A sequence of recent works have constructed constant-size quasi-adaptive (QA) NIZK arguments of membership in linear subspaces ofĜ m , whereĜ is a group equipped with a bilinear map e :Ĝ ×Ȟ → T. Although applicable to any bilinear group, these techniques are less useful in the asymmetric case. For example, Jutla and Roy (Crypto 2014) show how to do QA aggregation of Groth-Sahai proofs, but the types of equations which can be aggregated are more restricted in the asymmetric setting. Furthermore, there are natural statements which cannot be expressed as membership in linear subspaces, for example the satisfiability of quadratic equations. In this paper we develop specific techniques for asymmetric groups. We introduce a new computational assumption, under which we can recover all the aggregation results of Groth-Sahai proofs known in the symmetric setting. We adapt the arguments of membership in linear spaces of G m to linear subspaces ofĜ m ×Ȟ n . In particular, we give a constantsize argument that two sets of Groth-Sahai commitments, defined over different groupsĜ,Ȟ, open to the same scalars in Zq, a useful tool to prove satisfiability of quadratic equations in Zq. We then use one of the arguments for subspaces inĜ m ×Ȟ n and develop new techniques to give constant-size QA-NIZK proofs that a commitment opens to a bit-string. To the best of our knowledge, these are the first constant-size proofs for quadratic equations in Zq under standard and falsifiable assumptions. As a result, we obtain improved threshold Groth-Sahai proofs for pairing product equations, ring signatures, proofs of membership in a list, and various types of signature schemes.

We revisit the problem of anonymous communication, in which users wish to send messages to each other without revealing their identities. We propose a novel framework to organize and compare anonymity definitions. In this framework, we present simple and practical definitions for anonymous channels in the context of computational indistinguishability. The notions seem to capture the intuitive properties of several types of anonymous channels (Pfitzmann and Köhntopp 2001) (eg. sender anonymity and unlinkability). We justify these notions by showing they naturally capture practical scenarios where information is unavoidably leaked in the system. Then, we compare the notions and we show they form a natural hierarchy for which we exhibit non-trivial implications. In particular, we show how to implement stronger notions from weaker ones using cryptography and dummy traffic -in a provably optimal way. With these tools, we revisit the security of previous anonymous channels protocols, in particular constructions based on broadcast networks (Blaze et al. 2003), anonymous broadcast (Chaum 1981), and mix networks (Groth 2003, Nguyen et al. 2004). Our results give generic, optimal constructions to transform known protocols into new ones that achieve the strongest notions of anonymity.

Abstract. We study Pseudorandom Number Generators (PRNGs) as used in practice. We first give a general security framework for PRNGs, incorporating the attacks that users are typically concerned about. We then analyze the most popular ones, including the ANSI X9.17 PRNG and the FIPS 186 PRNG. Our results also suggest ways in which these PRNGs can be made more efficient and more secure.

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Phishing email fraud has been considered as one of the main cyber-threats over the last years. Its development has been closely related to social engineering techniques, where different fraud strategies are used to deceit a naïve email user. In this work, a latent semantic analysis and text mining methodology is proposed for the characterisation of such strategies, and further classification using supervised learning algorithms. Results obtained showed that the feature set obtained in this work is competitive against previous phishing feature extraction methodologies, achieving promising results over different benchmark machine learning classification techniques.