International Association for Cryptologic Research

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2012-12-27
19:17 [Pub][ePrint]

Many index calculus algorithms generate multiplicative relations

between smoothness basis elements by using a process called {\\it

Sieving}. This process allows to filter potential candidate

relations very quickly, without spending too much time to consider bad

candidates. However, from an asymptotic point of view, there is not

much difference between sieving and straightforward testing of

candidates. The reason is that even when sieving, some small amount

time is spend for each bad candidates. Thus, asymptotically, the total

number of candidates contributes to the complexity.

In this paper, we introduce a new technique: {\\it Pinpointing}, which

allows us to construct multiplicate relations much faster, thus

reducing the asymptotic complexity of relations\'

construction. Unfortunately, we only know how to implement this

technique for finite fields which contain a medium-sized

subfield. When applicable, this method improves the asymptotic

complexity of the index calculus algorithm in the cases where the sieving

phase dominates. In practice, it gives a very interesting boost to the

performance of state-of-the-art algorithms. We illustrate the

feasability of the method with a discrete logarithm record in a

medium prime finite field of total size 1175~bits.

19:17 [Pub][ePrint]

The Fiat-Shamir paradigm was proposed as a way to remove interaction from 3-round proof of knowledge protocols and derive secure signature schemes. This generic transformation leads to very efficient schemes and has thus grown quite popular. However, this transformation is proven secure only in the random oracle model. In FOCS 2003, Goldwasser and Kalai showed that this transformation is provably insecure in the standard model by presenting a counterexample of a 3-round protocol, the Fiat-Shamir transformation of which is (although provably secure in the random oracle model) insecure in the standard model, thus showing that the random oracle is uninstantiable. In particular, for every hash function that is used to replace the random oracle, the resulting signature scheme is existentially forgeable. This result was shown by relying on the non-black-box techniques of Barak (FOCS 2001).

An alternative to the Fiat-Shamir paradigm was proposed by Fischlin in Crypto 2005. Fischlin\\rq{}s transformation can be applied to any so called 3-round Fiat-Shamir proof of knowledge\\rq{}\\rq{} and can be used to derive non-interactive zero-knowledge proofs of knowledge as well as signature schemes. An attractive property of this transformation is that it provides online extractability (i.e., the extractor works without having to rewind the prover). Fischlin remarks that in comparison to the Fiat-Shamir transformation, his construction tries to decouple the hash function from the protocol flow\" and hence, the counterexample in the work of Goldwaaser and Kalai does not seem to carry over to this setting.

In this work, we show a counterexample to the Fischlin\'s transformation. In particular, we construct a 3-round Fiat-Shamir proof of knowledge (on which Fischlin\'s transformation is applicable), and then, present an adversary against both - the soundness of the resulting non-interactive zero-knowledge, as well as the unforegeability of the resulting signature scheme. Our attacks are successful except with negligible probability for any hash function, that is used to instantiate the random oracle, provided that there is an apriori (polynomial) bound on the running time of the hash function. By choosing the right bound, secure instantiation of Fischlin transformation with most practical cryptographic hash functions can be ruled out.

The techniques used in our work are quite unrelated to the ones used in the work of Goldwasser and Kalai. Our primary technique is to bind the protocol flow with the hash function if the code of the hash function is available. We believe that our ideas are of independent interest and maybe applicable in other related settings.

19:17 [Pub][ePrint]

A common method for increasing the usability and uplifting the security of pseudorandom function families (PRFs) is to hash\" the inputs into a smaller domain before applying the PRF. This approach, known as Levin\'s trick\", is used to achieve PRF domain extension\" (using a short, e.g., fixed, input length PRF to get a variable-length PRF), and more recently to transform non-adaptive PRFs to adaptive ones. Such reductions, however, are vulnerable to a birthday attack\": after $\\sqrt{|\\mathcal U|}$ queries to the resulting PRF, where $\\mathcal U$ being the hash function range, a collision (i.e., two distinct inputs have the same hash value) happens with high probability. As a consequence, the resulting PRF is insecure against an attacker making this number of queries.

In this work we show how to go beyond the birthday attack barrier, by replacing the above simple hashing approach with a variant of cuckoo hashing --- a hashing paradigm typically used for resolving hash collisions in a table, by using two hash functions and two tables, and cleverly assigning each element into one of the two tables. We use this approach to obtain: (i) A domain extension method that requires just two calls to the original PRF, can withstand as many queries as the original domain size and has a distinguishing probability that is exponentially small in the non cryptographic work. (ii) A security-preserving reduction from non-adaptive to adaptive PRFs.

19:17 [Pub][ePrint]

In order to prevent the SPA (Simple Power Analysis) attack against modular exponentiation algorithms, a multiply-always implementation is generally used. Witteman et al. introduced in \\cite{WI} a new cross-correlation power analysis attack against the multiply-always implementation. We suggest two new algorithms, resistant to this attack and also to other known attacks.

The first algorithm is an alternative approach to exponentiation algorithms used in cryptography, which usually receive as an input some representation (e.g. binary) of the exponent. In our approach both the exponent and the result are functions (not necessarily easily invertible) of the exponentiation algorithm input. We show that this approach can have a good performance and that it is also resistant to several known attacks, especially to the cross-correlation power analysis. It is particularly relevant for cryptographic schemes in which the private exponent can be chosen arbitrarily.

Another exponentiation algorithm that we present here may be preferable for use with RSA in certain settings. It is resistant to the cross-correlation power analysis attack, C safe error attack, and other attacks; although it involves squaring operations.

2012-12-23
16:51 [Job][New]

Description and Responsibilities:

Embedded platforms comprising SoCs, i.e., Tablets and Smartphones, have become important players in the products offered by Intel. One of the key component of SoC-based platforms is the baseband processor. The goal of this internship is to investigate the threats posed to/by baseband to a phone platform. In this position, you will be a member of the Security Center of Excellence (SeCoE) in the Intel Architecture Group to identify security vulnerabilities in the architecture, design and implementation of future SoC-based products. During this internship, your responsibilities will include but not be limited to:

- Performing the threat analysis of a current baseband processor,

- Driving the investigation of Intel baseband software and hardware for evaluating their security features,

- Researching literature from conferences (e.g.; BlackHat, PacSec) and technical forums to find out relevant attacks.

You will also be developing software and/or hardware solutions to hack the platform.

Qualifications:

You must be currently enrolled in a Ph.D or a Master\\\'s degree program in Computer Science, Computer Engineering or Electrical Engineering or equivalent discipline with knowledge of computer security, applied cryptography, computer architecture, hardware design (SystemVerilog or VHDL) and programming (C, Assembly). Demonstrable results in academic projects involving the above areas are essential. Exposure to security threat analysis and knowledge of existing attacks on security of System-on-chip (SoC) based platforms (For example, smartphones) would be highly desirable. Experience with hacking hardware (programming FPGAs, Flash chips etc) will be a plus. Behavioral skills include: solid analytical skills, willingness to tackle complex problems, attention to detail, self-driven, excellent written and verbal communication skills.

16:48 [Event][New]

Submission: 1 February 2013
From June 25 to June 28

16:47 [Event][New]

Submission: 1 March 2013
From September 2 to September 6
Location: Regensburg, Germany

16:47 [Event][New]

Submission: 13 May 2013
From November 18 to November 20
Location: Okinawa, Japan

16:46 [Job][New]

The Electronic Health Information Laboratory (EHIL) at the Children\\\'s Hospital of Eastern Ontario (CHEO) Research Institute is looking for a Post?Doctoral Fellow responsible for research and development of secure multi?party computation and privacy preserving protocols for applications in the area of health information and medical research.

Candidates should have a Ph.D. in Computer Science, Computer Engineering, Mathematics, Engineering, or a related field, and a strong research track record with journal or conference publications in secure computation and/or privacy preserving statistics and data?mining.

2012-12-19
19:17 [Pub][ePrint]

This paper studies the distinctness of primitive sequences over Z/(M) modulo 2, where M is an odd integer that is composite and square-free, and Z/(M) is the integer residue ring modulo M. A new sufficient condition is given for ensuring that primitive sequences generated by a primitive polynomial f(x) over Z/(M) are pairwise distinct modulo 2. Such result improves a recent result obtained in our previous paper [27] and consequently the set of primitive sequences over Z/(M) that can be proven to be distinct modulo 2 is greatly enlarged.

19:17 [Pub][ePrint]

The Random Oracle Model, introduced by Bellare and Rogaway, provides a method to heuristically argue about the security of cryptographic primitives and protocols. The basis of this heuristic is that secure hash functions are close enough to random functions in their behavior, and so, a primitive that is secure using a random function should continue to remain secure even when the random function is replaced by a real hash function. In the security proof, this setting is realized by modeling the hash function as a random oracle. However, this approach in particular also enables any reduction, reducing a hard problem to the existence of an adversary, to \\emph{observe} the queries the adversary makes to its random oracle and to \\emph{program} the responses that the oracle provides to these queries. While, the issue of programmability of query responses has received a lot of attention in the literature, to the best of our knowledge, observability of the adversary\'s queries has not been identified as an artificial artefact of the Random Oracle Model. In this work, we study the security of several popular schemes when the security reduction cannot observe\'\' the adversary\'s queries to the random oracle, but can (possibly) continue to program\'\' the query responses. We first show that RSA-PFDH and Schnorr\'s signatures continue to remain secure when the security reduction is non observing (NO reductions), which is not surprising as their proofs in the random oracle model rely on programmability. We also provide two example schemes, namely, Fischlin\'s NIZK-PoK \\cite{Fischlin05} and non interactive extractable commitment scheme, extractor algorithms of which seem to rely on observability in the random oracle model. While we prove that Fischlin\'s online extractors cannot exist when they are non observing, our extractable commitment scheme continues to be secure even when the extractors are non observing. We also introduce Non Observing Non Programming reductions which we believe are closest to standard model reductions.