International Association for Cryptologic Research

# IACR News Central

You can also access the full news archive.

Further sources to find out about changes are CryptoDB, ePrint RSS, ePrint Web, Event calender (iCal).

2012-06-04
00:17 [Pub][ePrint]

Broadcast encryption aims at sending a content to a large arbitrary group of users at once. Currently, the most efficient schemes provide constant-size headers, that encapsulate ephemeral session keys under which the payload is encrypted. However, in practice, and namely for pay-TV, providers have to send various contents to different groups of users. Headers are thus specific to each group, one for each channel: as a consequence, the global overhead is linear in the number of channels. Furthermore, when one wants to zap to and watch another channel, one has to get the new header and decrypt it to learn the new session key: either the headers are sent quite frequently or one has to store all the headers, even if one watches one channel only. Otherwise, the zapping time becomes unacceptably long.

In this paper, we consider encapsulation of several ephemeral keys, for various groups and thus various channels, in one header only, and we call this new primitive Multi-Channel Broadcast Encryption: one can hope for a much shorter global overhead and a short zapping time since the decoder already has the information to decrypt any available channel at once. Our candidates are private variants of the Boneh-Gentry-Waters scheme, with a constant-size global header, independently of the number of channels. In order to prove the CCA security of the scheme, we introduce a new dummy-helper technique and implement it in the random oracle model.

00:17 [Pub][ePrint]

Verified security provides a firm foundation for cryptographic proofs

by means of rigorous programming language techniques and verification

methods. EasyCrypt is a framework that realizes the verified security

paradigm and supports the machine-checked construction and

verification of cryptographic proofs using state-of-the-art SMT

solvers, automated theorem provers and interactive proof assistants.

Previous experiments have shown that EasyCrypt is effective for a

posteriori validation of cryptographic systems. In this paper, we

report on the first application of verified security to a novel

cryptographic construction, with strong security properties and

interesting practical features. Specifically, we use EasyCrypt to

prove the IND-CCA security of a redundancy-free public-key encryption

scheme based on trapdoor one-way permutations. Somewhat surprisingly,

we show that even with a zero-length redundancy, Boneh\'s SAEP scheme

(an OAEP-like construction with a single-round Feistel network rather

than two) converts a trapdoor one-way permutation into an

IND-CCA-secure scheme, provided the permutation satisfies two

additional properties. We then prove that the Rabin function and RSA

with short exponent enjoy these properties, and thus can be used to

instantiate the construction we propose to obtain efficient encryption

schemes. The reduction that justifies the security of our construction

is tight enough to achieve practical security with reasonable key

sizes.

00:17 [Pub][ePrint]

Elliptic curve cryptosystems have improved greatly in speed over the past few years. In this paper we outline a new elliptic curve signature and key agreement implementation which achieves record speeds while remaining relatively compact. For example, on Intel Sandy Bridge, a curve with about $2^{250}$ points produces a signature in just under 60k clock cycles, verifies in under 169k clock cycles, and computes a Diffie-Hellman shared secret in under 153k clock cycles. Our implementation has a small footprint: the library is under 55kB. We also post competitive timings on ARM processors, verifying a signature in under 626k Tegra-2 cycles. We introduce faster field arithmetic, a new point compression algorithm, an improved fixed-base scalar multiplication algorithm and a new way to verify signatures without inversions or coordinate recovery. Some of these improvements should be applicable to other systems.

00:17 [Pub][ePrint]

In this paper, we specify a class of mathematical problems, which we refer to as Function Density Problems\'\' (FDPs, in short), and point out novel connections of FDPs to the following two cryptographic topics; theoretical security evaluations of keyless hash functions (such as SHA-1), and constructions of provably secure pseudorandom generators (PRGs) with some enhanced security property introduced by Dubrov and Ishai [STOC 2006]. Our argument aims at proposing new theoretical frameworks for these topics (especially for the former) based on FDPs, rather than providing some concrete and practical results on the topics. We also give some examples of mathematical discussions on FDPs, which would be of independent interest from mathematical viewpoints. Finally, we discuss possible directions of future research on other cryptographic applications of FDPs and on mathematical studies on FDPs themselves.

00:17 [Pub][ePrint]

We construct the first public-key encryption scheme whose chosen-ciphertext (i.e., IND-CCA) security can be proved under a standard assumption and does not degrade in either the number of users or the number of ciphertexts. In particular, our scheme can be safely deployed in unknown settings in which no a-priori bound on the number of encryptions and/or users is known.

As a central technical building block, we devise the first structure-preserving signature scheme with a tight security reduction. (This signature scheme may be of independent interest.) Combining this scheme with Groth-Sahai proofs yields a tightly simulation-sound non-interactive zero-knowledge proof system for group equations. If we use this proof system in the Naor-Yung double encryption scheme, we obtain a tightly IND-CCA secure public-key encryption scheme from the Decision Linear assumption.

We point out that our techniques are not specific to public-key encryption security. Rather, we view our signature scheme and proof system as general building blocks that can help to achieve a tight security reduction.

00:17 [Pub][ePrint]

Recently, Chien et al. proposed a gateway-oriented password-based authenticated key exchange (GPAKE) protocol, through which a client and a gateway could generate a session key for future communication with the help of an authentication server. They also demonstrated that their scheme is provably secure in a formal model. However, in this letter, we will show that Chien et al.\'s protocol is vulnerable to the off-line password guessing attack. To overcome the weakness, we also propose an efficient countermeasure.

00:17 [Pub][ePrint]

The concept of proxy signature was introduced in 1996, up to now many proxy signature schemes have been proposed. In order to protect the proxy signer\'s privacy, the concept of anonymous proxy signature, which is also called proxy ring signature, was introduced in 2003. Some anonymous proxy signature schemes, which are provable secure in the random oracle model, have been proposed. However, provable security in the random oracle model is doubtful when the random oracles are instantiated with hash functions in their implementation. Hence, we propose the first secure anonymous proxy signature scheme without random oracles.

00:17 [Pub][ePrint]

The nonlinear feedback shift registers (NLFSR) are used to construct pseudorandom generators for stream ciphers. Their theory is not so complete as that of the linear feedback shift registers (LFSR). In general, it is not known how to construct NLFSRs with maximum period. The direct method is to search for such registers with suitable properties. We used the implementation of NLFSRs in Field Programmable Gate Arrays (FPGA) to perform a corresponding search. We also investigated local statistical properties of the binary sequences ganerated by NLFSRs of order 25 and 27.

2012-06-03
21:17 [Pub][ePrint]

We prove three optimal transference theorems on lattices possessing $n^{\\epsilon}$-unique shortest vectors which relate to the successive minima, the covering radius and the minimal length of

generating vectors respectively. The theorems result in reductions

between GapSVP$_{\\gamma\'}$ and GapSIVP$_\\gamma$ for this class of

lattices. Furthermore, we prove a new transference theorem giving an

optimal lower bound relating the successive minima of a lattice with

its dual. As an application, we compare the respective advantages of

current upper bounds on the smoothing parameter of discrete Gaussian

measures over lattices and show a more appropriate bound for lattices whose duals possess $\\sqrt{n}$-unique shortest vectors.

21:17 [Pub][ePrint]

Pollard\'s rho algorithm, along with parallelized, vectorized, and negating variants, is the standard method to compute discrete logarithms in generic prime-order groups.

This paper presents two reasons that Pollard\'s rho algorithm

is farther from optimality than generally believed.

First, higher-degree local anti-collisions\'\'

make the rho walk less random than the predictions made by the conventional Brent--Pollard heuristic.

Second, even a truly random walk is suboptimal,

because it suffers from global anti-collisions\'\' that can at least partially be avoided.

For example, after (1.5+o(1))\\sqrt(l) additions in a group of order l (without fast negation),

the baby-step-giant-step method has probability 0.5625+o(1)

of finding a uniform random discrete logarithm;

a truly random walk would have probability 0.6753\\ldots+o(1);

and this paper\'s new two-grumpy-giants-and-a-baby method has probability 0.71875+o(1).

21:17 [Pub][ePrint]

We present a formalisation of a category of schemes which we call \\emph{Broadcast-enhanced Key Predistribution Schemes}.

These schemes can be used instead of a key predistribution scheme in any network which has access to a trusted base station and broadcast channel.

In such networks, broadcast-enhanced key predistribution schemes can provide advantages over key predistribution schemes including flexibility and more efficient revocation.

There are many possible applications and ways to implement broadcast-enhanced key predistribution schemes, and we propose a framework for describing, comparing and analysing them.

In their paper From key predistribution to key redistribution\', Cicho\\\'{n}, Go{\\l}\\c{e}biewski and Kuty{\\l}owski propose a scheme for redistributing\' keys to a wireless sensor network using a broadcast channel after an initial key predistribution.

We classify this as a broadcast-enhanced key predistribution scheme and analyse it in that context.

We provide simpler proofs of some results from their paper, give a precise analysis of the resilience of their scheme, and discuss modifications based on defining a suitable keyring intersection threshold.

In the latter half of the paper we suggest two particular scenarios where broadcast-enhanced key predistribution schemes may be particularly desirable and the relevant design goals to prioritise in each case.

For each scenario we propose a suitable family of broadcast-enhanced key predistribution schemes and our analysis demonstrates their effectiveness in achieving their aims in resource-constrained networks.