International Association for Cryptologic Research

# IACR News Central

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2014-06-02
09:17 [Pub][ePrint]

A ciphertext-policy attribute-based encryption protocol uses bilinear pairings to provide

control access mechanisms, where the set of user\'s attributes is specified by means of a linear secret sharing scheme. In this paper we present the design of a software cryptographic library that achieves record timings for the computation of a 126-bit security level attribute-based encryption scheme. We developed all the required auxiliary building blocks and compared the computational weight that each of them adds to the overall performance of this protocol.

In particular, our single pairing and multi-pairing implementations achieve state-of-the-art

time performance at the 126-bit security level.

09:17 [Pub][ePrint]

A function f is extractable if it is possible to algorithmically extract,\'\' from any adversarial program that outputs a value y in the image of f, a preimage of y.

When combined with hardness properties such as one-wayness or collision-resistance, extractability has proven to be a powerful tool. However, so far, extractability has not been explicitly shown. Instead, it has only been considered as a non-standard *knowledge assumption* on certain functions.

We make two headways in the study of the existence of extractable one-way functions (EOWFs). On the negative side, we show that if there exist indistinguishability obfuscators for a certain class of circuits then

there do not exist EOWFs where extraction works for any adversarial program with auxiliary-input of unbounded polynomial length.

On the positive side, for adversarial programs with bounded auxiliary-input (and unbounded polynomial running time), we give the first construction of EOWFs with an explicit extraction procedure, based on relatively standard assumptions (e.g., sub-exponential hardness of Learning with Errors). We then use these functions to construct the first 2-message zero-knowledge arguments and 3-message zero-knowledge arguments of knowledge, against the same class of adversarial verifiers, from essentially the same assumptions.

09:17 [Pub][ePrint]

In this article, we study the security of iterative hash-based MACs, such as HMAC or NMAC, with regards to universal forgery attacks. Leveraging recent advances in the analysis of functional graphs built from the iteration of HMAC or NMAC, we exhibit the very first generic universal forgery attack against hash-based MACs. In particular, our work implies that the universal forgery resistance of an n-bit output HMAC construction is not 2^n queries as long believed by the community. The techniques we introduce extend the previous functional graphs-based attacks that only took in account the cycle structure or the collision probability: we show that one can extract much more meaningful secret information by also analyzing the distance of a node from the cycle of its component in the functional graph.

09:17 [Pub][ePrint]

We are interested in secure computation protocols in settings where the number of parties is huge and their data even larger. Assuming the existence of a single-use broadcast channel (per player), we demonstrate statistically secure computation protocols for computing (multiple) arbitrary dynamic RAM programs over parties\' inputs, handling (1/3-eps) fraction static corruptions, while preserving up to polylogarithmic factors the computation and memory complexities of the RAM program. Additionally, our protocol is load balanced and has polylogarithmic communication locality.

09:17 [Pub][ePrint]

In a recent celebrated breakthrough, Garg et al. (FOCS 2013) gave the first candidate for so-called indistinguishability obfuscation (iO) thereby reviving the interest in obfuscation for a general purpose. Since then, iO has been used to advance numerous sub-areas of cryptography. While indistinguishability obfuscation is a general purpose obfuscation scheme, several obfuscators for specific functionalities have been considered. In particular, special attention has been given to the obfuscation of so-called \\emph{point functions} that return zero everywhere, except for a single point $\\alpha$. A strong variant is point obfuscation with auxiliary input (AIPO), which allows an adversary to learn some non-trivial auxiliary information about the obfuscated point $\\alpha$ (Goldwasser, Tauman-Kalai; FOCS, 2005).

Multi-bit point functions are a strengthening of point functions, where on $\\alpha$, the point function returns a string $\\beta$ instead of $1$. Multi-bit point functions with auxiliary input (MB-AIPO) have been constructed by Canetti and Bitansky (Crypto 2010) and have been used by Matsuda and Hanaoka (TCC 2014) to construct CCA-secure public-key encryption schemes and by and Bitansky and Paneth (TCC 2012) to construct three-round weak zero-knowledge protocols for NP.

In this paper we present both positive and negative results. We show that if indistinguishability obfuscation exists, then MB-AIPO does not. Towards this goal, we build on techniques by Brzuska, Farshim and Mittelbach (Crypto 2014) who use indistinguishability obfuscation as a means of attacking a large class of assumptions from the Universal Computational Extractor framework (Bellare et al; Crypto 2013). On the positive side we introduce a weak version of MB-AIPO which we deem to be outside the reach of our impossibility result. We prove that this weak version of MB-AIPO suffices to construct a public-key encryption scheme that is secure even if the adversary can learn an arbitrary leakage function of the secret key, as long as the secret key remains computationally hidden. Thereby, we strengthen a result by Canetti et al. (TCC 2010) that showed a similar connection in the symmetric-key setting.

09:17 [Pub][ePrint]

In this paper we study the security of hash-based MAC algorithms (such as HMAC and NMAC) above the birthday bound. Up to the birthday bound, HMAC and NMAC are proven to be secure under reasonable assumptions on the hash function. On the other hand, if an $n$-bit MAC is built from a hash function with a $l$-bit state ($l \\ge n$), there is a well-known existential forgery attack with complexity $2^{l/2}$. However, the remaining security after $2^{l/2}$ computations is not well understood. In particular it is widely assumed that if the underlying hash function is sound, then a generic universal forgery attack should still require $2^{n}$ computations and some distinguishing (e.g. distinguishing-H but not distinguishing-R) and state-recovery attacks should still require $2^{l}$ (or $2^k$ if $k < l$) computations.

In this work, we show that above the birthday bound, hash-based MACs offer significantly less security than previously believed. Our main result is a generic distinguishing-H and state-recovery attack against hash-based MACs with a complexity of only $\\tilde O(2^{l/2})$. In addition, we show a key-recovery attack with complexity $\\tilde O(2^{3l/4})$ against HMAC used with a hash functions with an internal checksum, such as GOST. This surprising result shows that the use of a checksum might actually weaken a hash function when used in a MAC. We stress that our attacks are generic, and they are in fact more efficient than some previous attacks proposed on MACs instantiated with concrete hash functions.

We use techniques similar to the cycle-detection technique proposed by Peyrin et al. at Asiacrypt 2012 to attack HMAC in the related-key model. However, our attacks works in the single-key model for both HMAC and NMAC, and without restriction on the key size.

09:17 [Pub][ePrint]

We present a symmetric-key predicate-only functional encryption system, SP-FE, which supports functionality for regular languages describe by deterministic finite automata. In SP-FE, a data owner can encrypt a string of symbols as encrypted symbols for matching. Later, the data owner can generate predicate tokens of the transitions in a deterministic finite automaton. The server with these tokens can decrypt a sequence of encrypted symbols correctly and transfer from one state to another accordingly. If the final state belongs to the set of accept states, the server takes assigned operations or returns the corresponding encrypted data. We have proven SP-FE preserves both plaintext privacy and predicate privacy through security analysis and security games. To achieve predicate privacy, we put bounds on the length of a string and the number of states of a DFA. Due to these restrictions, SP-FE can capture only finite languages. Finally, we present the performance analysis of SP-FE and mention possible future work.

06:17 [Forum]

This looks like a paper of "how to tranform secret key algorithm into public key one using obfuscation". Take AES-X, and obfuscate. From: 2014-02-06 05:46:14 (UTC)

2014-05-30
15:17 [Pub][ePrint]

This paper presents a new projective coordinate system and new explicit algorithms which together boost the speed of arithmetic in the divisor class group of genus 2 curves. The proposed formulas generalise the use of Jacobian coordinates on elliptic curves, and their application improves the speed of performing cryptographic scalar multiplications in Jacobians of genus 2 curves over prime fields by an approximate factor of 1.25x. For example, on a single core of an Intel Core i7-3770M (Ivy Bridge), we show that replacing the previous best formulas with our new set improves the cost of generic scalar multiplications from 243,000 to 195,000 cycles, and drops the cost of specialised GLV-style scalar multiplications from 166,000 to 129,000 cycles.

15:17 [Pub][ePrint]

We propose Chaskey: a very efficient Message Authentication Code (MAC) algorithm for 32-bit microcontrollers. It is intended for applications that require 128-bit security, yet cannot implement standard MAC algorithms because of stringent requirements on speed, energy consumption, or code size. Chaskey is a permutation-based MAC algorithm that uses the Addition-Rotation-XOR (ARX) design methodology. We formally prove that Chaskey is secure in the standard model, based on the security of an underlying Even-Mansour block cipher. Chaskey is designed to perform well on a wide range of 32-bit microcontrollers. Our benchmarks show that on the ARM Cortex-M3/M4, our Chaskey implementation reaches a speed of 7.0 cycles/byte, compared to 89.4 cycles/byte for AES-128-CMAC. For the ARM Cortex-M0, our benchmark results give 16.9 cycles/byte and 136.5 cycles/byte for Chaskey and AES-128-CMAC respectively.

15:17 [Pub][ePrint]

We present a new method for building pairs of HFE polynomials of high degree, such that the map constructed with such a pair is easy to invert. The inversion is accomplished using a low degree polynomial of Hamming weight three, which is derived from a special reduction via Hamming weight three polynomials produced by these two HFE polynomials. This allows us to build new candidates for multivariate trapdoor functions in which we use the pair of HFE polynomials to fabricate the core map. We performed the security analysis for the case where the base field is $GF(2)$ and showed that these new trapdoor functions have high degrees of regularity, and therefore they are secure against the direct algebraic attack. We also give theoretical arguments to show that these new trapdoor functions over $GF(2)$ are secure against the MinRank attack as well.