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21:17 [Pub][ePrint] Phasing: Private Set Intersection using Permutation-based Hashing, by Benny Pinkas and Thomas Schneider and Gil Segev and Michael Zohner

  Private Set Intersection (PSI) allows two parties to compute the intersection of private sets while revealing nothing more than the intersection itself. PSI needs to be applied to large data sets in scenarios such as measurement of ad conversion rates, data sharing, or contact discovery. Existing PSI protocols do not scale up well, and therefore some applications use insecure solutions instead.

We describe a new approach for designing PSI protocols based on permutation-based hashing, which enables to reduce the length of items mapped to bins while ensuring that no collisions occur. We denote this approach as Phasing, for Permutation-based Hashing Set Intersection. Phasing can dramatically improve the performance

of PSI protocols whose overhead depends on the length of the representations of input items.

We apply Phasing to design a new approach for circuit-based PSI protocols. The resulting protocol is up to 5 times faster than the previously best Sort-Compare-Shuffle circuit of Huang et al. (NDSS 2012). We also apply Phasing to the OT-based PSI protocol of Pinkas et

al. (USENIX Security 2014), which is the fastest PSI protocol to date. Together with additional improvements that reduce the computation complexity by a logarithmic factor, the resulting protocol improves run-time by a factor of up to 20 and can also have better communication overhead than the previously best PSI protocol in that respect. The new protocol is only moderately less efficient

than an insecure PSI protocol that is currently used by real-world applications, and is therefore the first secure PSI protocol that is scalable to the demands and the constraints of current real-world settings.

21:17 [Pub][ePrint] Microcash: Efficient Off-Line Small Payments, by Chris Pavlovski and Colin Boyd

  An off-line electronic cash scheme is proposed that is suitable for

small payments. The approach is innovative, in that each coin may be

efficiently verified by the same or different merchants during payment. The scheme relies on a batch signature technique to efficiently sign and verify individually spent coins; coins may also be deposited in batch manner. The scheme outlined differs considerably from conventional micropayments schemes by servicing a number of cash-like properties, such as off-line processing, detection of double spent coins, and ability to spend at different merchants. Additionally, the scheme eliminates a number of processing overheads that are apparent to some existing micropayment schemes.

21:17 [Pub][ePrint] Analyzing Constructions for key-alternating Pseudorandom Functions with Applications to Stream Cipher Operation Modes, by Matthias Krause

  In the last years, much research work has been invested into the security analysis of key alternating ciphers in the random oracle model. These are pseudorandom permutations (PRPs), sometimes also called iterated Even-Mansour ciphers, which are defined by alternatingly adding $n$-bit sub-keys $k_i$ and calling public $n$-bit permutations $P_i$. Besides the fact, that results of this kind concern the fundamental questions of understanding the nature of pseudorandomness, a practical motivation for this study is that many modern block cipher designs correspond exactly to variants of iterated Even-Mansour ciphers.

In this paper, we study similar construction for pseudorandom functions (PRFs), where additionally the access to a public $n$-bit (one-way) function $F$ is allowed. In particular, we show a sharp $n/2$-security bound for the simplest possible construction $F(x\\oplus k)$ and a sharp $2/3\\cdot n$-bound for the $FP(1)$-construction $F(P(x\\oplus k)\\oplus k)$, both in the random oracle model. The latter result contrasts with a sharp bound of the same order for $P(P(x\\oplus k)\\oplus \\pi(k))\\oplus k$, recently proved by Chen et. al.

One practical motivation for our research is due to the fact that operation modes of key stream generator based (KSG-based) stream ciphers can be modeled in a very straightforward way by FP-constructions. Our research shows a way to save KSG inner state length by using operation modes, which yield provable security beyond the birthday bound against time-space-data tradeoff attacks. For instance, we demonstrate that a slight change in the operation mode of the Bluetooth cipher (adding the session key twice in the initialization phase) raises the security w.r.t. to generic time-space-data tradeoff attacks from $n/2$ to $2/3\\cdot n$, where $n$ denotes the KSG inner state length.

21:17 [Pub][ePrint] A Novel Cyberspace-Oriented Access Control Model, by Fenghua Li and Yanchao Wang and Rongna Xie and Fangfang Shan and Jinbo Xiong

  With the developments of mobile communication, networks and information technology, many new information service patterns and dissemination modes emerge with some security and privacy threats in access control, i.e., the ownership of data is separated from the administration of them, secondary/mutiple information distribution etc. Existing access control models, which are always proposed for some specific scenarios, are hardly to achieve fine-grained and adaptive access control. In this paper, we propose a novel Cyberspace-oriented Access Control model, termed as CoAC, which avoids the aforementioned threats by comprehensively considering some vital factors, such as the access requesting entity, general tense, access point, resource, device, networks, internet-based interactive graph and chain of resource transmission. By appropriately adjusting these factors, CoAC covers most of typical access control models and fulfills the requirements of new information service patterns and dissemination modes. We also present the administrative model of our proposed CoAC model and formally describe the administrative functions and methods used in the administrative model by utilizing Z-notation. Our CoAC is flexible and scalable, it can be further refined and expanded to figure out new opportunities and challenges in the upcoming access control techniques.

21:17 [Pub][ePrint] An Efficient Many-Core Architecture for Elliptic Curve Cryptography Security Assessment, by Marco Indaco and Fabio Lauri and Andrea Miele and Pascal Trotta

  Elliptic Curve Cryptography (ECC) is a popular tool to construct public-key crypto-systems.

The security of ECC is based on the hardness of the elliptic curve discrete logarithm problem (ECDLP).

Implementing and analyzing the performance of the best known methods to solve the ECDLP is useful to assess the security of ECC and choose security parameters in practice.

We present a novel many-core hardware architecture implementing the parallel version of Pollard\'s rho algorithm

to solve the ECDLP. This architecture results in a speed-up of almost 300% compared to the state of the art and we use it to estimate the monetary cost of solving the Certicom ECCp-131 challenge using FPGAs.

21:17 [Pub][ePrint] Polynomial time reduction from approximate shortest vector problem to principle ideal probelm for lattices in cyclotomic rings, by Hao Chen

  Many cryptographic schemes have been established based on the hardness of lattice problems. For the asymptotic efficiency, ideal lattices

in the ring of cyclotomic integers are suggested to be used in most such schemes. On the other hand in computational algebraic number theory one of the main problem

is called principle ideal problem (PIP). Its goal is to find a generators of any principle ideal in the ring of algebraic integers in any number field. In this paper we establish a polynomial time reduction from approximate shortest lattice vector problem for principle ideal lattices to their PIP\'s in many cyclotomic integer rings. Combining with the polynomial time quantum algorithm for PIP of arbitrary number fields, this implies that some approximate SVP problem for principle ideal lattices within a polynomial factor in some cyclotomic integer rings can be solved by polynomial time quantum algorithm.

21:17 [Pub][ePrint] Very-efficient simulatable flipping of many coins into a well, by Luís T. A. N. Brandão

  Secure two-party parallel coin-flipping is a cryptographic functionality that allows two mutually distrustful parties to agree on a common random bit-string of a certain target length. In coin-flipping into-a-well, one party learns the bit-string and then decides whether to abort or to allow the other party to learn it. It is well known that this functionality can be securely achieved in the ideal/real simulation paradigm, using commitment schemes that are simultaneously extractable (X) and equivocable (Q).

This paper presents two new constant-round simulatable coin-flipping protocols, based explicitly on one or a few X-commitments of short seeds and a Q-commitment of a short hash, independently of the large target length. A pseudo-random generator and a collision-resistant hash function are used to combine the separate X and Q properties (associated with short bit-strings) into a unified X&Q property amplified to the target length, thus amortizing the cost of the base commitments. In this way, the new protocols are significantly more efficient than an obvious batching or extension of coin-flippings designed (in the same security setting) for short bit-strings and based on inefficient X&Q commitments.

The first protocol, simulatable with rewinding, deviates from the traditional coin-flipping template in order to improve simulatability in case of unknown adversarial probabilities of abort, without having to use a X&Q commitment scheme. The second protocol, one-pass simulatable, derives from a new construction of a universally composable X&Q commitment scheme for large bit-strings, achieving communication-rate asymptotically close to 1. Besides the base X and Q commitments, the new commitment scheme only requires corresponding collision-resistant hashing, pseudo-random generation and application of a threshold erasure code. Alternative constructions found in recent work with comparable communication complexity require explicit use of oblivious transfer and use different encodings of the committed value.

21:17 [Pub][ePrint] Noise-free Symmetric Fully Homomorphic Encryption based on noncommutative rings, by Jing Li and Licheng Wang

  In this paper, we propose a noise-free symmetric fully homomorphic encryption (FHE) based on matrices over noncommutative rings. The scheme is secure against chosen plaintext attacks based on the factorization problem of matrices over noncommutative rings as well as the hardness of an overdefined system of multivariate polynomial equations over the given non-commutative algebraic structure. Meanwhile, the new proposal is efficient in terms of computational cost and the sizes of plaintext/ciphertext. On the basis of this framework, a verifiable FHE is proposed, where the receiver can check the validity of ciphertexts. Furthermore, any attacker fails to construct a valid ciphertext without making query of encryption oracle, then the verifiable FHE scheme maybe secure against non-adaptively chosen ciphertext attacks (IND-CCA1).

21:17 [Pub][ePrint] A New Partial Key Exposure Attack on Multi-power RSA, by Muhammed F. Esgin and Mehmet S. Kiraz and Osmanbey Uzunkol

  An important attack on multi-power RSA ($N=p^rq$) was introduced by Sarkar in 2014, by extending the small private exponent attack of Boneh and Durfee on classical RSA. In particular, he showed that $N$ can be factored efficiently for $r=2$ with private exponent $d$ satisfying $d

21:17 [Pub][ePrint] Short Accountable Ring Signatures Based on DDH, by Jonathan Bootle and Andrea Cerulli and Pyrros Chaidos and Essam Ghadafi and Jens Groth and Christophe Petit

  Ring signatures and group signatures are prominent cryptographic primitives offering a combination of privacy and authentication. They enable individual users to anonymously sign messages on behalf of a group of users. In ring signatures, the group, i.e.\\ the ring, is chosen in an ad hoc manner by the signer. In group signatures, group membership is controlled by a group manager.

Group signatures additionally enforce accountability by providing the group manager with a secret tracing key that can be used to identify the otherwise anonymous signer when needed.

Accountable ring signatures, introduced by Xu and Yung (CARDIS 2004), bridge the gap between the two notions. They provide maximal flexibility in choosing the ring, and at the same time maintain accountability by supporting a designated opener that can identify signers when needed.

We revisit accountable ring signatures and offer a formal security model for the primitive. Our model offers strong security definitions incorporating protection against maliciously chosen keys and at the same time flexibility both in the choice of the ring and the opener.

We give a generic construction using standard tools.

We give a highly efficient instantiation of our generic construction in the random oracle model by meticulously combining Camenisch\'s group signature scheme (CRYPTO 1997) with a generalization of the one-out-of-many proofs of knowledge by Groth and Kohlweiss (EUROCRYPT 2015). Our instantiation yields signatures of logarithmic size (in the size of the ring) while relying solely on the well-studied decisional Diffie-Hellman assumption.

In the process, we offer a number of optimizations for the recent Groth and Kohlweiss one-out-of-many proofs, which may be useful for other applications.

Accountable ring signatures imply traditional ring and group signatures. We therefore also obtain highly efficient instantiations of those primitives with signatures shorter than all existing ring signatures as well as existing group signatures relying on standard assumptions.

20:15 [Job][New] Post.doc., Norwegian University of Science and Technology (NTNU), Trondheim, Norway

  Malicious cryptography is about using cryptography for cyber attacks. We have already seen applications of malicious cryptography in so-called ransomware. Sophisticated attackers may want to use cryptography to hide an attack, the target of the attack, the source of the attack or to protect attack infrastructure (botnets).

The project goal is not to design sophisticated malware, but to understand the possible threats that we need to defend against.