*21:17* [Pub][ePrint]
Quo Vadis Quaternion? Cryptanalysis of Rainbow over Non-Commutative Rings, by Enrico Thomae
The Rainbow Signature Scheme is a non-trivial generalization of the well known Unbalanced Oil and Vinegar Signature Scheme (Eurocrypt \'99) minimizing the length of the signatures. Recently a new variant based on non-commutative rings, called NC-Rainbow, was introduced at CT-RSA 2012 to further minimize the secret key size.We disprove the claim that NC-Rainbow is as secure as Rainbow in general and show how to reduce the complexity of MinRank attacks from 2^288 to 2^112 and of HighRank attacks from 2^128 to 2^96 for the proposed instantiation over the ring of Quaternions. We further reveal some facts about Quaternions that increase the complexity of the signing algorithm. We show that NC-Rainbow is just a special case of introducing further structure to the secret key in order to decrease the key size. As the results are comparable with the ones achieved by equivalent keys, which provably do not decrease security, and far worse than just using a PRNG, we recommend not to use NC-Rainbow.

*21:17* [Pub][ePrint]
Passive Corruption in Statistical Multi-Party Computation, by Martin Hirt and Christoph Lucas and Ueli Maurer and Dominik Raub
The goal of Multi-Party Computation (MPC) is to perform an arbitrary computation in a distributed, private, and fault-tolerant way. For this purpose, a fixed set of n parties runs a protocol that tolerates an adversary corrupting a subset of the parties, preserving certain security guarantees like correctness, secrecy, robustness, and fairness. Corruptions can be either passive or active: A passively corrupted party follows the protocol correctly, but the adversary learns the entire internal state of this party. An actively corrupted party is completely controlled by the adversary, and may deviate arbitrarily from the protocol. A mixed adversary may at the same time corrupt some parties actively and some additional parties passively.In this work, we consider the statistical setting with mixed adversaries and study the exact consequences of active and passive corruptions on secrecy, correctness, robustness, and fairness separately (i.e., hybrid security). Clearly, the number of passive corruptions affects the thresholds for secrecy, while the number of active corruptions affects all thresholds. It turns out that in the statistical setting, the number of passive corruptions in particular also affects the threshold for correctness, i.e., in all protocols there are (tolerated) adversaries for which a single additional passive corruption is sufficient to break correctness. This is in contrast to both the perfect and the computational setting, where such an influence cannot be observed. Apparently, this effect arises from the use of information-theoretic signatures, which are part of most (if not all) statistical protocols.

*21:17* [Pub][ePrint]
Public-Key Cryptography from New Multivariate Quadratic Assumptions, by Yun-Ju Huang and Feng-Hao Liu and Bo-Yin Yang
In this work, we study a new multivariate quadratic (MQ) assumption that can be used to construct public-key encryption schemes. In particular, we research in the following two directions:

We establish a precise \\emph{asymptotic} formulation of a family of hard MQ problems, and provide empirical evidence to confirm the hardness. %Since there are many practical solvers studied and implemented during the studies of algebraic attacks, we use

We construct public-key encryption schemes, and prove their security under the hardness assumption of this family. Also, we provide a new \\emph{perspective} to look at MQ systems that plays a key role to our design and proof of security.

As a consequence, we construct the \\emph{first} public-key encryption scheme that is \\emph{provably secure} under the MQ assumption.

Moreover, our public-key encryption scheme is efficient in the sense that it only needs a ciphertext length $L + \\poly(k)$ to encrypt a message $M\\in \\{0, 1 \\}^{L}$ for any un-prespecified polynomial $L$, where $k$ is the security parameter. This is essentially \\emph{optimal} since an additive overhead is the best we can hope for.

*21:17* [Pub][ePrint]
Boomerang and Slide-Rotational Analysis of the SM3 Hash Function, by Aleksandar Kircanski and Amr M. Youssef
SM3 is a hash function designed by Xiaoyun Wang et al., andpublished by the Chinese Commercial Cryptography Administration Office

for the use of electronic authentication service system. The design of

SM3 builds upon the design of the SHA-2 hash function, but introduces

additional strengthening features. In this paper, using a higher order

differential cryptanalysis approach, we present a practical 4-sum

distinguisher against the compression function of SM3 reduced to 32

rounds. In addition, we point out a slide-rotational property of

SM3-XOR, which exists due to the fact that constants used in the rounds

are not independent.

*21:17* [Pub][ePrint]
Official Arbitration and its Application to Secure Cloud Storage, by Alptekin Küpçü
Many cryptographic protocols exist that enable two parties to exchange items (e.g., e-commerce) or agree on something (e.g., contract-signing). In such settings, disputes may arise. Official arbitration refers to the process of resolving disputes between two (or more) parties by a trusted and authorized Judge, based on evidence provided. As an example, consider the secure cloud storage scenario where there needs to be an official arbitration process between the client and the server in case of data loss or corruption. Without such a mechanism that can be officially used by the Judge in the court, the barrier on the enterprise adoption of such systems is high.In this paper we first formally define official arbitration, and then provide several general purpose official arbitration protocols. Later, we focus on secure cloud storage, and provide efficient official arbitration schemes that can be used on top of any secure cloud storage scheme. We furthermore present a completely automated system where the Judge can just be a computer instead of a human being. All our constructions have security proofs, and we conclude with performance measurements showing that our overhead for official arbitration is roughly 2 ms and 80 bytes for each update on the stored data.

*21:17* [Pub][ePrint]
Cyptanalysis CDHP , BDHP and Tate pairing under certain conditions The Tate pairing is less secure than Weil, by Rkia Aouinatou (1) Mostafa Belkasmi (2)
This work fall within the cadre of Cryptanalysis. Because, undercertain condition, we would give a fairly simple method to solve

the CDHP (the Problem Computational of Diffie and Hellman) and

others problems associated to it. Since, solving this problem, will help

us to provide a solution to the BDH (Problem Bilinear of Diffie and

Hellman). The CDHP and BDHP are the heart of many cryptosystems

in the point of view security, so solving it may be a threat to this

cryptosystem\'s. To elucidate this, we use a concept of geometry

algebraic named Tate Pairing.

This work is purely theoretical, we give firstly an overview on the

idea and we illustrate it by an examples to see its efficiency.

*21:17* [Pub][ePrint]
Concurrent Zero Knowledge in the Bounded Player Model, by Abhishek Jain, Rafail Ostrovsky, Silas Richelson, Ivan Visconti
In this paper we put forward the Bounded Player Model for secure computation. In this new model, the number of players that will ever be involved in secure computations is bounded, but the number of computations has no a-priori bound. Indeed, while the number of devices and people on this planet can be realistically estimated and bounded, the number of computations these devices will run can not be realistically bounded.We stress that in the Bounded Player model, in addition to no apriori bound on the number of sessions, there is no synchronization barrier, no trusted party, and simulation must be performed in polynomial time.

In this setting, we achieve concurrent Zero Knowledge (cZK) with sub-logarithmic round complexity.

Our security proof is (necessarily) non-black-box, our simulator is straight-line and works as long as the number of rounds is $\\omega(1)$.

We further show that unlike previously studied relaxations of the standard model (e.g., timing assumptions, super-polynomial simulation), concurrent-secure computation is impossible to achieve in the Bounded Player model. This gives evidence that our model is closer to the standard model than previously studied models, and we believe might have additional applications.