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15:17 [Pub][ePrint] Higher-Order Side Channel Security and Mask Refreshing, by Jean-Sebastien Coron and Emmanuel Prouff and Matthieu Rivain and Thomas Roche

  Masking is a widely used countermeasure to protect block cipher implementations against side-channel attacks. The principle is to split every sensitive intermediate variable occurring in the computation into d + 1 shares, where d is called the masking order and plays the

role of a security parameter. A masked implementation is then said to achieve dth-order security if any set of d (or less) intermediate variables does not reveal key-dependent information. At CHES 2010, Rivain and Prouff have proposed a higher-order masking scheme for AES that works for any arbitrary order d. This scheme, and its subsequent extensions, are based on an improved version of the shared multiplication processing published by Ishai et al. at CRYPTO 2003. This improvement enables better memory/timing performances but its security relies on the refreshing of the masks at some points in the algorithm. In this paper, we show that the method proposed at CHES 2010 to do such mask refreshing introduces a security flaw in the overall masking scheme. Specically, we show that it is vulnerable to an attack of order d/2 + 1 whereas the scheme is supposed to achieve dth-order security. After exhibiting and analyzing the flaw, we propose a new solution which avoids the use of mask refreshing, and we prove its security. We also provide some implementation trick that makes our proposed solution, not only secure, but also faster than the original scheme.

15:17 [Pub][ePrint] Achieving Differential Privacy with New Imperfect Randomness, by Yanqing Yao and Zhoujun Li

  We revisit the question of achieving differential privacy with realistic imperfect randomness. In the design of differentially private mechanisms, it\'s usually assumed that uniformly random source is available. However, in many situations it seems unrealistic, and one must deal with various imperfect random sources. Dodis et al. (CRYPTO\'12) proposed that differential privacy can be achieved with Santha-Vazirani(SV) source via adding a stronger property called SV-consistent sampling and left open the question if differential privacy is possible with more realistic

(i.e., less structured) sources than SV source. A new source, called

Bias-Control Limited (BCL) source, introduced by Dodis (ICALP\'01),

as a generalization of the SV source and sequential bit-fixing source, is

more realistic. Unfortunately, if we nationally expand SV-consistent sampling to the BCL source, the expansion is hopeless to achieve differential privacy. One main reason is that SV-consistent sampling requires \"consecutive\"strings, while some strings can\'t be generated from \"non-trivial\"BCL source.

Motivated by this question, we introduce a new appealing property, called

compact BCL-consistent sampling, the degeneration of which is different

from SV-consistent sampling proposed by Dodis et al. We prove that if

the mechanism based on the BCL source satisfies this property, then it\'s

differentially private. Even if the BCL source is degenerated into the SV source,our proof is much more intuitive and simpler than that of Dodis

et al. Further, we construct explicit mechanisms using a new truncation

technique as well as arithmetic coding. We also propose its concrete

results for differential privacy and accuracy. While the results of [DY14]imply that if there exist differentially private mechanisms for imperfect randomness, then some parameters should have some constraints, ours show explicit construction of such mechanisms whose parameters match

the prior constraints.

15:17 [Pub][ePrint] Computationally binding quantum commitments, by Dominique Unruh

  We present a new definition of computationally binding commitment

schemes in the quantum setting, which we call \"collapse-binding\". The

definition applies to string commitments, composes in parallel, and

works well with rewinding-based proofs. We give simple constructions

of collapse-binding commitments in the random oracle model, giving

evidence that they can be realized from hash functions like SHA-3. We

evidence the usefulness of our definition by constructing three-round

statistical zero-knowledge quantum arguments of knowledge for all NP


15:17 [Pub][ePrint] Oblivious Transfer from weakly Random Self-Reducible Public-Key Cryptosystem, by Claude Crepeau and Raza Ali Kazmi

  In this work, we define a new notion of weakly Random-Self-Reducibile cryptosystems and show how it can be used to implement secure Oblivious Transfer. We also show that two recent (Post-quantum) cryptosystems (based on Learning with errors and Approximate Integer GCD) can be considered as weakly Random-Self-Reducible.

15:17 [Pub][ePrint] Optimally Secure Tweakable Blockciphers, by Bart Mennink

  We consider the generic design of a tweakable blockcipher from one or more evaluations of a classical blockcipher, in such a way that all input and output wires are of size n bits. As a first contribution, we show that any tweakable blockcipher with one primitive call and arbitrary linear pre- and postprocessing functions can be distinguished from an ideal one with an attack complexity of about 2^{n/2}. Next, we introduce the tweakable blockcipher tilde{F}[1]. It consists of one multiplication and one blockcipher call with tweak-dependent key, and achieves 2^{2n/3} security. Finally, we introduce tilde{F}[2], which makes two blockcipher calls, one of which with tweak-dependent key, and achieves optimal 2^n security. Both schemes are more efficient than all existing beyond birthday bound tweakable blockciphers known to date, as long as one blockcipher key renewal is cheaper than one blockcipher evaluation plus one universal hash evaluation.

15:17 [Pub][ePrint] Privacy-preserving Context-aware Recommender Systems: Analysis and New Solutions, by Qiang Tang and Jun Wang

  Nowadays, recommender systems have become an indispensable part of our daily life and provide personalized services for almost everything. However, nothing is for free -- such systems have also upset the society with severe privacy concerns because they accumulate a lot of personal information in order to provide recommendations. In this work, we construct privacy-preserving recommendation protocols by incorporating cryptographic techniques and the inherent data characteristics in recommender systems. We first revisit the protocols by Jeckmans et al. at ESORICS 2013 and show a number of security and usability issues. Then, we propose two privacy-preserving protocols, which compute predicted ratings for a user based on inputs from both the user\'s friends and a set of randomly chosen strangers. A user has the flexibility to retrieve either a predicted rating for an unrated item or the Top-N unrated items. The proposed protocols prevent information leakage from both protocol executions and the protocol outputs: a somewhat homomorphic encryption scheme is used to make all computations run in encrypted form, and inputs from the randomly-chosen strangers guarantee that the inputs of a user\'s friends will not be compromised even if this user\'s outputs are leaked. Finally, we use the well-known MovieLens 100k dataset to evaluate the performances for different parameter sizes.

15:17 [Pub][ePrint] On the (im)possibility of receiving security beyond 2^l using an l-bit PRNG: the case of Wang et. al. protocol, by Masoumeh Safkhani and Nasour Bagheri and Mehdi Hosseinzadeh and Mojtaba Eslamnezhad N

  Recently,Wang et al. analyzed the security of two EPC C1-G2 compliant RFID authentication protocols, called RAPLT and SRP^+, and proved that these protocols are vulnerable against de-synchronization and secret disclosure attacks. The time complexity of their attacks were O(2^{16}). In addition, they proposed an improved version of SRP^+ entitled SRP^{++}, for which they claim the security would be O(2^{32}). However, in this letter, we analyze the security of SRP^{++} and show that the complexity of retrieving all secret parameters of a given tag is $O(2^{16})$, similar to its predecessor protocol.

15:17 [Pub][ePrint] A random zoo: sloth, unicorn, and trx, by Arjen K. Lenstra and Benjamin Wesolowski

  Many applications require trustworthy generation of public random numbers. It is shown how this can be achieved using a hash function

that is timed to be as slow as desired (sloth), while the correctness

of the resulting hash can be verified quickly. It is shown how sloth

can be used for uncontestable random number generation (unicorn),

and how unicorn can be used for a new trustworthy random elliptic

curves service (trx) and random-sample voting.

15:17 [Pub][ePrint] Improved Higher-Order Differential Attacks on MISTY1, by Achiya Bar-On

  MISTY1 is a block cipher designed by Matsui in 1997. It is widely

deployed in Japan, and is recognized internationally as an European

NESSIE-recommended cipher and an ISO standard. Since its introduction,

MISTY1 was subjected to extensive cryptanalytic

efforts, yet no attack significantly faster than exhaustive key search is

known on its full version. The best currently

known attack is a higher-order differential attack presented by Tsunoo

et al. in 2012 which breaks a reduced variant of MISTY1 that contains

7 of the 8 rounds and 4 of the 5 $FL$ layers in $2^{49.7}$ data and $2^{116.4}$


In this paper, we present improved higher-order differential attacks on

reduced-round MISTY1. Our attack on the variant considered by Tsunoo et al.

requires roughly the same amount of data and only $2^{100.4}$ time

(i.e., is $2^{16}$ times faster). Furthermore, we present the first attack

on a MISTY1 variant with 7 rounds and all 5 $FL$ layers, requiring

$2^{51.4}$ data and $2^{121}$ time. To achieve our results, we use a new

higher-order differential characteristic for 4-round MISTY1, as well as

enhanced key recovery algorithms based on the {\\it partial sums} technique.

15:17 [Pub][ePrint] Breaking the Rabin-Williams digital signature system implementation in the Crypto++ library, by Evgeny Sidorov

  This paper describes a bug in the implementation of the Rabin-Williams digital signature in the \\texttt{Crypto++} framework. The bug is in the misuse of blinding technique that is aimed at preventing timing attacks on the digital signature system implementation, but eventually results in an opportunity to find the private key having only two different signatures of the same message. The CVE identifier of the issue is \\texttt{CVE-2015-2141}.

15:17 [Pub][ePrint] On Non-Black-Box Simulation and the Impossibility of Approximate Obfuscation, by Nir Bitansky and Omer Paneth

  The introduction of a non-black-box simulation technique by Barak (FOCS 2001) has been a major landmark in cryptography, breaking the previous barriers of black-box impossibility. Barak\'s technique has given rise to various powerful applications and it is a key component in all known protocols with non-black-box simulation.

We present the first non-black-box simulation technique that does not rely on Barak\'s technique (or on nonstandard assumptions). Invoking this technique, we obtain new and improved protocols resilient to various resetting attacks. These improvements include weaker computational assumptions and better round complexity.

A prominent feature of our technique is its compatibility with rewinding techniques from classic black-box zero-knowledge protocols. The combination of rewinding with non-black-box simulation has proven instrumental in coping with challenging goals as: simultaneously-resettable zero-knowledge, proofs of knowledge, and resettable-security from one-way functions. While previous works required tailored modifications to Barak\'s technique, we give a general recipe for combining our technique with rewinding. This yields simplified resettable protocols in the above settings, as well as improvements in round complexity and required computational assumptions.

The main ingredient in our technique is a new impossibility result for general program obfuscation. The results extend the impossibility result of Barak et al. (CRYPTO 2001) to the case of obfuscation with approximate functionality; thus, settling a question left open by Barak et al..

In the converse direction, we show a generic transformation from any resettably-sound zero-knowledge protocol to a family of functions that cannot be obfuscated.