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2012-07-06
21:17 [Pub][ePrint] How to Store some Secrets, by Reto E. Koenig and Rolf Haenni

  This paper introduces a special type of symmetric cryptosystem called multi-encryption scheme. It allows users to encrypt multiple plaintexts into a single ciphertext. Each plaintext is protected with its own secret key, meaning that they can be decrypted individually by applying the decryption function with the corresponding key to the ciphertext. Compared to encrypting the ciphertexts one-by-one using a standard symmetric cryptosystem, the main advantage of using a multi-encryption scheme is the no-search property, which guarantees that knowing the key is sufficient for decrypting a single plaintext. We show how to construct a multi-encryption scheme based on polynomials over finite fields. A possible application area is coercion-resistant electronic voting. To ensure a strong form of privacy, voters are equipped with multiple fake credentials, which are indistinguishable from the proper one. While theoretically sound, this requires a voter to perfectly recall multiple lengthy random numbers, and to know which of them is the proper one. To ensure 100\\% recall, users need to manage these numbers and keep them secret. A multi-encryption scheme is an elegant solution for this problem.



21:17 [Pub][ePrint] Combinatorial Solutions Providing Improved Security for the Generalized Russian Cards Problem, by Colleen M. Swanson and Douglas R. Stinson

  We present the first formal mathematical presentation of the generalized Russian cards problem, and provide rigorous security definitions that capture both basic and extended versions of weak and perfect security notions. In the generalized Russian cards problem, three players, Alice, Bob, and Cathy, are dealt a deck of $n$ cards, each given $a$, $b$, and $c$ cards, respectively. The goal is for Alice and Bob to learn each other\'s hands via public communication, without Cathy learning the fate of any particular card. The basic idea is that Alice announces a set of possible hands she might hold, and Bob, using knowledge of his own hand, should be able to learn Alice\'s cards from this announcement, but Cathy should not. Using a combinatorial approach, we are able to give a nice characterization of informative strategies (i.e., strategies allowing Bob to learn Alice\'s hand), having optimal communication complexity, namely the set of possible hands Alice announces must be equivalent to a large set of $t-(n, a, 1)$-designs, where $t=a-c$. We also provide some interesting necessary conditions for certain types of deals to be simultaneously informative and secure. That is, for deals satisfying $c = a-d$ for some $d \\geq 2$, where $b \\geq d-1$ and the strategy is assumed to satisfy a strong version of security (namely perfect $(d-1)$-security), we show that $a = d+1$ and hence $c=1$. We also give a precise characterization of informative and perfectly $(d-1)$-secure deals of the form $(d+1, b, 1)$ satisfying $b \\geq d-1$ involving $d-(n, d+1, 1)$-designs.



21:17 [Pub][ePrint] Distributed Key Generation in the Wild, by Aniket Kate and Yizhou Huang and Ian Goldberg

  Distributed key generation (DKG) has been studied extensively in the cryptographic literature. However, it has never been examined outside of the synchronous setting, and the known DKG protocols cannot guarantee safety or liveness over the Internet.

In this work, we present the first realistic DKG protocol for use over the Internet. We propose a practical system model for the Internet and define an efficient verifiable secret sharing (VSS) scheme in it. We observe the necessity of Byzantine agreement for asynchronous DKG and analyze the difficulty of using a randomized protocol for it. Using our VSS scheme and a leader-based agreement protocol, we then design a provably secure DKG protocol. We also consider and achieve cryptographic properties such as uniform randomness of the shared secret and compare static versus adaptive adversary models. Finally, we implement our DKG protocol, and establish its efficiency and reliability by extensively testing it on the PlanetLab platform. Counter to a general non-scalability perception about asynchronous systems, our experiments demonstrate that our asynchronous DKG protocol scales well with the system size and it is suitable for realizing multiparty computation and threshold cryptography over the Internet.



21:17 [Pub][ePrint] Multiparty Proximity Testing with Dishonest Majority from Equality Testing, by Ran Gelles and Rafail Ostrovsky and Kina Winoto

  Motivated by the recent widespread emergence of location-based services (LBS) over mobile devices, we explore efficient protocols for proximity-testing. Such protocols allow a group of friends to discover if they are all close to each other in some physical location, without revealing their individual locations to each other. We focus on hand-held devices and aim at protocols with very small communication complexity and a small number of rounds.

The proximity-testing problem can be reduced to the private equality testing (PET) problem, in which parties find out whether or not they hold the same input (drawn from a low-entropy distribution) without revealing any other information about their inputs to each other. While previous works analyze the 2-party PET special case (and its LBS application), in this work we consider highly-efficient schemes for the multiparty case with no honest majority. We provide schemes for both a direct-communication setting and a setting with a honest-but-curious mediating server that does not learn the users\' inputs. Our most efficient scheme takes 2 rounds, where in each round each user sends only a couple of ElGamal ciphertexts.



21:17 [Pub][ePrint] A Framework for Efficient Fully-Equipped UC Commitments, by Eiichiro Fujisaki

  We present a general framework for constructing non-interactive universally composable (UC) commitment schemes that are secure against adaptive adversaries in the erasure-free setting under a single re-usable common reference string.

Previously, such fully-equipped UC commitment schemes are

only known in \\cite{CF01,CLOS02}, with an unavoidable overhead of $O(\\spar)$; meaning that to commit $\\lambda$ bit, the communication and computational costs are $O(\\lambda\\spar)$. Efficient construction of a fully-equipped UC commitment scheme was a long-standing open problem. We introduce a new cryptographic primitive, called all-but-many encryptions (ABMEs), and prove that it is a translation of fully-equipped UC commitment in the algorithmic level. We implement ABMEs from two primitives, called probabilistic pseudo random functions

and extractable sigma protocols, where the former is a probabilistic version of pseudo random function and the latter is a special kind of sigma (i.e., canonical 3-round public-coin HVSZK) protocols with some extractability.

Interestingly, ABEs are not chosen-ciphertext secure, but still suffice to construct UC commitments without an additional zero-knowledge protocol.

We provide efficient fully-equipped UC commitment schemes

from ABMEs under DDH and DCR-based assumptions. The former is at least as efficient as the arguably most efficient UC commitment scheme~\\cite{Lin11:UCCom} (which is interactive and not erasure-free) in reasonable security parameters.

The latter is the first fully-equipped UC commitment scheme

with optimal expansion factor $O(1)$.

We also construct a fully-equipped UC commitment scheme from

a general assumption (that trap-door permutations exist), converted from a weak ABME in an non-black-box manner, which is far more efficient than the previous general construction~\\cite{CLOS02}, because it does not require any non-interactive zero knowledge protocol.



21:17 [Pub][ePrint] Several Weak Bit-Commitments Using Seal-Once Tamper-Evident Devices, by Ioana Boureanu and Serge Vaudenay

  Following both theoretical and practical arguments, we construct UC-secure bit-commitment protocols that place their strength on the sender\'s side and are built using tamper-evident devices, e.g., a type of distinguishable, sealed envelopes.

We show that by using a second formalisation of tamper-evident distinguishable envelopes we can attain better security guarantees, i.e., EUC-security.

We show the relations between several flavours of weak bit-commitments, bit-commitments and distinguishable tamper-evident envelopes.

We focus, at all points, on the lightweight nature of the underlying mechanisms and on the end-to-end human verifiability.



15:03 [Job][New] Post-Doc Applied Cryptography, University of Twente, Enschede, The Netherlands

  Job description

The main task of the candidate will be to do research in applied cryptography. He/she will also be responsible for the daily supervision of a number of PhD students. The candidate will be offered the opportunity to gain some teaching experience, related to his/her background and interests.

Requirements

We are looking for an excellent and independent researcher who has (1) a PhD degree in Applied Cryptography or a related discipline, (2) a good publication record, and (3) good communication skills.

Conditions of employment

The Post-Doc will be appointed as a Researcher for a period of two years, with the possibility of an extension for a further three years. The monthly salary of the Researcher will amount to, depending on the experience, maximum 3755 euro gross per month according to salary scale 10 of the Dutch Universities Labour Agreement.

09:43 [Job][New] Research Fellows and Ph.D. students, Nanyang Technological University, Singapore

  Coding and Cryptograph Research Group (http://www1.spms.ntu.edu.sg/~ccrg/index.html) at Nanyang Technological University (NTU), Singapore, is seeking candidates for 1 or 2 research fellow positions (from fresh post-docs to senior research fellows) and a few Ph.D. student positions in the areas of symmetric key cryptography and lightweight cryptography. The future research team will be funded by the 5-year National Research Foundation (NRF) Fellowship grant from Singapore (started in April 2012).

Salaries are very competitive and are determined according to the successful applicants accomplishments, experience and qualifications. The duration of the contracts are very flexible. Interested applicants are encouraged to send their detailed CVs, cover letter and references.

Review of applications starts immediately and will continue until positions are filled.

00:15 [Event][New] ACNS'13: 11th International Conference on Applied Cryptography and Network Security

  From June 25 to June 28
Location: Banff, Alberta, Canada
More Information: http://acns2013.cpsc.ucalgary.ca/




2012-07-05
14:45 [Conf][Crypto] Early registration deadline for CRYPTO is July 8!

  Link to online registration --

http://www.iacr.org/conferences/crypto2012/registration-2012.html

05:40 [Job][New] Post-Doc Positions, IT University of Copenhagen, Denmark

  The IT University of Copenhagen invites applications for several postdoctoral fellow positions on trustworthy electronic election technology. The positions are part of a larger effort to prove that it is possible to modernize the democratic process without losing the trust of the voters.

We are looking for experts in epistemic logical framework technology and cryptographic methods, such as full homomorphic encryption. The research will be conducted under the supervision of Profs. Joseph Kiniry and Carsten Schürmann. A successful applicant will be hired initially for one year with the option to renew. The start date is flexible. Candidates are also encouraged to explore research ideas beyond the project description. The positions provide significant opportunities for professional development.

Postdoctoral candidates should have a Ph.D. in Computer Science or Mathematics and an established research record in one or more of the following fields:

  • applied formal methods
  • cryptography
  • electronic voting systems (of primary importance)
  • rigorous software engineering
  • trust and trustworthiness
  • logic and semantics
  • logical frameworks and type theory
  • proof theory and higher-order theorem proving
  • program verification

Early expressions of interest are encouraged: Carsten Schuermann (carsten (at) itu.dk), Joseph Kiniry (kiniry (at) itu.dk).