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Random oracles are powerful cryptographic objects. They facilitate the security proofs of an impressive number of practical cryptosystems ranging from KDM-secure and deterministic encryption to point-function obfuscation and many more. However, due to an uninstantiability result of Canetti, Goldreich, and Halevi (STOC 1998) random oracles have become somewhat controversial. Recently, Bellare, Hoang, and Keelveedhi (BHK; CRYPTO 2013 and ePrint 2013/424, August 2013) introduced a new abstraction called Universal Computational Extractors (UCEs), and showed that they suffice to securely replace random oracles in a number of prominent applications, including all those mentioned above, without suffering from the aforementioned uninstantiability result. This, however, leaves open the question of constructing UCEs in the standard model.

We show that the existence of indistinguishability obfuscation (iO) implies (non-black-box) attacks on all the definitions that BHK proposed within their UCE framework in the original version of their paper, in the sense that no concrete hash function can satisfy them. We also show that this limitation can be overcome, to some extent, by restraining the class of admissible adversaries via a statistical notion of unpredictability. Following our attack, BHK (ePrint 2013/424, September 2013), independently adopted this approach in their work.

In the updated version of their paper, BHK (ePrint 2013/424, September 2013) also introduce two other novel source classes, called bounded parallel sources and split sources, which aim at recovering the computational applications of UCEs that fall outside the statistical fix. These notions keep to a computational notion of unpredictability, but impose structural restrictions on the adversary so that our original iO attack no longer applies. We extend our attack to show that indistinguishability obfuscation is sufficient to also break the UCE security of any hash function against bounded parallel sources. Towards this goal, we use the randomized encodings paradigm of Applebaum, Ishai, and Kushilevitz (STOC 2004) to parallelize the obfuscated circuit used in our attack, so that it can be computed by a bounded parallel source whose second stage consists of constant-depth circuits. We conclude by discussing the composability and feasibility of hash functions secure against split sources.

Name: Nizamuddin

Topic: On the Design of signcryption Schemes

Category: public-key cryptography

2014-02-13

2014-02-11

We are looking for an excellent PhD candidate to work in the area of information and communication security with a focus on authentication problems in constrained settings. This is particularly important for applications involving mobile phones, wireless communication and RFID systems, which suffer from restrictions in terms of power resources, network connectivity, computational capabilities, as well as potential privacy issues. The overall aim of the project will be to develop nearly optimal algorithms for achieving security and privacy while minimising resource use.

More concretely, part of the research will involve the analysis and development of authentication protocols in specific settings. This will include investigating resistance of both existing and novel protocols against different types of attacks, theoretically and experimentally. In addition to investigating established settings, such as RFID authentication, the research will also explore more general authentication problems, such as those that arise in the context of trust in social networks, smartphone applications and collaborative data processing. This will be done by grounding the work in a generalised decision-making framework. The project should result in the development of theory and authentication mechanisms for noisy, constrained settings that strike an optimal balance between reliable authentication, privacy-preservation and resource consumption. Some previous research related to this research project can be found here: http://lasecwww.epfl.ch/~katerina/Publications.html

Applicants for the position shall have a Master’s Degree or corresponding in Computer Science, Informatics, Telecommunications or in a related discipline. A master\\\'s degree in information security or cryptography is a bonus.

2014-02-10

We consider secret key agreement by multiple parties observing correlated data and communicating interactively over an insecure communication channel. Our main contribution is a single-shot upper bound on the length of the secret keys that can be generated, without making any assumptions on the distribution of the underlying data. Heuristically, we bound the secret key length in terms of ``how far\" is the joint distribution of the initial observations of the parties and the eavesdropper from a distribution that renders the observations of the parties conditionally independent across some partition, when conditioned on the eavesdropper\'s side information.

The closeness of the two distributions is measured in terms of the exponent of the probability of error of type II for a binary hypothesis testing problem, thus bringing out a structural connection between secret key agreement and binary hypothesis testing. When the underlying data consists of an independent and identically distributed sequence, an application of our bound recovers several known upper bounds for the asymptotic rate of a secret key that can be generated, without requiring the agreement error probability or the security index to vanish to 0 asymptotically.

Also, we consider the following problem of secure function computation with trusted parties: Multiple parties observing correlated data seek to compute a function of their collective data. To this end, they communicate interactively over an insecure communication channel. It is required that the value of the function be concealed from an eavesdropper with access to the communication. When is such a secure computation of a given function feasible? Using the aforementioned upper bound, we derive a necessary condition for the existence of a communication protocol that allows the parties to reliably recover the value of a given function, while keeping this value concealed from an eavesdropper with access to (only) the communication.

PRINCE is a lightweight block cipher proposed by Borghoff et al. at Asiacrypt 2012. Due to its originality, novel design and low number of rounds, it has already attracted the attention of a large number of

cryptanalysts. Several results on reduced versions have been published

to date; the best one is an attack on 8 rounds out of the total number

of 12. In this paper we improve this result by two rounds: we provide

an attack on 10 rounds of the cipher with a data complexity of $2^{57.94}$ and a time complexity of $2^{60.62}$, corresponding to 118.56 security bits, instead of 126 for the generic attacks. Our attack uses multiple differentials and exploits some properties of PRINCE for recovering the whole key. PRINCE is defined as a member of a family of ciphers, differing by the choice of an Sbox among a distinguished set. We also show that the security offered by all the members of the family is not equivalent, by identifying an Sbox for which our attack can be extended up to 11 rounds with a data complexity of $2^{59.81}$ and a time complexity of $2^{62.43}$.

Due to the recent emergence of resource-constrained devices, cryptographers are facing the problem of designing dedicated lightweight ciphers. KLEIN is one of the resulting primitives, proposed at RFIDSec in 2011 by Gong et al. This family of software-oriented block ciphers has an innovative structure, as it combines 4-bit Sboxes with the AES MixColumn transformation, and has woken up the attention of cryptanalysts. Several security analyses have been published, in particular on the 64-bit key version. The best of these results could attack up to 8 rounds out of the total number of 12. In this paper we propose a new family of attacks that can cryptanalyze for the first time all the 12 rounds of the complete version of KLEIN-64. Our attacks use truncated differential paths and are based both on some of the notions developed in previous attacks and on our new ideas that allow to considerably improve the performance. To prove the validity of our attacks, we have implemented reduced-round versions of them. In particular we were able to reproduce a practical attack that recovers the whole key on 10 rounds, which also corresponds to the best practical attack against KLEIN-64.

In this paper, we investigate matrices acting on finite commutative groups and rings; in fact, we study modules on ring of matrices over Z_N and also modules over the ring (F_2^t,\\oplus,\\land); these new algebraic constructions are a generalization of some of the constructions which were previously presented by the authors of this paper. We present new linearized and nonlinear MDS diffusion layers, based on this mathematical investigation. Then, we study some types of nonlinear number generators over Z_(2^n ) and we present a lower bound on the period of these new nonlinear number generators. As a consequence, we present nonlinear recurrent sequences over Z_(2^n ) with periods which are multiples of the period of the corresponding

sigma-LFSR\'s.

In this paper, we present a new type of PKC, system-oriented PKC,referred to as K(I)SOPKC that can be well adapted to a secure and a high speed communication between various systems and organizations of the present day network society.

K(I)SOPKC is constructed on the basis of K(XIV)SE(1)PKC, a modified version of K(XII)SE(1)PKC, K(XIII)SE(1)PKC and ${\\rm K_p(XIII)SE(1)PKC}$.

It is well known that the classical three- and four-round Feistel constructions are provably secure under chosen-plaintext and chosen-ciphertext attacks, respectively. However, irrespective of the number of rounds, no Feistel construction can resist related-key attacks where the keys can be offset by a constant. In this paper we show that, under suitable reuse of round keys, security under related-key attacks can be provably attained. Our modification is substantially simpler and more efficient than alternatives obtained using generic transforms, namely the PRG transform of Bellare and Cash (CRYPTO 2010) and its random-oracle analogue outlined by Lucks (FSE 2004).

Additionally we formalize Luck\'s transform and show that it does not always work if related keys are derived in an oracle-dependent way, and then prove it sound under appropriate restrictions.