International Association for Cryptologic Research

# IACR News Central

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2012-11-11
19:17 [Pub][ePrint]

We present high performance non-deterministic fully-homomorphic methods for practical randomization of data (over commutative ring), and symmetric-key encryption of random mod-N data (over ring of reidues mod-N) well suited for crypto applications. These methods secure, for example, the multivariate input or the coefficients of a polynomial function running in an open untrusted environment. We show that random plaintext is the sufficient condition for proof of security for the homomorphic encryption. The efficient nature of the methods - one large-numbers multiplication per encryption and six for the product of two encrypted values - motivates and enables the use of low cost collaborative security platforms for crypto applications such as keyed-hash or private key derivation algorithms. Such a platform is comprised of a low-cost and low performance security element supported by an untrusted high performance server running the homomorpic algorithms. The methods employed may also provide enhanced protection for some existing crypto algorithms against certain attacks. Specifically, it is shown how to secure OSS public-key signature against Pollard attack. Further, we demonstrate how the homomorphic randomization of data can offer protection for an AES-key against side-channel attacks. Finally, the methods provide both fault detection and verification of computed-data integrity.

19:17 [Pub][ePrint]

Generalized signcryption (GSC) scheme can adaptively work as an encryption scheme, a signature scheme or a signcryption scheme with only one algorithm. It is very suitable for storage-constrained environments. In this paper, we analyze a multi-receiver GSC scheme, and show that it cannot achieve indistinguishability-adaptive chosen ciphertext attack (IND-CCA2) secure in the pure encryption mode and hybrid encryption mode. We further propose a revised version of the scheme, which resolves the security issues of the original scheme without sacrificing its high efficiency and simple design. Our improved scheme can be proved to be IND-CCA2 secure and existentially unforgeable-adaptive chosen message attack (EUF-CMA) under computational Diffie-Hellman (CDH) assumption.

19:17 [Pub][ePrint]

We count ]B, C]-grained, k-factor integers which are simultaneously B-rough and C-smooth and have a fixed number k of prime factors. Our aim is to exploit explicit versions of the prime number theorem as much as possible to get good explicit bounds for the count of such integers. This analysis was inspired by certain inner procedures in the general number field sieve. The result should at least provide some insight in what happens there.

We estimate the given count in terms of some recursively defined functions. Since they are still difficult to handle, only another approximation step reveals their orders.

Finally, we use the obtained bounds to perform numerical experiments that show how good the desired count can be approximated for the parameters of the general number field sieve in the mentioned inspiring application.

19:17 [Pub][ePrint]

SM3~\\cite{SM3hf} is the Chinese cryptographic hash standard which was announced in 2010 and designed by Wang $et\\ al.$. It is based on the Merkle-Damg\\r{a}rd design and its compression function can be seen as a block cipher used in Davies-Meyer mode. It uses message block of length 512 bits and outputs hash value of length 256 bits.

This paper studies the security of SM3 hash function against preimage attack and pseudo-collision attack. We propose preimage attacks on 29-step and 30-step SM3, and pseudo-preimage attacks on 31-step and 32-step SM3 out of 64 steps. The complexities of these attacks are $2^{245}$ 29-step operations, $2^{251.1}$ 30-step operations, $2^{245}$ 31-step operations and $2^{251.1}$ 32-step operations, respectively. These (pseudo) preimage attacks are all from the first step of the reduced SM3. Meanwhile, these (pseudo) preimage attacks can be converted into pseudo-collision attacks on SM3 reduced to 29 steps, 30 steps, 31 steps and 32 steps with complexities of $2^{122}$, $2^{125.1}$, $2^{122}$ and $2^{125.1}$ respectively. As far as we know, the previously best known preimage attacks on SM3 cover 28 steps (from the first step) and 30 steps (from the 7-th step), and there is no publicly published result on (pseudo) collision attack on SM3.

19:17 [Pub][ePrint]

Recently, Fang et al. [6] introduced an interactive(bidirectional) conditional proxy re-encryption(C-PRE) scheme such that a proxy can only convert ciphertexts that satisfy access policy set by a delegator. Their scheme supports monotonic access policy expressed by \"OR\" and \"AND\" gates. In addition, their scheme is called interactive since generation of re-encryption keys requires interaction between the delegator and delegatee. In this paper, we study the problem of constructing a unidirectional(non-interactive) C-PRE scheme supporting non-monotonic access policy expressed by \"NOT\", \"OR\" and \"AND\" gates. A security model for unidirectional C-PRE schemes is also proposed in this paper. To yield a unidirectional C-PRE scheme supporting non-monotonic access policy, we extend the unidirectional PRE scheme presented by Libert et al. [8] by using the ideas from the non-monotonic attributed based encryption (ABE) scheme presented by Ostrovsky et al. [9]. Furthermore, the security of our C-PRE scheme is proved under the modified 3-weak Decision Bilinear Diffie-Hellman Inversion assumption in the standard model.

19:17 [Pub][ePrint]

SPDZ (pronounced \"Speedz\") is the nickname of the MPC protocol of Damg°ard et al. from Crypto 2012. SPDZ provided various efficiency innovations on both the theoretical and practical sides compared to previous work in the preprocessing model. In this paper we both resolve a number of open problems with SPDZ; and present several

theoretical and practical improvements to the protocol.

In detail, we start by designing and implementing a covertly secure key generation protocol for distributed BGV secret keys. In prior work this was assumed to be provided by a given setup functionality. Protocols for distributingBGV secret keys are likely to be of wider applicability than to the SPDZ protocol alone.

We then construct both a covertly and actively secure preprocessing phase, both of which compare favourably with previous work in terms of efficiency and provable security. We also build a new online phase, which solves a major problem of the SPDZ protocol: namely prior to this work preprocessed data could be used for only one function evaluation and then had to be recomputed from scratch for the next evaluation, while our online phase can support reactive functionalities. This improvement comes mainly from the fact that our construction does not require players to reveal the MAC keys to check correctness of MAC\'d values.

Since our focus is also on practical instantiations, our implementation offloads as much computation as possible into the preprocessing phase, thus resulting in a faster online phase. Moreover, a better analysis of the parameters of the underlying cryptoscheme and a more specific choice of the field where computation is performed allow us to obtain a better optimized implementation. Improvements are also due to the fact that our construction is in the random oracle model, and the practical implementation is multi-threaded.

2012-11-08
19:10 [Event][New]

Submission: 23 May 2013
From October 23 to October 25
Location: Melaka, Malaysia

16:17 [Pub][ePrint]

We present explicit formulae and complexities of bit-parallel shifted polynomial basis (SPB)

squarers in finite field $GF(2^{n})$s generated by general irreducible trinomials

$x^{n}+x^{k}+1$ (\$0< k

16:17 [Pub][ePrint]

In a group signature scheme, group members are able to sign on behalf of the group. Since the introduction of this cryptographic authentication mechanism, several schemes have been proposed

but only few of them enjoy a security in the standard model. Moreover, those provided in the standard model suffer the recourse to non standard-assumptions, or the expensive cost and bandwidth of the resulting signature.

We provide three practical group signature schemes that are provably secure in the standard model under standard assumptions. The three schemes permit dynamic enrollment of new members while keeping a constant size for both keys and group signatures, and they improve the state-of-the art by several orders of magnitude.

16:17 [Pub][ePrint]

Key management in multicast dynamic groups, where users can leave or join at their ease is one of the most crucial and essential part of secure communication. Various efficient management strategies have been proposed during last decade that aim to decrease encryption costs and transmission overheads. In this report, two different types of key management schemes are proposed. First proposed scheme is based on One-way function tree (OFT).

The proposed scheme fulfills the security gaps that have been pointed out in recent years. Second proposed scheme is based on logical key hierarchy (LKH). This proposed scheme provides better performance for, rather inflexible and expensive, LKH scheme.

16:17 [Pub][ePrint]

Secure two-party and multiparty computation has long stood at the center of the foundations of theoretical cryptography. Recently, however, interest has grown regarding the efficiency of such protocols and their application in practice. As a result, there has been significant progress on this problem and it is possible to actually carry out secure computation for non-trivial tasks on reasonably large inputs. Part of this research goal of making secure computation practical has also involved \\emph{implementations}. Such implementations are of importance for two reasons: first, they demonstrate the real efficiency of known and new protocols; second, they deepen our understanding regarding where the bottlenecks in efficiency lie. However, it is very hard to compare between implementations by different research groups since they are carried out on different platforms and using different infrastructures. In addition, most implementations have been carried out without the goal of code reuse, and so are not helpful to other researchers. The difficulty of beginning implementation projects is further compounded by the fact that existing cryptographic libraries (like openSSL, Bouncy Castle, and others) are tailored for tasks like encryption, authentication and key-exchange, and not for secure computation. We have developed SCAPI in order to address these problems. SCAPI is an \\emph{open-source} general library tailored for secure computation implementations. Our aim in developing SCAPI has been to provide a flexible and efficient infrastructure for secure computation implementations, that is both easy to use and robust. Great care has been taken in the design of the library, in writing clean code, and in documentation. We hope that this library will be useful to the community interested in implementations of secure protocols, and will help to promote the goal of making secure computation practical.