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Collaborative Multi-Authority Key-Policy Attribute-Based Encryption for Shorter Keys and Parameters

Mon, 12/11/2017 - 09:11
Architectures relying on a single central authority often offer a great efficiency, but suffer of resiliency problems and are quite vulnerable to attacks. In our proposal, a Multiple-Authorities Key-Policy Attribute-Based Encryption scheme is constructed including a collaboration phase between the authorities, in order to achieve shorter keys and parameters, thus enhancing the efficiency of encryption and decryption. \\ We prove our system secure under a variation of the bilinear Diffie-Hellman assumption, providing a lower bound on its complexity.

Another Look at Success Probability in Linear Cryptanalysis

Sat, 12/09/2017 - 01:50
This work studies the success probability of key recovery attacks based on using a single linear approximation. Previous works had analysed success probability under different hypotheses on the distributions of correlations for the right and wrong key choices. This work puts forward a unifying framework of general key randomisation hypotheses. All previously used key randomisation hypotheses as also zero correlation attacks can be seen to special cases of the general framework. Derivations of expressions for the success probability are carried out under both the settings of the plaintexts being sampled with and without replacements. Compared to previous analysis, we uncover several new cases which have not been considered in the literature. For most of the cases which have been considered earlier, we provide complete expressions for the respective success probabilities. Finally, the complete picture of the dependence of the success probability on the data complexity is revealed. Compared to the extant literature, our work provides a deeper and more thorough understanding of the success probability of single linear cryptanalysis.

Logical loophole in random 3-bit sequence generator

Fri, 12/08/2017 - 15:24
In this note, we will provide an information-theoretic framework for the random 3-bit sequence {111, 110, 101, 100, 011, 010, 001, 000} that causes a logical loophole in the Bell-CHSH inequality [1,2,3], as also has recently been found experimentally for triples measurements [4]. As a consequence of this, both classical and quantum regimes can share their bounds within the same environment, which shows that maximally entangled states used in cryptography secure systems can critically be subverted via EPR (Einstein-Podolsky-Rosen) paradox [5].

EFLASH: A New Multivariate Encryption Scheme

Fri, 12/08/2017 - 14:09
Multivariate Public Key Cryptography is a leading option for security in a post quantum society. In this paper we propose a new encryption scheme, EFLASH, and analyze its efficiency and security.

Round2: KEM and PKE based on GLWR

Fri, 12/08/2017 - 14:09
Cryptographic primitives that are secure against quantum computing are receiving growing attention with recent, steady advances in quantum computing and standardization initiatives in post-quantum cryptography by NIST and ETSI. Lattice-based cryptography is one of the families in post-quantum cryptography, demonstrating desirable features such as well-understood security, efficient performance, and versatility. In this work, we present Round2 that consists of a key-encapsulation mechanism and a public-key encryption scheme. Round2 is based on the General Learning with Rounding problem, that unifies the Learning with Rounding and Ring Learning with Rounding problems. Round2's construction using the above problem allows for a unified description and implementation. The key-encapsulation mechanism and public-key encryption scheme furthermore share common building blocks, simplifying (security and operational) analysis and code review. Round2's reliance on prime cyclotomic rings offers a large design space that allows fine-tuning of parameters to required security levels. The use of rounding reduces bandwidth requirements and the use of sparse-trinary secrets improves CPU performance and decryption success rates. Finally, Round2 includes various approaches of refreshing the system public parameter A, allowing efficient ways of preventing precomputation and back-door attacks.

Distributed Computing Made Secure: A New Cycle Cover Theorem

Fri, 12/08/2017 - 14:03
In the area of distributed graph algorithms a number of network's entities with local views solve some computational task by exchanging messages with their neighbors. Quite unfortunately, an inherent property of most existing distributed algorithms is that throughout the course of their execution, the nodes get to learn not only their own output but rather learn quite a lot on the inputs or outputs of many other entities. This leakage of information might be a major obstacle in settings where the output (or input) of network's individual is a private information (e.g., distributed networks of selfish agents, decentralized digital currency such as Bitcoin, voting systems). While being quite an unfamiliar notion in the classical distributed setting, the notion of secure multi-party computation (MPC) is one of the main themes in the Cryptography community. Yet despite all extensive work in the area, no existing algorithm fits the framework of classical distributed models in which there are no assumptions on the graph topologies and only messages of bounded size are sent on the edges in each round. In this paper, we introduce a new framework for \emph{secure distributed graph algorithms} and provide the first \emph{general compiler} that takes any "natural" non-secure distributed algorithm that runs in $r$ rounds, and turns it into a secure algorithm that runs in $\widetilde{O}(r \cdot D \cdot poly(\Delta))$ rounds where $\Delta$ is the maximum degree in the graph and $D$ is its diameter. We also show that this is nearly (existentially) optimal for any round-by-round compiler for bounded degree graphs. The main technical part of our compiler is based on a new cycle cover theorem: We show that the edges of every bridgeless graph $G$ of diameter $D$ can be covered by a collection of cycles such that each cycle is of length $\widetilde{O}(D)$ and each edge of the graph $G$ appears in $\widetilde{O}(1)$ many cycles. In fact, our construction can be made instance optimal with respect to each single edge. Letting $C_e$ be the shortest cycle containing $e$ in $G$, our cycle collection contains a cycle of length $\widetilde{O}(|C_e|)$ that covers $e$ for every $e \in G$, and in addition, each edge appears on $\widetilde{O}(1)$ many cycles. As a result, our compiler becomes instance optimal for bounded degree graphs.

Implementing Joux-Vitse's Crossbred Algorithm for Solving MQ Systems over GF(2) on GPUs

Fri, 12/08/2017 - 14:01
The hardness of solving multivariate quadratic (MQ) systems is the underlying problem for multivariate-based schemes in the field of post-quantum cryptography. The concrete, practical hardness of this problem needs to be measured by state-of-the-art algorithms and high-performance implementations. We describe, implement, and evaluate an adaption of the Crossbred algorithm by Joux and Vitse for solving MQ systems over GF(2). Our adapted algorithm is highly parallelizable and is suitable for solving MQ systems on GPU architectures. Our implementation is able to solve an MQ system of 134 equations in 67 variables in 98.39 hours using one single commercial Nvidia GTX 980 graphics card, while the original Joux-Vitse algorithm requires 6200 CPU-hours for the same problem size. We used our implementation to solve all the Fukuoka Type-I MQ challenges for n = 55,...,74. Based on our implementation, we estimate that the expected computation time for solving an MQ system of 80 equations in 84 variables is about one year using a cluster of 3647 GTX 980 graphics cards. These parameters have been proposed for 80-bit security by, e.g., Sakumoto, Shirai, and Hiwatari in 2011.

FPGA-based Niederreiter Cryptosystem using Binary Goppa Codes

Fri, 12/08/2017 - 14:01
This paper presents an FPGA implementation of the Niederreiter cryptosystem using binary Goppa codes, including modules for encryption, decryption, and key generation. We improve over previous implementations in terms of efficiency (time-area product and raw performance) and security level. Our implementation is constant time in order to protect against timing side-channel analysis. The design is fully parameterized, using code-generation scripts, in order to support a wide range of parameter choices for security, including binary field size, the degree of the Goppa polynomial, and the code length. The parameterized design allows us to choose design parameters for time-area trade-offs in order to support a wide variety of applications ranging from smart cards to server accelerators. For parameters that are considered to provide 128-bit ‘’post-quantum security'', our time-optimized implementation requires 966,400 cycles for the generation of both public and private portions of a key and 14,291 cycles to decrypt a ciphertext. The time-optimized design uses only 121,806 ALMs (52% of the available logic) and 961 RAM blocks (38% of the available memory), and results in a design that runs at about 250 MHz on a medium-size Stratix V FPGA.

On the exponents of APN power functions and Sidon sets, sum-free sets, and Dickson polynomials

Fri, 12/08/2017 - 13:58
We derive necessary conditions related to the notions, in additive combinatorics, of Sidon sets and sum-free sets, on those exponents $d\in {\mathbb Z}/(2^n-1){\mathbb Z}$ which are such that $F(x)=x^d$ is an APN function over ${\mathbb F}_{2^n}$ (which is an important cryptographic property). We study to which extent these new conditions may speed up the search for new APN exponents $d$. We also show a new connection between APN exponents and Dickson polynomials: $F(x)=x^d$ is APN if and only if the reciprocal polynomial of the Dickson polynomial of index $d$ is an injective function from $\{y\in {\Bbb F}_{2^n}^*; tr_n(y)=0\}$ to ${\Bbb F}_{2^n}\setminus \{1\}$. This also leads to a new and simple connection between Reversed Dickson polynomials and reciprocals of Dickson polynomials in characteristic 2 (which generalizes to every characteristic thanks to a small modification): the squared Reversed Dickson polynomial of some index and the reciprocal of the Dickson polynomial of the same index are equal.

Comparison analysis and efficient implementation of reconciliation-based RLWE key exchange protocol

Fri, 12/08/2017 - 13:56
Error reconciliation is an important technique for Learning With Error (LWE) and Ring-LWE (RLWE)-based constructions. In this paper, we present a comparison analysis on two error reconciliation-based RLWE key exchange protocols: Ding et al. in 2012 (DING12) and Bos et al. in 2015 (BCNS15). We take them as examples to explain core idea of error reconciliation, building key exchange over RLWE problem, implementation, real-world performance and compare them comprehensively. We also analyse a LWE key exchange “Frodo” that uses an improved error reconciliation mechanism in BCNS15. To the best of our knowledge, our work is the first to present at least 128-bit classic (80-bit quantum) and 256-bit classic (>200-bit quantum) secure parameter choices for DING12 with efficient portable C/C++ implementations. Benchmark shows that our efficient implementation is 11x faster than BCNS15 and one key exchange execution only costs 0.07ms on a 4-year-old middle range CPU. Error reconciliation is 1.57x faster than BCNS15.

Reusable Authentication from the Iris

Fri, 12/08/2017 - 13:55
Mobile platforms use biometrics for authentication. Unfortunately, biometrics exhibit noise between repeated readings. Due to the noise, biometrics are stored in plaintext, so device compromise completely reveals the user's biometric value. To limit privacy violations, one can use fuzzy extractors to derive a stable cryptographic key from biometrics (Dodis et al., Eurocrypt 2004). Unfortunately, fuzzy extractors have not seen wide deployment due to insufficient security guarantees. Current fuzzy extractors provide no security for real biometric sources and no security if a user enrolls the same biometric with multiple devices or providers. Previous work claims key derivation systems from the iris but only under weak adversary models. In particular, no known construction securely handles the case of multiple enrollments. Canetti et al. (Eurocrypt 2016) proposed a new fuzzy extractor called sample-then-lock. We construct biometric key derivation for the iris starting from sample-then-lock. Achieving satisfactory parameters requires modifying and coupling of the image processing and the cryptography. Our construction is implemented in Python and being open-sourced. Our system has the following novel features: -- 45 bits of security. This bound is pessimistic, assuming the adversary can sample strings distributed according to the iris in constant time. Such an algorithm is not known. -- Secure enrollment with multiple services. -- Natural incorporation of a password, enabling multifactor authentication. The structure of the construction allows the overall security to be sum of the security of each factor (increasing security to 79 bits).

Cyclic Locking and Memristor-based Obfuscation Against CycSAT and Inside Foundry Attacks

Fri, 12/08/2017 - 13:51
The high cost of IC design has made chip protection one of the first priorities of the semiconductor industry. Although there is a common impression that combinational circuits must be designed without any cycles, circuits with cycles can be combinational as well. Such cyclic circuits can be used to reliably lock ICs. Moreover, since memristor is compatible with CMOS structure, it is possible to efficiently obfuscate cyclic circuits using polymorphic memristor-CMOS gates. In this case, the layouts of the circuits with different functionalities look exactly identical, making it impossible even for an inside foundry attacker to distinguish the defined functionality of an IC by looking at its layout. In this paper, we propose a comprehensive chip protection method based on cyclic locking and polymorphic memristor-CMOS obfuscation. The robustness against state-of-the-art key-pruning attacks is demonstrated and the overhead of the polymorphic gates is investigated.

Improved Garbled Circuit Building Blocks and Applications to Auctions and Computing Minima

Fri, 12/08/2017 - 05:56
We consider generic Garbled Circuit (GC)-based techniques for Secure Function Evaluation (SFE) in the semi-honest model. We describe efficient GC constructions for addition, subtraction, multiplication, and comparison functions. Our circuits for subtraction and comparison are approximately two times smaller (in terms of garbled tables) than previous constructions. This implies corresponding computation and communication improvements in SFE of functions using our efficient building blocks. The techniques rely on recently proposed ``free XOR'' GC technique. Further, we present concrete and detailed improved GC protocols for the problem of secure integer comparison, and related problems of auctions, minimum selection, and minimal distance. Performance improvement comes both from building on our efficient basic blocks and several problem-specific GC optimizations. We provide precise cost evaluation of our constructions, which serves as a baseline for future protocols.

Cryptanalysis of an Authenticated Image Encryption Scheme Based on Chaotic Maps and Memory Cellular Automata

Wed, 12/06/2017 - 09:44
In this paper, the security of an authenticated image encryption scheme based on chaotic maps and memory cellular automata is evaluated. It is demonstrated that the scheme can be broken by chosen plain-text attack. Furthermore, the authentication algorithm of the scheme is faulty and reveals information about the plain-image and it also results in a brute search attack with efficient time complexity. Also the scheme suffers from differential attacks because of low sensitivity to the plain-image. We provide experimental results to support the proposed attacks. Finally, we suggest some remedial methods to fix the weaknesses and enhance sensitivity to the plain-image modifications.

Fully Verifiable Secure Delegation of Pairing Computation: Cryptanalysis and An Efficient Construction

Wed, 12/06/2017 - 05:00
We address the problem of secure and verifiable delegation of general pairing computation. We first analyze some recently proposed pairing delegation schemes and present several attacks on their security and/or verifiability properties. In particular, we show that none of these achieve the claimed security and verifiability properties simultaneously. We then provide a fully verifiable secure delegation scheme ${\sf VerPair}$ under one-malicious version of a two-untrusted-program model (OMTUP). ${\sf VerPair}$ not only significantly improves the efficiency of all the previous schemes, such as fully verifiable schemes of Chevallier-Mames et al. and Canard et al. by eliminating the impractical exponentiation- and scalar-multiplication-consuming steps, but also offers for the first time the desired full verifiability property unlike other practical schemes. Furthermore, we give a more efficient and less memory consuming invocation of the subroutine ${\sf Rand}$ for ${\sf VerPair}$ by eliminating the requirement of offline computations of modular exponentiations and scalar-multiplications. In particular, ${\sf Rand}$ includes a fully verifiable partial delegation under the OMTUP assumption. The partial delegation of ${\sf Rand}$ distinguishes ${\sf VerPair}$ as a useful lightweight delegation scheme when the delegator is resource-constrained (e.g. RFID tags, smart cards or sensor nodes).

Non-malleable Codes from Additive Combinatorics

Wed, 12/06/2017 - 03:06
Non-malleable codes provide a useful and meaningful security guarantee in situations where traditional error-correction (and even error-detection) is impossible; for example, when the attacker can completely overwrite the encoded message. Informally, a code is non-malleable if the message contained in a modified codeword is either the original message, or a completely unrelated value. Although such codes do not exist if the family of "tampering functions" \cF is completely unrestricted, they are known to exist for many broad tampering families \cF. One such natural family is the family of tampering functions in the so called {\em split-state} model. Here the message m is encoded into two shares L and R, and the attacker is allowed to arbitrarily tamper with L and R {\em individually}. The split-state tampering arises in many realistic applications, such as the design of non-malleable secret sharing schemes, motivating the question of designing efficient non-malleable codes in this model. Prior to this work, non-malleable codes in the split-state model received considerable attention in the literature, but were either (1) constructed in the random oracle model [DPW10], or (2) relied on advanced cryptographic assumptions (such as non-interactive zero-knowledge proofs and leakage-resilient encryption) [LL12], or (3) could only encode 1-bit messages [DKO13]. As our main result, we build the first efficient, multi-bit, information-theoretically-secure non-malleable code in the split-state model. The heart of our construction uses the following new property of the inner-product function <L,R> over the vector space F_p^n (for any prime p and large enough dimension n): if L and R are uniformly random over F_p^n, and f,g: F_p^n \rightarrow F_p^n are two arbitrary functions on L and R, the joint distribution (<L,R>,<f(L),g(R)>) is ``close'' to the convex combination of "affine distributions" {(U,c U+d)| c,d \in F_p}, where U is uniformly random in F_p. In turn, the proof of this surprising property of the inner product function critically relies on some results from additive combinatorics, including the so called {\em Quasi-polynomial Freiman-Ruzsa Theorem} (which was recently established by Sanders [San12] as a step towards resolving the Polynomial Freiman-Ruzsa conjecture [Gre05]).

Short Solutions to Nonlinear Systems of Equations

Tue, 12/05/2017 - 23:01
This paper presents a new hard problem for use in cryptography, called Short Solutions to Nonlinear Equations (SSNE). This problem generalizes the Multivariate Quadratic (MQ) problem by requiring the solution be short; as well as the Short Integer Solutions (SIS) problem by requiring the underlying system of equations be nonlinear. The joint requirement causes common solving strategies such as lattice reduction or Gröbner basis algorithms to fail, and as a result SSNE admits shorter representations of equally hard problems. We show that SSNE can be used as the basis for a provably secure hash function. Despite failing to find public key cryptosystems relying on SSNE, we remain hopeful about that possibility.

Efficient Optimal Ate Pairing at 128-bit Security Level

Tue, 12/05/2017 - 23:01
Following the emergence of Kim and Barbulescu's new number field sieve (exTNFS) algorithm at CRYPTO'16 [21] for solving discrete logarithm problem (DLP) over the finite field; pairing-based cryptography researchers are intrigued to find new parameters that confirm standard security levels against exTNFS. Recently, Barbulescu and Duquesne have suggested new parameters [3] for well-studied pairing-friendly curves i.e., Barreto-Naehrig (BN) [5], Barreto-Lynn-Scott (BLS-12) [4] and Kachisa-Schaefer-Scott (KSS-16) [19] curves at 128-bit security level (twist and sub-group attack secure). They have also concluded that in the context of Optimal-Ate pairing with their suggested parameters, BLS-12 and KSS-16 curves are more efficient choices than BN curves. Therefore, this paper selects the atypical and less studied pairing-friendly curve in literature, i.e., KSS-16 which offers quartic twist, while BN and BLS-12 curves have sextic twist. In this paper, the authors optimize Miller's algorithm of Optimal-Ate pairing for the KSS-16 curve by deriving efficient sparse multiplication and implement them. Furthermore, this paper concentrates on the Miller's algorithm to experimentally verify Barbulescu et al.'s estimation. The result shows that Miller's algorithm time with the derived pseudo 8-sparse multiplication is most efficient for KSS-16 than other two curves. Therefore, this paper defends Barbulescu and Duquesne's conclusion for 128-bit security.

There Goes Your PIN: Exploiting Smartphone Sensor Fusion Under Single and Cross User Setting

Tue, 12/05/2017 - 23:01
A range of zero-permission sensors are found in modern smartphones to enhance user experience. These sensors can lead to unintentional leakage of user private data. In this paper, we combine leakage from a pool of zero-permission sensors, to reconstruct user's secret PIN. By harvesting the power of machine learning algorithms, we show a practical attack on the full four-digit PIN space. Able to classify all 10,000 PIN combinations, results show up to 83.7% success within 20 tries in a single user setting. Latest previous work demonstrated 74% success on a reduced space of 50 chosen PINs, where we report 99.5% success with a single try in a similar setting. Moreover, we extend the PIN recovery attack from a single user to a cross-user scenario. Firstly, we show that by training on several users, the PIN recovery success can be boosted, when a target user is part of the training pool. On the other hand, PIN recovery is still possible when training pool is mutually exclusive to the target user, albeit with low success rate.

A Note on Stream Ciphers that Continuously Use the IV

Tue, 12/05/2017 - 23:00
Time-memory-data tradeoff (TMD-TO) attacks limit the security level of many classical stream ciphers (like $E_0$, A5/1, Trivium, Grain) to $n/2$, where $n$ denotes the inner state length of the underlying keystream generator. This implies that to withstand TMD tradeoff attacks, the state size should be at least double the key size. In 2015, Armknecht and Mikhalev introduced a new line of research, which pursues the goal of reducing the inner state size of lightweight stream ciphers below this boundary by deploying a key-dependent state update function in a Grain-like stream cipher. Although their design Sprout was broken soon after publication, it has raised interest in the design principle, and a number of related ciphers have been suggested since, including Plantlet, a follow-up of Sprout, and the cipher Fruit. In 2017, Hamann et al. showed that the initial hope of achieving full security against TMD-TO attacks by continuously using the secret key has failed. In particular, they demonstrated that there are generic distinguishing attacks against such ciphers with a complexity significantly smaller than that of exhaustive key search. However, by studying the assumptions underlying the applicability of these attacks, they came up with a new design idea for small-state stream ciphers, which is based on also continuously using the public IV as part of the state update. The authors conjectured that this design principle might allow to finally achieve full security against TMD-TO attacks. In this note, we take their idea one step further. While Hamann et al. aimed for improving the security of small-state stream ciphers that continuously use the secret key against distinguishing, we explain here that also other stream cipher constructions can benefit from continuously using the IV. In particular, our approach allows for thwarting the well-known TMD-TO inner state recovery attacks of Babbage and Biryukov and Shamir without using the secret key more than once.


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