

only on the security parameter and is independent of the number of

secret key queries. This partially answers an open problem posed by

Waters (Eurocrypt 2005). Our construction combines Waters\' dual

system encryption methodology (Crypto 2009) with the Naor-Reingold

pseudo-random function (J. ACM, 2004) in a novel way. The security

of our scheme relies on the DLIN assumption in prime-order groups.