International Association for Cryptologic Research

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2014-01-28
19:17 [Pub][ePrint]

A lot of cryptographic protocols have been proposed for semi-honest model. In general, they are much more efficient than those proposed for the malicious model. In this paper, we propose a method that allows to detect the parties that have violated the protocol rules after the computation has ended, thus making the protocol secure against covert attacks. This approach can be useful in the settings where for any party it is fatal to be accused in violating protocol rules. In this way, up to the verification, all the computation can be performed in semi-honest model, which makes it very efficient in practice. The verification is statistical zero-knowledge, and it is based on linear probabilistically checkable proofs ($\\PCP$) for verifiable computation. Each malicious party is detected with probability $1 - \\varepsilon$ for a negligible $\\varepsilon$ that is defined by the failure of the corresponding linear $\\PCP$. The initial protocol has to be executed only once, and the verification requires in total $3$ additional rounds (if some parties act dishonestly, in the worst case they may force the protocol to substitute each round with $4$ rounds, due to the transmission functionality that prevents the protocol from stopping). The verification also ensures that all the parties have sampled all the randomness from an appropriate distribution. Its efficiency does not depend on whether the inputs of the parties have been shared, or each party uses its own private input.

The major drawback of the proposed scheme is that the number of values sent before and after the protocol is exponential in the number of parties. Nevertheless, the settings make the verification very efficient for a small number of parties.

19:17 [Pub][ePrint]

Identity-based encryption (IBE) is a special case of public-key encryption where user identities replace public keys. Every user is given a corresponding secret key for decryp- tion, and encryptions for his or her identity must remain confidential even to attackers who learn the secret keys associated with other identities. Several IBE constructions are known to date, but their security relies on specific assumptions, such as quadratic residuosity, as well as different pairing-based and lattice-based assumptions.

To circumvent the lack of generic constructions, Dodis et al. (EUROCRYPT \'02) introduced the notion of bounded-collusion IBE (BC-IBE), where attackers only learn secret keys of an a-priori bounded number t of identities. They provided a generic BC-IBE construction from any semantically-secure encryption scheme which, however, suffers from a ω(t) blow-up in ciphertext size. Goldwasser et al. (TCC 2012) recently presented a generic construction with no ciphertext-length blow-up. Their construction requires an underlying public-key scheme with a key homomorphism, as well as a hash-proof-style security definition that is strictly stronger than semantic security. This latter requirement in particular reduces the applicability of their construction to existing schemes.

In this paper, we present the first generic constructions of BC-IBE from semantically-secure encryption schemes with no ciphertext-length blow-up. Our constructions require different degrees of key-homomorphism and malleability properties that are usually easy to verify. We provide concrete instantiations based on the DDH, QR, NTRU, and LWE assumptions. For all of these assumptions, our schemes present the smallest BC-IBE ciphertext size known to date. Our NTRU-based construction is particularly interesting, due to the lack of NTRU- based IBE constructions as well as the fact that it supports fully-homomorphic evaluation. Our results also yield new constructions of bounded CCA-secure cryptosystems.

19:17 [Pub][ePrint]

We conduct a theoretical and practical comparison of two Ring-LWE-based, scale-invariant, leveled homomorphic encryption schemes -- Fan and Vercauteren\'s adaptation of BGV and the YASHE scheme proposed by Bos, Lauter, Loftus and Naehrig. In particular, we explain how to choose parameters to ensure correctness and security against lattice attacks. Our parameter selection improves the approach of van de Pol and Smart to choose parameters for schemes based on the Ring-LWE problem by using the BKZ-2.0 simulation algorithm.

We implemented both schemes in C++, using the arithmetic library FLINT, and compared them in practice to assess their respective strengths and weaknesses. In particular, we performed a homomorphic evaluation of the lightweight block cipher SIMON. Combining block ciphers with homomorphic encryption allows to solve the gargantuan ciphertext expansion in cloud applications.

19:17 [Pub][ePrint]

Recently, Baseri et al. proposed a secure untraceable off-line electronic cash system. They claimed that their scheme could achieve security requirements of an e-cash system such as, untraceability, anonymity, unlinkability, double spending checking, un-forgeability, date-attachability, and prevent forging coins. They further prove the un-forgeability security feature by using the hardness of discrete logarithm problems. However, after cryptanalysis, we found that the scheme cannot attain the security feature, untraceability. We, therefore, modify it to comprise this desired requirement, which is very important in an e-cash system.

19:17 [Pub][ePrint]

We give a polynomial time attack on the McEliece public key cryptosystem based on algebraic geometry codes. Roughly speaking, this attacks runs in $O(n^4)$ operations in $\\mathbb F_q$, where $n$ denotes the code length. Compared to previous attacks, allows to recover a decoding algorithm for the public key even for codes from high genus curves.

13:17 [Pub][ePrint]

we propose an elegant memory-hard proof-of-work system based on cuckoo hashing

04:48 [PhD][New]

Name: Claude Carlet

04:47 [PhD][New]

Name: Abdelaziz Elaabid
Topic: Side channel attacks: advanced experimentations on template attacks
Category: secret-key cryptography

Description: In the 90\'s, the emergence of new cryptanalysis methods revolutionized the security of cryptographic devices. These attacks are based on power consumption analysis, when the microprocessor is running the cryptographic algorithm. Especially, we analyse in this thesis some properties of the template attack, with examples from attacks against an unprotected ASIC implementation. We point out that the efficiency of template attacks can be unleashed by using a relevent power model, and we provide some practical improvements by the use of different signal processing techniques. Furthermore, we investigate the relevance of the theoretical framework on profiled SCAs presented by F.-X. Standaert et al. at Eurocrypt 2009. The analyse consists in a case-study based on side-channel measurements acquired experimentally from a hardwired cryptographic accelerator. Therefore, with respect to previous formal analyses carried out on software measurements or on simulated data, the investigations we describe are more complex, due to the underlying chip\'s architecture and to the large amount of algorithmic noise.In this context, we explore the appropriateness of different choices for the sensitive variables, and we show that a skilled attacker aware of the register transfers occurring during the cryptographic operations can select the most adequate distinguisher, thus increasing its success rate. The principal component analysis (PCA) is used to represent the templates in some dimensions, and we give a physical interpretation of the templates eigenvalues and eigenvectors. We introduce a method based on the thresholding of leakage data to accelerate the profiling or the matching stages. This method empowers an attacker, in that it saves traces when converging towards correct values of the secret. Concretely, we demonstrate a 5 times speed-up in the on-line phase of the attack. Also, it has been underlined that the various samples garnered during the same acquisition can carry complement[...]

04:47 [PhD][Update]

2014-01-27
14:48 [Job][New]

Department of Applied Mathematics and Computer Science, Technical University of Denmark, would like to invite applications for a Postdoc position of 18 months, starting 1 April 2014 or soon thereafter. The topic of the project is lightweight cryptology, which regards scenarios involving strongly resource-constrained devices.

Candidates for the position should have a solid background in hardware design and automation and be able to work on the physical constraints and optimization of the hardware implementations or, alternatively, we will consider candidates with a strong cryptanalytic and mathematical background who are able to analyse the security of ciphers to be designed.

14:44 [Job][New]

The Chair for Information Security and Cryptography at the University of Trier, Germany, offers

a full-time position for a postdoctoral researcher

in a project funded by the German Research Foundation (DFG). The goal of the project is to develop methods for the modular analysis of real-world cryptographic protocols, such as TLS, SSH, WPA2, etc., based on the approach of universal composability, and to apply the developed methods to such protocols.

The position is available immediately, with an internationally competitive salary. The starting date is negotiable. Contracts can initially be offered for up to three years, with the perspective of an extension.

There are no teaching obligations.

The successful candidate must have a Master`s degree (or an equivalent degree) in Computer Science, Mathematics, or a related discipline, and have completed, or be near completion of a PhD degree relevant to the research area of the project. You should have a proven high level of analytical capability and mathematical skills. Good English skills are expected; knowledge of German is not required.

Applications will be considered until the position is filled.