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15:17 [Pub][ePrint] Centrally Banked Cryptocurrencies, by George Danezis and Sarah Meiklejohn

  Current cryptocurrencies, starting with Bitcoin, build a decentralized blockchain-based transaction ledger, maintained through proofs-of-work that also generate a monetary supply. Such decentralization has benefits, such as independence from national political control, but also significant limitations in terms of scalability and computational cost. We introduce RSCoin, a cryptocurrency framework in which central banks maintain complete control over the monetary supply, but rely on a distributed set of authorities, or mintettes, to prevent double-spending. While monetary policy is centralized, RSCoin still provides strong transparency and auditability guarantees. We demonstrate, both theoretically and experimentally, the benefits of a modest degree of centralization, such as the elimination of wasteful hashing and a scalable system for avoiding double-spending attacks.

09:17 [Pub][ePrint] Quantifying Location Privacy Leakage from Transaction Prices, by Arthur Gervais and Hubert Ritzdorf and Mario Lucic and Srdjan Capkun

  Large-scale datasets of consumer behavior might revolutionize the way we gain competitive advantages and increase our knowledge in the respective domains. At the same time, valuable datasets pose potential privacy risks that are difficult to foresee. In this paper we study the impact that the prices from consumers\' purchase histories have on the consumers\' location privacy. We show that using a small set of low-priced product prices from the consumers\' purchase histories, an adversary can determine the country, city, and local retail store where the transaction occurred with high confidence. Our paper demonstrates that even when the product category, precise time of purchase, and currency are removed from the consumers\' purchase history (e.g., for privacy reasons), information about the consumers\' location is leaked. The results are based on three independent datasets containing thousands of low-priced and frequently-bought consumer products. In addition, we show how to identify the local currency, given only the total price of a consumer purchase in a global currency (e.g., in Bitcoin). The results show the existence of location privacy risks when releasing consumer purchase histories. As such, the results highlight the need for systems that hide transaction details in consumer purchase histories.

09:17 [Pub][ePrint] Efficient Zero-Knowledge Proofs of Non-Algebraic Statements with Sublinear Amortized Cost, by Zhangxiang Hu and Payman Mohassel and Mike Rosulek

  We describe a zero-knowledge proof system in which a prover holds a large dataset $M$ and can repeatedly prove NP relations about that dataset. That is, for any (public) relation $R$ and $x$, the prover can prove that $\\exists w: R(M,x,w)=1$. After an initial setup phase (which depends only on $M$), each proof requires only a constant number of rounds and has communication/computation cost proportional to that of a {\\em random-access machine (RAM)} implementation of $R$, up to polylogarithmic factors. In particular, the cost per proof in many applications is sublinear in $|M|$. Additionally, the storage requirement between proofs for the verifier is constant.

02:01 [Event][New] CryptoBG*2015: CryptoBG*2015: Cryptology and Cyber Resilience

  Submission: 10 June 2015
Notification: 20 June 2015
From July 19 to July 26
Location: Oriahovitza, Bulgaria
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15:17 [Pub][ePrint] Cryptanalysis Of Dynamic ID Based Remote User Authentication Scheme With Key Agreement, by Sonam Devgan Kaul and Amit K. Awasthi

  In 2012, Wen and Li proposed a secure and robust dynamic identity based remote user authentication scheme with key agreement using smart cards. They claimed that their scheme is efficient and secure. But in this paper, we demonstrate that their scheme is completely insecure and vulnerable to various known attacks like offline and online password guessing attack, impersonation attack, server masquerading attack, denial of service attack and an insider attack. Also we point out that there are loopholes in password change phase and online secret renew phase which leads to the desynchronization between user and the server and even the legitimate user is rejected by the server. In addition, an adversary can easily generate the common session key of further transmission between user and the server. Thus the entire system collapses and authors claims are proven to be wrong and their scheme will not be secure and efficient for practical purpose.

15:17 [Pub][ePrint] Re-encryption, functional re-encryption, and multi-hop re-encryption: A framework for achieving obfuscation-based security and instantiations from lattices, by Nishanth Chandran and Melissa Chase and

  In this work we define multiple relaxations to the definition of correctness in secure obfuscation. While still remaining meaningful, these relaxations provide ways to obfuscate many primitives in a more direct and efficient way. In particular, we first show how to construct a secure obfuscator for the re-encryption primitive from the Decisional Learning with Errors (DLWE) assumption, without going through fully homomorphic encryption. This can be viewed as a meaningful way to trade correctness for efficiency.

Next, we show how our tools can be used to construct secure obfuscators for the functional re-encryption and multi-hop unidirectional re-encryption primitives. In the former case, we improve upon the efficiency of the only previously known construction that satisfies the stronger notion of collusion-resistant obfuscation (due to Chandran et al. - TCC 2012) and obtain a construction with input ciphertexts of constant length. In the latter case, we provide the first known obfuscation-based definition and construction; additionally, our scheme is the first scheme where the size of the ciphertexts does not grow with every hop.

15:17 [Pub][ePrint] Masking vs. Multiparty Computation: How Large is the Gap for AES?, by Vincent Grosso and François-Xavier Standaert and Sebastian Faust

  In this paper, we evaluate the performances of state-of-the-art higher-order masking schemes for the AES. Doing so, we pay a particular attention to the comparison between specialized solutions introduced exclusively as countermeasures against side-channel analysis, and a recent proposal by Roche and Prouff exploiting MultiParty Computation (MPC) techniques. We show that the additional security features this latter scheme provides (e.g. its glitch-freeness) comes at the cost of large performance overheads. We then study how exploiting standard optimization techniques from the MPC literature can be used to reduce this gap. In particular, we show that ``packed secret sharing\" based on a modified multiplication algorithm can speed up MPC-based masking when the order of the masking scheme increases. Eventually, we discuss the randomness requirements of masked implementations. For this purpose, we first show with information theoretic

arguments that the security guarantees of masking are only preserved if this randomness is uniform, and analyze the consequences of a deviation from this requirement. We then conclude the paper by including the cost of randomness generation in our performance evaluations. These results should help actual designers to choose a masking scheme based on security and performance~constraints.

15:17 [Pub][ePrint] Fault Tolerant Infective Countermeasure for AES, by Sikhar Patranabis and Abhishek Chakraborty and Debdeep Mukhopadhyay

  Infective countermeasures have been a promising class of fault attack countermeasures. However, they have been subjected to several attacks owing to lack of formal proofs of security and improper implementations. In this paper, we first provide a formal information theoretic proof of security for one of the most recently proposed infective countermeasures against DFA, under the assumption that the adversary does not change the flow sequence or skip any instruction. Subsequently, we identify weaknesses in the infection

mechanism of the countermeasure that could be exploited by attacks which change the flow sequence. We propose suitable randomizations to reduce the success probabilities of such attacks. Furthermore, we develop a fault tolerant implementation of the countermeasure using the x86 instruction set to make such attacks which attempt to change the control flow of the algorithm practically infeasible. All the claims have been validated by supporting simulations and real life experiments on a SASEBO-W platform. We also compare the performance and security provided by the proposed countermeasure against that provided by the existing scheme.

15:17 [Pub][ePrint] Cryptanalysis of the LSH and SHA-V Hash Functions, by Yonglin Hao and Hongbo Yu

  In this paper, we study the security of two hash function families LSH and SHA-V.

We find that the wide-pipe MD structural LSH hash functions do not apply the traditional feeding forward operation.

This structural weakness enables us to launch free-start collision and pseudo-preimage attacks on full-round LSH hash functions with negligible complexities.

In order to evaluate the quality of the LSH round functions, we launch 14-round boomerang attacks on LSH-512 and LSH-256 hash functions with complexities $2^{308}$ and $2^{242}$ respectively.

We verify the correctness of our boomerang attacks by giving practical 11-round boomerang quartets.

These boomerang results indicate that the round functions of LSH are well designed.

Based on our analysis, we stress that the adoption of the feeding forward operation should be essential to the LSH hash functions despite of their well designed round functions.

The PMD structural SHA-V parallelizes two SHA-1-like streams and each stream processes independent 512-bit message blocks.

This structure enable us to utilize the divide-and-conquer strategy to find preimages and collisions.

Our preimage attack can be applied to full-round SHA-V with time \\& memory complexities $O(2^{80})$.

Our trivial collision attacks also requires $O(2^{80})$ complexities but, utilizing existing results on SHA-1, we can find a SHA-V collision with a time complexity $O(2^{61})$ and a negligible memory complexity.

These results indicate that there are weaknesses in both the structure and the round function of SHA-V.

15:17 [Pub][ePrint] Powers of Subfield Polynomials and Algebraic Attacks on Word-Based Stream Ciphers, by Sondre R{\\o}njom

  In this paper we investigate univariate algebraic attacks on filter generators over extension fields $\\F_q=\\F_{2^n}$ with focus on the Welch-Gong (WG) family of stream ciphers. Our main contribution is to break WG-5, WG-7, WG-8 and WG-16 by combining results on the so-called spectral immunity (minimum distance of certain cyclic codes) with properties of the WG type stream cipher construction. The spectral immunity is the univariate analog of algebraic immunity and instead of measuring degree of multiples of a multivariate polynomial, it measures the minimum number of nonzero coefficients of a multiple of a univariate polynomial. Based on the structure of the general WG-construction, we deduce better bounds for the spectral immunity and the univariate analog of algebraic attacks.

00:49 [Event][New] SPiCy: 1st Workshop on Security and Privacy in Cybermatics

  Submission: 3 July 2015
Notification: 3 August 2015
From September 30 to September 30
Location: Florence, Italy
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