International Association for Cryptologic Research

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2014-06-15
06:17 [Pub][ePrint]

In a homomorphic signature scheme, a user Alice signs some large data $x$ using her secret signing key and stores the signed data on a server. The server can then run some computation $y=g(x)$ on the signed data and homomorphically produce a short signature $\\sigma$. Anybody can verify the signature using Alice\'s public verification key and become convinced that $y$ is the correct output of the computation $g$ over Alice\'s data, without needing to have the underlying data itself.

In this work, we construct the first leveled fully homomorphic signature schemes that can evaluate arbitrary circuits over signed data, where only the maximal depth $d$ of the circuit needs to be fixed a priori. The size of the evaluated signature grows polynomially in $d$, but is otherwise independent of the circuit size or the data size. Our solutions are based on the hardness of the small integer solution (SIS) problem, which is in turn implied by the worst-case hardness of problems in standard lattices. We get a scheme in the standard model, albeit with large public parameters whose size must exceed the total size of all signed data. In the random-oracle model, we get a scheme with short public parameters. These results offer a significant improvement in capabilities and assumptions over the best prior homomorphic signature scheme due to Boneh and Freeman (Eurocrypt \'11).

As a building block of independent interest, we introduce a new notion called homomorphic trapdoor functions (HTDF). We show to how construct homomorphic signatures using HTDFs as a black box. We construct HTDFs based on the SIS problem by relying on a recent technique developed by Boneh et al. (Eurocrypt \'14) in the context of attribute based encryption.

06:17 [Pub][ePrint]

We propose a new protocol for a cryptocurrency, that builds upon the Bitcoin protocol by combining its Proof of Work component with a Proof of Stake type of system. Our Proof of Activity (PoA) protocol offers good security against possibly practical future attacks on Bitcoin, and has a relatively low penalty in terms of network communication and storage space. We explore various attack scenarios and suggest remedies to potential vulnerabilities of the PoA protocol, as well as evaluate the performance of its core subroutine.

06:17 [Pub][ePrint]

The linear layer is a core component in any substitution-permutation network block cipher. Its design significantly influences both the security and the efficiency of the resulting block cipher. Surprisingly, not many general constructions are known that allow to choose trade-offs between security and efficiency. Especially, when compared to Sboxes, it seems that the linear layer is crucially understudied. In this paper, we propose a general methodology to construct good, sometimes optimal, linear layers allowing for a large variety of trade-offs. We give several instances of our construction and on top underline its value by presenting a new block cipher. PRIDE is optimized for 8-bit micro-controllers and significantly outperforms all academic solutions both in terms of code size and cycle count.

06:17 [Pub][ePrint]

This work deals with DPA-resistant logic styles, i.e., cell-level countermeasures against power analysis attacks that are known as a serious threat to cryptographic devices. Early propagation and imbalanced routings are amongst the well-known issues of such countermeasures, that - if not considered during the design process - can cause the underlying cryptographic device to be vulnerable to certain attacks. Although most of the DPA-resistant logic styles target an ASIC design process, there are a few attempts to apply them in an FPGA platform. This is due to the missing freedom in FPGA design tools required to deal with the aforementioned problems. Our contribution in this work is to provide solutions for both early propagation and imbalanced routings considering a modern Xilinx FPGA as the target platform. Foremost, based on the WDDL concept we design a new FPGA-based logic style without early propagation in both precharge and evaluation phases. Additionally, with respect to the limited routing resources within an FPGA we develop a customized router to nd the best appropriate dual-rail routes for a given dual-rail circuit. Based on practical experiments on a Virtex-5 FPGA our evaluations verify the efficiency of each of our proposed approaches. They significantly improve the resistance of the design compared to cases not benefiting from our schemes.

2014-06-14
15:17 [Pub][ePrint]

Secret sharing (SS) is one of the most important cryptographic primitives used for data outsourcing. The (t, n) SS was introduced by Shamir and Blakley separately in 1979. The secret sharing policy of the (t, n) threshold SS is far too simple for many applications because it assumes that every shareholder has equal privilege to the secret or every share-holder is equally trusted. Ito et al. introduced the concept of a general secret sharing scheme (GSS). In a GSS, a secret is divided among a set of shareholders in such a way that any \"qualified\" subset of shareholders can access the secret, but any \"unqualified\" subset of shareholders cannot access the secret. The secret access structure of GSS is far more flexible than threshold SS. In this paper, we propose an optimized implementation of GSS. Our proposed scheme first uses Boolean logic to derive two important subsets, one is called which is the minimal positive access subset and the other is called which is the maximal negative access subset, of a given general secret sharing structure. Then, condi-tions of parameters of a GSS are established based on these two important subsets. Fur-thermore, integer linear/non-linear programming is used to optimize the size of shares of a GSS. The complexity of linear/non-linear programming is where is the number of shares generated by the dealer. This proposed design can be applied to implement GSS based on any classical SS. We use two GSSs, one is based on Shamir\'s weighted SS (WSS) using linear polynomial and the other is based on Asmuth-Bloom\'s SS using Chinese Re-mainder Theorem (CRT), to demonstrate our design. In comparing with existing GSSs, our proposed scheme is more efficient and can be applied to all classical SSs.

06:17 [Pub][ePrint]

Private set intersection (PSI) allows two parties to compute the intersection of their sets without revealing any information about items that are not in the intersection. It is one of the best studied applications of secure computation and many PSI protocols have been proposed. However, the variety of existing PSI protocols makes it

difficult to identify the solution that performs best in a respective scenario, especially since they were not all implemented and compared in the same setting.

In this work, we give an overview on existing PSI protocols that are secure against semi-honest adversaries. We take advantage of the most recent efficiency improvements in OT extension to propose significant optimizations to previous PSI protocols and to suggest a new PSI protocol whose runtime is superior to that of existing protocols. We compare the performance of the protocols both theoretically and experimentally, by implementing all protocols on the same platform, and give recommendations on which protocol to use in a particular setting.

06:17 [Pub][ePrint]

SIMON is a family of lightweight block ciphers which are designed by the U.S National Security Agency in 2013. In this paper, we improve the previous differential attacks on SIMON family of block ciphers by considering some bit-difference equations. Combining with some new observations on key guess policies of SIMON family, we mount differential attacks on 21-round SIMON32/64, 22-round SIMON$48/72$, 22-round SIMON48/96, 28-round SIMON$64/96$ and SIMON$64/128$ with time complexity about $2^{46}$, $2^{63}$, $2^{71}$, $2^{60}$ and $2^{60}$ encryptions respectively. As far as we know, these results are the best attacks on reduced-round SIMON versions.

06:17 [Pub][ePrint]

In this paper, we present a construction of public key encryption

secure against related key attacks from hash proof systems in the

standard model. We show that the schemes presented by Jia et

al. (Provsec2013) are special cases of our general theory, and also

give other instantiations based on the QR and DCR assumptions. To

fulfill the related key security, we require the hash proof systems

to satisfy the key homomorphism and computational finger-printing properties. Compared with the construction given by Wee (PKC2012), our construction removed the use of one-time

signatures.

2014-06-13
18:17 [Pub][ePrint]

A usual way to construct block ciphers is to apply several rounds of a given structure. Many kinds of attacks are

mounted against block ciphers. Among them, differential and linear attacks are widely used. In~\\cite{V98,V03}, it is

shown that ciphers that achieve perfect pairwise decorrelation are secure against linear and differential attacks. It is possible to obtain such schemes by introducing at least one random affine permutation as a round function in the design of the scheme.

In this paper, we study attacks on schemes based on classical Feistel schemes where we introduce one or two affine

permutations.

Since these schemes resist against linear and differential

attacks, we will study stronger attacks based on specific equations on 4-tuples of

cleartext/ciphertext messages. We give the

number of messages needed to distinguish a permutation produced by such schemes from a random

permutation, depending on the number of rounds used in the schemes, the number and the position of the random affine permutations introduced in the schemes.

15:17 [Pub][ePrint]

At Eurocrypt 2010, Freeman presented a framework to convert cryptosystems based on composite-order groups into ones that use prime-order groups. Such a transformation is interesting not only from a conceptual point of view, but also since for relevant parameters, operations in prime-order groups are faster than composite-order operations by an order of magnitude. Since Freeman\'s work, several other works have shown improvements, but also lower bounds on the efficiency of such conversions.

In this work, we present a new framework for composite-to-prime-order conversions. Our framework is in the spirit of Freeman\'s work; however, we develop a different, polynomial\'\' view of his approach, and revisit several of his design decisions. This eventually leads to significant efficiency improvements, and enables us to circumvent previous lower bounds. Specifically, we show how to implement Groth-Sahai proofs in a prime-order environment (with a symmetric pairing) almost twice as efficiently as the state of the art.

We also show that our new conversions are optimal in a very broad sense. Besides, our conversions also apply in settings with a multilinear map, and can be instantiated from a variety of computational assumptions (including, e.g., the $k$-linear assumption).

12:59 [News]

Starting in 2014, IACR will sponsor a small number of Cryptology Schools providing intensive training on clearly identified topics in cryptology. The aim of this program is to develop awareness and increased capacity for research in cryptology.

A Cryptology School is typically held full-time for 4-5 days of intensive learning and constitutes an efficient way to provide high-quality training for graduate students, as well as for professionals. Attendance should be open to anyone who is interested and qualified. In order to facilitate learning, a school is usually taught by a few domain experts with a focus on educating the audience rather than impressing with results. In line with the mission of IACR, a Cryptology School should enable the audience to advance the theory and practice of cryptology and related fields.

There are two rounds of submissions every year. The submission deadlines are:

• December 31st of year X-1: For schools that take place between March of year X and February of year X + 1.
• June 30th of year X: For schools that take place between September of year X and August of year X + 1.
Submissions must be sent by email to schools /at/ iacr.org.