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21:17 [Pub][ePrint] Key Homomorphic PRFs and Their Applications, by Dan Boneh and Kevin Lewi and Hart Montgomery and Ananth Raghunathan

  A pseudorandom function F : K x X -> Y is said to be key homomorphic if given F(k1, x) and F(k2, x) there is an efficient algorithm to compute F(k1 xor k2, x), where xor denotes a group

operation on k1 and k2 such as xor. Key homomorphic PRFs are natural objects to study and have a number of interesting applications: they can simplify the process of rotating encryption keys for encrypted data stored in the cloud, they give one round distributed PRFs, and they can be the basis of a symmetric-key proxy re-encryption scheme. Until now all known constructions

for key homomorphic PRFs were only proven secure in the random oracle model. We construct the first provably secure key homomorphic PRFs in the standard model. Our main construction

is based on the learning with errors (LWE) problem. In the proof of security we need a variant of LWE where query points are non-uniform and we show that this variant is as hard as the standard LWE. We also construct key homomorphic PRFs based on the decision linear assumption in groups with an l-linear map. We leave as an open problem the question of constructing standard model key homomorphic PRFs from more general assumptions.

21:17 [Pub][ePrint] Tighter, faster, simpler side-channel security evaluations beyond computing power, by Daniel J. Bernstein and Tanja Lange and Christine van Vredendaal

  A Eurocrypt 2013 paper \"Security evaluations beyond computing power: How to analyze side-channel attacks you cannot mount?\" by Veyrat-Charvillon, Gérard, and Standaert proposed a \"Rank Estimation Algorithm\" (REA) to estimate the difficulty of finding a secret key given side-channel information from independent subkeys, such as the 16 key bytes in AES-128 or the 32 key bytes in AES-256. The lower and upper bounds produced by the algorithm are far apart for most key ranks. The algorithm can produce tighter bounds but then becomes exponentially slower; it also becomes exponentially slower as the number of subkeys increases.

This paper introduces two better algorithms for the same problem. The first, the \"Extended Rank Estimation Algorithm\" (EREA), is an extension of REA using statistical sampling as a second step to increase the speed of tightening the bounds on the rank. The second, the \"Polynomial Rank Outlining Algorithm\" (PRO), is a new approach to computing the rank. PRO can handle a much larger number of subkeys efficiently, is easy to implement in a computer-algebra system such as Sage, and produces much tighter bounds than REA in less time.

21:17 [Pub][ePrint] Towards Understanding the Known-Key Security of Block Ciphers, by Elena Andreeva, Andrey Bogdanov, and Bart Mennink

  Known-key distinguishers for block ciphers were proposed by Knudsen and Rijmen at ASIACRYPT 2007 and have been a major research topic in cryptanalysis since then. A formalization of known-key attacks in general is known to be difficult. In this paper, we tackle this problem for the case of block ciphers based on ideal components such as random permutations and random functions as well as propose new generic known-key attacks on generalized Feistel ciphers. We introduce the notion of known-key indifferentiability to capture the security of such block ciphers under a known key. To show its meaningfulness, we prove that the known-key attacks on block ciphers with ideal primitives to date violate security under known-key indifferentiability. On the other hand, to demonstrate its constructiveness, we prove the balanced Feistel cipher with random functions and the multiple Even-Mansour cipher with random permutations known-key indifferentiable for a sufficient number of rounds. We note that known-key indifferentiability is more quickly and tightly attained by multiple Even-Mansour which puts it forward as a construction provably secure against known-key attacks.

21:17 [Pub][ePrint] New Distinguishers for Reduced Round Trivium and Trivia-SC using Cube Testers, by Anubhab Baksi and Subhamoy Maitra and Santanu Sarkar

  In this paper we experiment with cube testers on reduced round Trivium that can act as a distinguisher. Using heuristics, we obtain several distinguishers for Trivium running more than 800 rounds (maximum 829) with cube sizes not exceeding 27. In the process, we also exploit state biases that has not been explored before. Further, we apply our techniques to analyse Trivia-SC, a stream cipher proposed by modifying the parameters of Trivium and used as a building block for TriviA-ck (an AEAD scheme, which is submitted to the ongoing CAESAR

competition). We obtain distinguishers till 900 rounds of Trivia-SC with a cube size of 21 only and our results refute certain claims made by the designers. These are the best results reported so far, though our work does not affect the security claims for the ciphers with full initialization rounds, namely 1152.

21:17 [Pub][ePrint] Privacy and Access Control for Outsourced Personal Records, by Matteo Maffei and Giulio Malavolta and Manuel Reinert and Dominique Schröder

  Cloud storage has rapidly become a cornerstone of many IT infrastructures, constituting a seamless solution for the backup, synchronization, and sharing of large amounts of data. Putting user data in the direct control of cloud service providers, however, raises security and privacy concerns related to the integrity of outsourced data, the accidental or intentional leakage of sensitive information, the profiling of user activities and so on. Furthermore, even if the cloud provider is trusted, users having access to outsourced files might be malicious and misbehave. These concerns are particularly serious in sensitive applications like personal health records and credit score systems.

To tackle this problem, we present GORAM, a cryptographic system that protects the secrecy and integrity of outsourced data with respect to both an untrusted server and malicious clients, guarantees the anonymity and unlinkability of accesses to such data, and allows the data owner to share outsourced data with other clients, selectively granting them read and write permissions. GORAM is the first system to achieve such a wide range of security and privacy properties for outsourced storage. In the process of designing an efficient construction, we developed two new, generally applicable cryptographic schemes, namely, batched zero-knowledge proofs of shuffle and an accountability technique based on chameleon signatures, which we consider of independent interest. We implemented GORAM in Amazon Elastic Compute Cloud (EC2) and ran a performance evaluation demonstrating the scalability and efficiency of our construction.

21:17 [Pub][ePrint] Bitwise Linear Mappings with Good Cryptographic Properties and Efficient Implementation, by S. M. Dehnavi and A. Mahmoodi Rishakani and M. R. Mirzaee Shamsabad

  Linear mappings are crucial components of symmetric ciphers. A special type of linear mappings are

(0,1)-matrices which have been used in symmetric ciphers such as ARIA, E2 and Camellia as diffusion

layers with efficient implementation. Bitwise linear maps are also used in symmetric ciphers such as

SHA family of hash functions and HC family of stream ciphers. In this article, we investigate a special

kind of linear mappings: based upon this study, we propose several linear mappings with only XOR and

rotation operations. The corresponding matrices of these mappings can be used in either the former case

as (0,1)-matrices of maximal branch number or in the latter case as linear mappings with good cryptographic

properties. The proposed mappings and their corresponding matrices can be efficiently implemented both

in software and hardware.

04:47 [Job][New] • Research Fellow/Postdoctoral Researcher in Applied Crypto, University of Auckland, Auckland, New Zealand

  The Computer Science department at the University of Auckland seeks a Research Fellow/Postdoctoral Researcher to join the cloud security team led by Dr Giovanni Russello.

This research will take place in a new MBIE-funded Cyber Security STRATUS (Security Technologies Returning Accountability, Transparency and User-centric Services to the Cloud) project and will be in collaboration with University of Waikato, UniTech, the Cloud Security Alliance, and several New Zealand-based industrial partners ( The aim is to research novel yet practical cloud security tools to be adopted by the industry partners.

The research conducted by the University of Auckland’s team will focus on applied cryptography for retrieval and processing of encrypted data in outsourced and untrusted environments. This involves a substantial program of research to develop, implement and apply to industrial case studies.

This is a full time post for a fixed-term of 2 years. Salary starts at 74,000 NZD per annum.

Applicants should have a PhD in computer science in a relevant field (cloud security with emphasis on crypto solutions) a demonstrable research interest in the area of applied crypto with emphasis in homomorphic encryption for encrypted data processing and retrieval focusing on cloud computing, and experience in designing, analysing, and efficiently implement novel crypto algorithms. Previous experience in the area of big data with emphasis on privacy/confidentiality would be advantageous.

Host Institution: The University of Auckland is New Zealand\'s leading university. In the 2013 QS survey, the Computer Science Department ranked 38th. The University of Auckland has a strong international focus and is the only New Zealand member of Universitas 21 and the Association of Pacific Rim Universities - international consortia of research-led universities. Auckland is ranked third out of 221 world citie

22:39 [Event][New] ISC: The Eighteenth Information Security Conference

  Submission: 27 April 2015
Notification: 12 June 2015
From September 9 to September 11
Location: Trondheim, Norway
More Information:

09:17 [Pub][ePrint] GCM Security Bounds Reconsidered, by Yuichi Niwa and Keisuke Ohashi and Kazuhiko Minematsu and Tetsu Iwata

  A constant of $2^{22}$ appears in the security bounds of the Galois/Counter Mode of Operation, GCM. In this paper, we first develop an algorithm to generate nonces that have a high counter-collision probability. We show concrete examples of nonces with the counter-collision probability of about $2^{20.75}/2^{128}$. This shows that the constant in the security bounds, $2^{22}$, cannot be made smaller than $2^{19.74}$ if the proof relies on ``the sum bound.\'\' We next show that it is possible to avoid using the sum bound, leading to improved security bounds of GCM. One of our improvements shows that the constant of $2^{22}$ can be reduced to 32.

09:17 [Pub][ePrint] A revocable anonymity in Tor, by Amadou Moctar Kane

  This new protocol is based on the idea of introducing a revocable anonymity in Tor, which was presented in our recent paper entitled \"Another Tor is possible\". Compared to that previous paper, this present scheme simplify the first protocol and reduce the power of the directory server, while maintaining the ability for the Tor community, to break the anonymity of a sender in case of misconduct.

We also take the opportunity of this paper, to appeal the majors internet companies, to help in the creation of a responsible Tor network (without pedophiles, spies, ....), by mixing billions of data flowing through their networks with those of Tor.

09:17 [Pub][ePrint] Quasi-Adaptive NIZK for Linear Subspaces Revisited, by Eike Kiltz and Hoeteck Wee

  Non-interactive zero-knowledge (NIZK) proofs for algebraic relations in a group, such as the Groth-Sahai proofs, are an extremely powerful tool in pairing-based cryptography. A series of recent works focused on obtaining very efficient NIZK proofs for linear spaces in a weaker quasi-adaptive model. We revisit recent quasi-adaptive NIZK constructions, providing clean, simple, and improved constructions via a conceptually different approach inspired by recent developments in identity-based encryption. We then extend our techniques also to linearly homomorphic structure-preserving signatures, an object both of independent interest and with many applications.