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2014-10-13
12:17 [Pub][ePrint] Tweaks and Keys for Block Ciphers: the TWEAKEY Framework, by Jérémy Jean and Ivica Nikolić and Thomas Peyrin

  We propose the TWEAKEY framework with goal to unify the design of tweakable block ciphers and of block ciphers resistant to related-key attacks. Our framework is simple, extends the key-alternating construction, and allows to build a primitive with arbitrary tweak and key sizes, given the public round permutation (for instance, the AES round). Increasing the sizes renders the security analysis very difficult and thus we identify a subclass of TWEAKEY, that we name STK, which solves the size issue by the use of finite field

multiplications on low hamming weight constants. We give very efficient instances of STK, in particular, a 128-bit tweak/key/state block cipher Deoxys-BC that is the first AES-based ad-hoc tweakable block cipher. At the same time, Deoxys-BC could be seen as a secure alternative to AES-256, which is known to be insecure in the related-key model. As another member of the TWEAKEY framework, we describe Kiasu-BC, which is a very simple and even more efficient tweakable variation of AES-128 when the tweak size is limited to 64 bits.

In addition to being efficient, our proposals, compared to the previous schemes that use AES as a black box, offer security beyond the birthday bound. Deoxys-BC and Kiasu-BC represent interesting pluggable primitives for authenticated encryption schemes, for instance, OCB instantiated with Kiasu-BC runs at about 0.75 c/B on Intel Haswell. Our work can also be seen as

advances on the topic of secure key schedule design for AES-like ciphers, describing several proposals in this direction.



09:38 [Job][Update] Tenure-Track Faculty Positions, Shanghai Jiao Tong University, Shanghai, China

  The Laboratory of Cryptology and Computer Security (LoCCS) at the CS Department of Shanghai Jiao Tong University invites applications for several tenure-track faculty positions in the area of cryptology, in particular (but not limited to), authenticated encryptions, leakage-resilient cryptography, side-channel attacks, obfuscation. Candidates are expected to have the following: (1) a PhD in a relevant area; (2) a proven track record (especially publications at top venues); (3) preferably a postdoctoral training for two years or more. Salaries will be globally competitive and commensurate with candidates\\\' accomplishments and experience. Shanghai Jiao Tong University is a member of China\\\'s C9 League and she has one of the country\\\'s best CS schools.

09:34 [Job][New] Post Doc, Université de Caen Basse-Normandie

  Applications are invited a post-doctoral position in pairing-based cryptography at Caen University. The successful applicant will participate in the project SIMPATIC (SIM and PAiring Theory for Information and Communications security) financed by the French governemental research funding agency ANR (Agence Nationale de la Recherche) and organized by Orange Labs, Caen. He/she will be a member of one of the research teams in the Computer Science (GREYC) or Mathematics (LMNO) laboratories at Caen University, France.

The position is open for one year. The starting date can be arranged as convenient, but in any case not later than 1st July 2015.

The partners involved in the SIMPATIC project are the crypto teams of the Laboratoire d\'Informatique de l\'ENS Paris, IMB (Bordeaux), University Paris 8, University Rennes 1, Oberthur, INVIA, STmicroelectronics (Le Mans) and Orange Labs (Caen).

The successful applicant will work on one of the following priorities of the project:

(i) The conception of cryptographic primitives suitable for SIMs and other small supports. Candidates are expected to have a high quality potential in theoretical cryptography. He/she will be expected to interact with members of the Applied Crypto Group (ACG) at Orange Labs (OL) in Caen.

(ii) The study of suitable pairing-friendly curves, both theoretical and algorithmic aspects. Candidates should therefore have a very strong background in relevant number theory and algebraic geometry. Some experience in software implementation (for example in Pari, Magma, Sage, ...) would be useful.

Preference will be given to candidates working on priority (i), but all applications related to the project themes will be examined.

Candidates must hold a PhD thesis or equivalent in mathematics or computer science, together with a strong research record.





2014-10-12
15:17 [Forum] [2013 Reports] 2013/235 by Ben.Smyth

  Galindo & Cortier (private communication) have shown that our original presentation of ballot secrecy (ESORICS\'13) is incompatible with verifiability and revision https://eprint.iacr.org/2013/235/20141010:082554 overcomes this limitation. From: 2014-12-10 13:35:30 (UTC)

03:17 [Pub][ePrint] Riding on Asymmetry: Efficient ABE for Branching Programs, by Sergey Gorbunov and Dhinakaran Vinayagamurthy

  In an Attribute-Based Encryption (ABE) a ciphertext, encrypting message $\\mu$, is associated with a public attribute vector $\\vecx$ and a secret key $\\sk_P$ is associated with a predicate $P$. The decryption returns $\\mu$ if and only if $P(\\vecx) = 1$. ABE provides efficient and simple mechanism for data sharing supporting fine-grained access control. Moreover, it is used as a critical component in constructions of succinct functional encryption, reusable garbled circuits, token-based obfuscation and more.

In this work, we describe a new efficient ABE scheme for a family of branching programs with short secret keys over a small ring. In particular, in our constriction the size of the secret key for a branching program $P$ is $|P| + \\poly(\\secp)$, where $\\secp$ is the security parameter. Our construction is secure assuming $n^{\\omega(1)}$-hardness of standard Learning With Errors (LWE) problem, resulting in small ring modulo. Previous constructions relied on $n^{O(\\log n)}$-hardness of LWE (resulting in large ring modulo) or had large secret keys of size $|P| \\times \\poly(\\secp)$. We rely on techniques developed by Boneh et al. (EUROCRYPT\'14) and Brakerski et al. (ITCS\'14) in the context of ABE for circuits and fully-homomorphic encryption.



03:17 [Pub][ePrint] Operational Signature Schemes, by Michael Backes and Ozgur Dagdelen and Marc Fischlin and Sebastian Gajek and Sebastian Meiser and Dominique Schroeder

  Functional encryption, as introduced by Boneh, Sahai, and Waters (TCC\'11),

generalizes public-key encryption systems to include functional decryption

capabilities. Recently, Boyle et al. as well as Bellare and

Fuchsbauer (both PKC\'14) formalized analogous notions for signature schemes. Here

we discuss that both their notions are limited in terms of expressiveness in the

sense that they cannot cast known signature schemes supporting operations on

data in their frameworks. We therefore propose a notion of what we call, for

sake of distinctiveness, operational signature schemes which captures

functional signatures, policy-based signatures, sanitizable signatures, P-homomorphic signatures, ring

signatures, aggregate signatures etc., and also their message authentication code counterparts.

We discuss possible instantiations for operational signatures.

We give some positive result about achieving our general notion of operational signatures presenting a

compact construction that relies on a new combination of indistinguishability

obfuscation and random oracles. We then indicate that it is unlikely to be able to instantiate

operational signature schemes in general using one-wayness and, under some

circumstances, even using specific ``non-interactive\'\' assumptions like RSA.



03:17 [Pub][ePrint] Non-malleable Reductions and Applications, by Divesh Aggarwal and Yevgeniy Dodis and Tomasz Kazana and Maciej Obremski

  Non-malleable codes, introduced by Dziembowski, Pietrzak and Wichs~\\cite{DPW10}, provide a useful message integrity guarantee in situations where traditional error-correction (and even error-detection) is impossible; for example, when the attacker can completely overwrite the encoded message. Informally, a code is non-malleable if the message contained in a modified codeword is either the original message, or a completely ``unrelated value\'\'. Although such codes do not exist if the family of ``tampering functions\'\' $\\cF$ allowed to modify the original codeword is completely unrestricted, they are known to exist for many broad tampering families $\\cF$.

The family which received the most attention~\\cite{DPW10,LL12,DKO13,ADL14,CG14a,CG14b} is the family of tampering functions in the so called {\\em split-state} model: here the message $x$ is encoded into two shares $L$ and $R$,

and the attacker is allowed to {\\em arbitrarily} tamper with each $L$ and $R$ {\\em individually}.

Despite this attention, the following problem remained open:

\\begin{center}

{\\em Build efficient, information-theoretically secure non-malleable codes in the split-state model with constant encoding rate}: $|L|=|R|=O(|x|)$.

\\end{center}

In this work, we resolve this open problem. Our technique for getting our main result is of independent interest. We

\\begin{itemize}

\\item[(a)] develop a generalization of non-malleable codes, called {\\em non-malleable reductions};

\\item[(b)] show simple composition theorem for non-malleable reductions;

\\item[(c)] build a variety of such reductions connecting various (independently interesting) tampering families $\\cF$ to each other; and

\\item[(d)] construct our final, constant-rate, non-malleable code in the split-state model by applying the composition theorem to a series of easy to understand reductions.

\\end{itemize}



03:17 [Pub][ePrint] Ballot secrecy with malicious bulletin boards, by Ben Smyth

  This letter proposes a formal definition of ballot secrecy in the computational model of cryptography. The definition builds upon and strengthens earlier definitions by Bernhard et al. (ASIACRYPT\'12, ESORICS\'11 & ESORICS\'13). The new definition is intended to ensure that ballot secrecy is preserved in the presence of malicious bulletin boards, whereas earlier definitions by Bernhard et al. only consider honest bulletin boards.



03:17 [Pub][ePrint] On the Oblivious Transfer Capacity of Generalized Erasure Channels against Malicious Adversaries, by Rafael Dowsley and Anderson C. A. Nascimento

  Noisy channels are a powerful resource for cryptography as they can be used to obtain

information-theoretically secure key agreement, commitment and oblivious transfer protocols, among others. Oblivious transfer (OT) is a fundamental primitive since it is complete for secure multi-party computation, and the OT capacity characterizes how efficiently a channel can be used for obtaining string oblivious transfer. Ahlswede and Csisz\\\'{a}r (\\emph{ISIT\'07}) presented upper and lower bounds on the OT capacity of generalized erasure channels (GEC) against passive adversaries. In the case of GEC with erasure probability at least 1/2, the upper and lower bounds match and therefore the OT capacity was determined. It was later proved by Pinto et al. (\\emph{IEEE Trans. Inf. Theory 57(8)}) that in this case there is also a protocol against malicious adversaries achieving the same lower bound, and hence the OT capacity is identical for passive and malicious adversaries. In the case of GEC with erasure probability smaller than 1/2, the known lower bound against passive adversaries that was established by Ahlswede and Csisz\\\'{a}r does not

match their upper bound and it was unknown whether this OT rate could be achieved against malicious adversaries as well. In this work we show that there is a protocol against malicious adversaries achieving the same OT rate that was obtained against passive adversaries.

In order to obtain our results we introduce a novel use of interactive hashing that is suitable for dealing with the case of low erasure probability ($p^*

03:17 [Pub][ePrint] Accountable Tracing Signatures, by Markulf Kohlweiss and Ian Miers

  Demands for lawful access to encrypted data are a long standing obstacle to integrating cryptographic protections into communication systems. A common approach is to allow a trusted third party (TTP) to gain access to private data. However, there is no way to verify that this trust is well place as the TTP may open all messages indiscriminately. Moreover, existing approaches do not scale well when, in addition to the content of the conversation, one wishes to hide ones identity. Given the importance of metadata this is a major problem. We propose a new signature scheme as an accountable replacement for group signatures, accountable forward and backward tracing signatures.



03:17 [Pub][ePrint] Towards Optimal Bounds for Implicit Factorization Problem, by Yao Lu and Liqiang Peng and Rui Zhang and Dongdai Lin

  We propose a new algorithm to solve the Implicit Factorization Problem, which was introduced by May and Ritzenhofen at PKC\'09. In 2011, Sarkar and Maitra (IEEE TIT 57(6): 4002-4013, 2011) improved May and Ritzenhofen\'s results by making use of the technique for solving multivariate approximate common divisors problem. In this paper, based on the observation that the desired root of the equations that derived by Sarkar and Maitra contains large prime factors, which are already determined by some known integers, we develop new techniques to acquire better bounds. We show that our attack is the best among all known attacks, and give experimental results to verify the correctness. Additionally, for the first time, we can experimentally handle the implicit factorization for the case of balanced RSA moduli.