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A Framework for Constructing Fast MPC over Arithmetic Circuits with Malicious Adversaries and an Honest-Majority

Wed, 12/12/2018 - 06:55
Protocols for secure multiparty computation enable a set of parties to compute a function of their inputs without revealing anything but the output. The security properties of the protocol must be preserved in the presence of adversarial behavior. The two classic adversary models considered are \emph{semi-honest} (where the adversary follows the protocol specification but tries to learn more than allowed by examining the protocol transcript) and \emph{malicious} (where the adversary may follow any arbitrary attack strategy). Protocols for semi-honest adversaries are often far more efficient, but in many cases the security guarantees are not strong enough. In this paper, we present a new efficient method for ``compiling'' a large class of protocols that are secure in the presence of semi-honest adversaries into protocols that are secure in the presence of malicious adversaries. Our method assumes an honest majority (i.e., that $t<n/2$ where $t$ is the number of corrupted parties and $n$ is the number of parties overall), and is applicable to many semi-honest protocols based on secret-sharing. In order to achieve high efficiency, our protocol is \emph{secure with abort} and does not achieve fairness, meaning that the adversary may receive output while the honest parties~do~not. We present a number of instantiations of our compiler, and obtain protocol variants that are very efficient for both a small and large number of parties. We implemented our protocol variants and ran extensive experiments to compare them with each other. Our results show that secure computation with an honest majority can be practical, even with security in the presence of malicious adversaries. For example, we securely compute a large arithmetic circuit of depth 20 with 1,000,000 multiplication gates, in approximately 0.5 seconds with three parties, and approximately 29 seconds with 50 parties, and just under 1 minute with 90 parties.

Batching Techniques for Accumulators with Applications to IOPs and Stateless Blockchains

Tue, 12/11/2018 - 17:02
We present batching techniques for cryptographic accumulators and vector commitments in groups of unknown order. Our techniques are tailored for decentralized settings where no trusted accumulator manager exists and updates to the accumulator are processed in batches. We develop techniques for non-interactively aggregating membership proofs that can be verified with a constant number of group operations. We also provide a constant sized batch non-membership proof for a large number of elements. These proofs can be used to build a positional vector commitment with constant sized openings and constant sized public parameters. As a core building block for our batching techniques we develop several succinct proofs for groups of unknown order. These include a proof that an exponentiation was done correctly and a zero-knowledge proof of knowledge of an integer discrete logarithm between two group elements. We use these new constructions to design a stateless blockchain, where nodes only need a constant storage. Further we show that our vector commitment can be used to significantly reduce the size of IOP instantiations, such as STARKs.

A Systematic Study of the Impact of Graphical Models on Inference-based Attacks on AES

Tue, 12/11/2018 - 11:22
Belief propagation, or the sum-product algorithm, is a powerful and well known method for inference on probabilistic graphical models, which has been proposed for the specific use in side channel analysis by Veyrat-Charvillon et al. We define a novel metric to capture the importance of variable nodes in factor graphs, we propose two improvements to the sum-product algorithm for the specific use case in side channel analysis, and we explicitly define and examine different ways of combining information from multiple side channel traces. With these new considerations we systematically investigate a number of graphical models that "naturally" follow from an implementation of AES. Our results are unexpected: neither a larger graph (i.e. more side channel information) nor more connectedness necessarily lead to significantly better attacks. In fact our results demonstrate that in practice the (on balance) best choice is to utilise an acyclic graph in an independent graph combination setting, which gives us provable convergence to the correct key distribution. We provide evidence using both extensive simulations and a final confirmatory analysis on real trace data.

Middle-Product Learning With Errors

Tue, 12/11/2018 - 11:11
We introduce a new variant $\MPLWE$ of the Learning With Errors problem ($\LWE$) making use of the Middle Product between polynomials modulo an integer~$q$. We exhibit a reduction from the Polynomial-$\LWE$ problem ($\PLWE$) parametrized by a polynomial~$f$, to $\MPLWE$ which is defined independently of any such~$f$. The reduction only requires~$f$ to be monic with constant coefficient coprime with~$q$. It incurs a noise growth proportional to the so-called expansion factor of~$f$. We also describe a public-key encryption scheme with quasi-optimal asymptotic efficiency (the bit-sizes of the keys and the run-times of all involved algorithms are quasi-linear in the security parameter), which is secure against chosen plaintext attacks under the $\MPLWE$ hardness assumption. The scheme is hence secure under the assumption that $\PLWE$ is hard for at least one polynomial~$f$ of degree~$n$ among a family of~$f$'s which is exponential in~$n$.

Post-Quantum EPID Signatures from Symmetric Primitives

Mon, 12/10/2018 - 23:04
EPID signatures are used extensively in real-world systems for hardware enclave attestation. As such, there is a strong interest in making these schemes post-quantum secure. In this paper we initiate the study of EPID signature schemes built only from symmetric primitives, such as hash functions and PRFs. We present two constructions in the random oracle model. The first is a scheme satisfying the EPID signature syntax and security definitions needed for private hardware attestation used in Intel’s SGX. The second achieves significantly shorter signatures for many applications, including the use case of remote hardware attestation. While our EPID signatures for attestation are longer than standard post-quantum signatures, they are short enough for applications where the data being signed is large, such as analytics on large private data sets, or streaming media to a trusted display. We evaluate several instantiations of our schemes so that the costs and benefits of these constructions are clear. Along the way we also give improvements to the zero-knowledge Merkle inclusion proofs of Derler et al. (2017).

Efficient Range ORAM with $\mathbb{O}(\log^{2}{N})$ Locality

Mon, 12/10/2018 - 20:51
Oblivious RAM protocols (ORAMs) allow a client to access data from an untrusted storage device without revealing to that device any information about their access pattern. Typically this is accomplished through random shuffling of the data such that the storage device cannot determine where individual blocks are located, resulting in a highly randomized access pattern. Storage devices however, are typically optimized for \emph{sequential} access. A large number of random disk seeks during standard ORAM operation induce a substantial overhead. In this paper, we introduce rORAM, an ORAM specifically suited for accessing ranges of \emph{sequentially logical blocks} while \emph{minimizing the number of random physical disk seeks}. rORAM obtains significantly better asymptotic efficiency than prior designs (Asharov et al., ePrint 2017, Demertzis et al., CRYPTO 2018) reducing {\em both} the number of seeks and communication complexity by a multiplicative factor of $\mathbb{O}(\log N)$. An rORAM prototype is 30-50x times faster than Path ORAM for similar range-query workloads on local HDDs, 30x faster for local SSDs, and 10x faster for network block devices. rORAM's novel disk layout can also speed up standard ORAM constructions, e.g., resulting in a 2x faster Path ORAM variant. Importantly, experiments demonstrate suitability for real world applications -- rORAM is up to 5x faster running a file server and up to 11x faster running a range-query intensive video server workloads compared to standard Path ORAM.

Cryptanalysis of 2-round KECCAK-384

Mon, 12/10/2018 - 17:19
In this paper, we present a cryptanalysis of round reduced Keccak-384 for 2 rounds. The best known preimage attack for this variant of Keccak has the time complexity $2^{129}$. In our analysis, we find a preimage in the time complexity of $2^{89}$ and almost same memory is required.

Large Universe Subset Predicate Encryption Based on Static Assumption (without Random Oracle)

Mon, 12/10/2018 - 17:19
In a recent work, Katz et al. (CANS'17) generalized the notion of Broadcast Encryption to define Subset Predicate Encryption (SPE) that emulates \emph{subset containment} predicate in the encrypted domain. They proposed two selective secure constructions of SPE in the small universe settings. Their first construction is based on $q$-type assumption while the second one is based on DBDH. % which can be converted to large universe using random oracle. Both achieve constant size secret key while the ciphertext size depends on the size of the privileged set. They also showed some black-box transformation of SPE to well-known primitives like WIBE and ABE to establish the richness of the SPE structure. This work investigates the question of large universe realization of SPE scheme based on static assumption without random oracle. We propose two constructions both of which achieve constant size secret key. First construction $\mathsf{SPE}_1$, instantiated in composite order bilinear groups, achieves constant size ciphertext and is proven secure in a restricted version of selective security model under the subgroup decision assumption (SDP). Our main construction $\mathsf{SPE}_2$ is adaptive secure in the prime order bilinear group under the symmetric external Diffie-Hellman assumption (SXDH). Thus $\mathsf{SPE}_2$ is the first large universe instantiation of SPE to achieve adaptive security without random oracle. Both our constructions have efficient decryption function suggesting their practical applicability. Thus the primitives like WIBE and ABE resulting through black-box transformation of our constructions become more practical.

The Role of the Adversary Model in Applied Security Research

Mon, 12/10/2018 - 17:18
Adversary models have been integral to the design of provably-secure cryptographic schemes or protocols. However, their use in other computer science research disciplines is relatively limited, particularly in the case of applied security research (e.g., mobile app and vulnerability studies). In this study, we conduct a survey of prominent adversary models used in the seminal field of cryptography, and more recent mobile and Internet of Things (IoT) research. Motivated by the findings from the cryptography survey, we propose a classification scheme for common app-based adversaries used in mobile security research, and classify key papers using the proposed scheme. Finally, we discuss recent work involving adversary models in the contemporary research field of IoT. We contribute recommendations to aid researchers working in applied (IoT) security based upon our findings from the mobile and cryptography literature. The key recommendation is for authors to clearly define adversary goals, assumptions and capabilities.

Automatic Search for A Variant of Division Property Using Three Subsets (Full Version)

Mon, 12/10/2018 - 17:17
The division property proposed at Eurocrypt'15 is a novel technique to find integral distinguishers, which has been applied to most kinds of symmetric ciphers such as block ciphers, stream ciphers, and authenticated encryption,~\textit{etc}. The original division property is word-oriented, and later the bit-based one was proposed at FSE'16 to get better integral property, which is composed of conventional bit-based division property (two-subset division property) and bit-based division property using three subsets (three-subset division property). Three-subset division property has more potential to achieve better integral distinguishers compared with the two-subset division property. The bit-based division property could not be to apply to ciphers with large block sizes due to its unpractical complexity. At Asiacrypt'16, the two-subset division property was modeled using Mixed Integral Linear Programming (MILP) technique, and the limits of block sizes were eliminated. However, there is still no efficient method searching for three-subset division property. The propagation rule of the \texttt{XOR} operation for $\mathbb{L}$ \footnote{The definition of $\mathbb{L}$ and $\mathbb{K}$ is introduced in Section 2.}, which is a set used in the three-set division property but not in two-set one, requires to remove some specific vectors, and new vectors generated from $\mathbb{L}$ should be appended to $\mathbb{K}$ when \texttt{Key-XOR} operation is applied, both of which are difficult for common automatic tools such as MILP, SMT or CP. In this paper, we overcome one of the two challenges, concretely, we address the problem to add new vectors into $\mathbb{K}$ from $\mathbb{L}$ in an automatic search model. Moreover, we present a new model automatically searching for a variant three-subset division property (VTDP) with STP solver. The variant is weaker than the original three-subset division property (OTDP) but it is still powerful in some ciphers. Most importantly, this model has no constraints on the block size of target ciphers, which can also be applied to ARX and S-box based ciphers. As illustrations, some improved integral distinguishers have been achieved for SIMON32, SIMON32/48/64(102), SPECK32 and KATAN/KTANTAN32/48/64 according to the number of rounds or number of even/odd-parity bits.

MILP Method of Searching Integral Distinguishers Based on Division Property Using Three Subsets

Mon, 12/10/2018 - 17:16
Division property is a generalized integral property proposed by Todo at EUROCRYPT 2015, and then conventional bit-based division property (CBDP) and bit-based division property using three subsets (BDPT) were proposed by Todo and Morii at FSE 2016. The huge time and memory complexity that once restricted the applications of CBDP have been solved by Xiang et al. at ASIACRYPT 2016. They extended Mixed Integer Linear Programming (MILP) method to search integral distinguishers based on CBDP. BDPT can find more accurate integral distinguishers than CBDP, but it can not be modeled efficiently. Thus it cannot be applied to block ciphers with block size larger than 32 bits. In this paper, we focus on the feasibility of applying MILP-aided method to search integral distinguishers based on BDPT. We firstly study how to get the BDPT propagation rules of an S-box. Based on that we can efficiently describe the BDPT propagation of cipher which has S-box. Moreover, we propose a technique called ``fast propagation", which can translate BDPT into CBDP, then the balanced bits based on BDPT can be presented. Together with the propagation properties of BDPT, we can use MILP method based on CBDP to search integral distinguishers based on BDPT. In order to prove the efficiency of our method, we search integral distinguishers on SIMON, SIMECK, PRESENT, RECTANGLE, LBlock, and TWINE. For SIMON64, PRESENT, and RECTANGLE, we find more balanced bits than the previous longest distinguishers. For LBlock, we find a 17-round integral distinguisher which is one more round than the previous longest integral distinguisher, and a better 16-round integral distinguisher with less active bits can be obtain. For other ciphers, our results are in accordance with the previous longest distinguishers.

On Quantum Chosen-Ciphertext Attacks and Learning with Errors

Mon, 12/10/2018 - 17:16
Large-scale quantum computing is a significant threat to classical public-key cryptography. In strong “quantum access” security models, numerous symmetric-key cryptosystems are also vulnerable. We consider classical encryption in a model which grants the adversary quantum oracle access to encryption and decryption, but where the latter is restricted to non-adaptive (i.e., pre-challenge) queries only. We define this model formally using appropriate notions of ciphertext indistinguishability and semantic security (which are equivalent by standard arguments) and call it QCCA1 in analogy to the classical CCA1 security model. Using a bound on quantum random-access codes, we show that the standard PRF- and PRP-based encryption schemes are QCCA1-secure when instantiated with quantum-secure primitives. We then revisit standard IND-CPA-secure Learning with Errors (LWE) encryption and show that leaking just one quantum decryption query (and no other queries or leakage of any kind) allows the adversary to recover the full secret key with constant success probability. In the classical setting, by contrast, recovering the key uses a linear number of decryption queries, and this is optimal. The algorithm at the core of our attack is a (large-modulus version of) the well-known Bernstein-Vazirani algorithm. We emphasize that our results should not be interpreted as a weakness of these cryptosystems in their stated security setting (i.e., post-quantum chosen-plaintext secrecy). Rather, our results mean that, if these cryptosystems are exposed to chosen-ciphertext attacks (e.g., as a result of deployment in an inappropriate real-world setting) then quantum attacks are even more devastating than classical ones.

Uncontrolled Randomness in Blockchains: Covert Bulletin Board for Illicit Activities

Mon, 12/10/2018 - 17:15
The blockchain technology represents a new paradigm to realize persistent distributed ledgers globally. While the blockchain technology is promising in a great number of fields, it can be abused to covertly store and disseminate potentially harmful digital content. Consequently, using blockchains as uncensored decentralized networks for arbitrary data distribution poses a serious regulatory issue. In this work, we show the severity of the problem by demonstrating a new technique that can be exploited to use the blockchain as a covert bulletin board to secretly store and distribute objectionable content. More speci cally, all major blockchain systems use randomized cryptographic primitives, such as digital signatures and non-interactive zero-knowledge proofs, and we illustrate how the uncontrolled randomness in such primitives can be maliciously manipulated to enable covert communication and hidden persistent storage. We also demonstrate how the same technique can be extended to launch subversion attacks on the wallets of most top-ranked cryptocurrencies, such as Bitcoin, Ethereum, Monero, etc. To clarify the potential risk of uncontrolled randomness, we design, implement and evaluate our technique against the widely-used ECDSA signature scheme, the CryptoNote's ring signature scheme, and Monero's ring con dential transactions. Note that the signi cance of the demonstrated attacks stems from their undetectability, their adverse effect on the future of decentralized blockchains, and their serious repercussions on users' privacy and crypto funds. Finally, besides presenting the attacks, we provide a discussion of current countermeasures and suggest some countermeasures to mitigate the threat of such attacks.

Steady: A Simple End-to-End Secure Logging System

Mon, 12/10/2018 - 11:11
We present Steady: an end-to-end secure logging system engineered to be simple in terms of design, implementation, and assumptions for real-world use. Steady gets its name from being based on a steady (heart)beat of events from a forward-secure device sent over an untrusted network through untrusted relays to a trusted collector. Properties include optional encryption and compression (with loss of confidentiality but significant gain in goodput), detection of tampering, relays that can function in unidirectional networks (e.g., as part of a data diode), cost-effective use of cloud services for relays, and publicly verifiable proofs of event authenticity. The design is formalized and security proven in the standard model. Our prototype implementation (about 2,200 loc) shows reliable goodput of over 1M events/s (about 160 MiB/s) for a realistic dataset with commodity hardware for a device on a GigE network using 16 MiB of memory connected to a relay running at Amazon EC2.

A Simpler Variant of Universally Composable Security for Standard Multiparty Computation

Mon, 12/10/2018 - 10:15
In this paper, we present a simpler and more restricted variant of the universally composable security (UC) framework that is suitable for ``standard'' two-party and multiparty computation tasks. Many of the complications of the UC framework exist in order to enable more general tasks than classic secure computation. This generality may be a barrier to entry for those who are used to the stand-alone model of secure computation and wish to work with universally composable security but are overwhelmed by the differences. The variant presented here (called simplified universally composable security, or just SUC) is closer to the definition of security for multiparty computation in the stand-alone setting. The main difference is that a protocol in the SUC framework runs with a \emph{fixed set of parties} who know each other's identities ahead of time, and machines \emph{cannot be added dynamically} to the execution. As a result, the definitions of polynomial time and protocol composition are much simpler. In addition, the SUC framework has authenticated channels built in, as is standard in previous definitions of security, and all communication is done via the adversary in order to enable arbitrary scheduling of messages. Due to these differences, not all cryptographic tasks can be expressed in the SUC framework. Nevertheless, standard secure computation tasks (like secure function evaluation) can be expressed. Importantly, we show a natural security-preserving transformation from protocols in the SUC model to protocols in the full-fledged UC model. Consequently, the UC composition theorem holds in the SUC model, and any protocol that is proven secure under SUC can be transformed to a protocol that is secure in the UC model.

On the Practical (In-)Security of 64-bit Block Ciphers: Collision Attacks on HTTP over TLS and OpenVPN

Sat, 12/08/2018 - 14:26
While modern block ciphers, such as AES, have a block size of at least 128 bits, there are many 64-bit block ciphers, such as 3DES and Blowfish, that are still widely supported in Internet security protocols such as TLS, SSH, and IPsec. When used in CBC mode, these ciphers are known to be susceptible to collision attacks when they are used to encrypt around $2^{32}$ blocks of data (the so-called birthday bound). This threat has traditionally been dismissed as impractical since it requires some prior knowledge of the plaintext and even then, it only leaks a few secret bits per gigabyte. Indeed, practical collision attacks have never been demonstrated against any mainstream security protocol, leading to the continued use of 64-bit ciphers on the Internet. In this work, we demonstrate two concrete attacks that exploit collisions on short block ciphers. First, we present an attack on the use of 3DES in HTTPS that can be used to recover a secret session cookie. Second, we show how a similar attack on Blowfish can be used to recover HTTP BasicAuth credentials sent over OpenVPN connections. In our proof-of-concept demos, the attacker needs to capture about 785GB of data, which takes between 19-38 hours in our setting. This complexity is comparable to the recent RC4 attacks on TLS: the only fully implemented attack takes 75 hours. We evaluate the impact of our attacks by measuring the use of 64-bit block ciphers in real-world protocols. We discuss mitigations, such as disabling all 64-bit block ciphers, and report on the response of various software vendors to our responsible disclosure of these attacks.

PIEs: Public Incompressible Encodings for Decentralized Storage

Sat, 12/08/2018 - 14:04
We present a provably secure approach to proving file replication (or other erasure coding) in distributed storage networks (DSNs). Storing multiple copies of a file $F$ is essential in DSNs to ensure against file loss in the event of faulty servers or corrupt data. The public nature of DSNs, however, makes this goal challenging. Files must be encoded and decoded using public coins—i.e., without encryption or other secret-key operations—and retention of files by servers in the network must be publicly verifiable. We introduce and formalize the notion of a public incompressible encoding (PIE), a primitive that supports file-replication proofs in the public setting. A PIE enables public verification that a server is storing a replicated encoding $G$ of a target file $F$, and has not compressed $G$ to save storage. In a DSN with monetary rewards or penalties, PIEs can help ensure that economically rational servers store $G$ and thus replicate $F$ honestly. Our PIE constructions are the first to achieve experimentally validated near-optimal performance—only a factor of 4 from optimal by one metric. They also support fast decoding which no other comparable construction does—as much as a factor of 800 in one experiment. PIEs additionally meet the critical security requirements for DSNs and related applications: they preclude demonstrated attacks involving parallelism via ASICs and other custom hardware. They achieve all of these properties using a graph construction we call a Dagwood Sandwich Graph (DSaG) that involves a novel interleaving of depth-robust graphs and superconcentrators. PIEs' performance makes them appealing for DSNs, such as the proposed Filecoin system, and other challenging file-storage needs in public settings. Conversely, their near optimality provides realistic measures of the (considerable) practical costs and limitations of DSNs and other applications.

The impact of error dependencies on Ring/Mod-LWE/LWR based schemes

Fri, 12/07/2018 - 04:54
Current estimation techniques for the probability of decryption failures in Ring/Mod-LWE/LWR based schemes assume independence of the failures in individual bits of the transmitted message to calculate the full failure rate of the scheme. In this paper we disprove this assumption both theoretically and practically for schemes based on Ring/Mod-Learning with Errors/Rounding. We provide a method to estimate the decryption failure probability, taking into account the bit failure dependency. We show that the independence assumption is suitable for schemes without error correction, but that it might lead to underestimating the failure probability of algorithms using error correcting codes. In the worst case, for LAC-128, the failure rate is $2^{48}$ times bigger than estimated under the assumption of independence. This higher-than-expected failure rate could lead to more efficient cryptanalysis of the scheme through decryption failure attacks.

PwoP: Intrusion-Tolerant and Privacy-Preserving Sensor Fusion

Thu, 12/06/2018 - 15:21
We design and implement, PwoP, an efficient and scalable system for intrusion-tolerant and privacy-preserving multi-sensor fusion. PwoP develops and unifies techniques from dependable distributed systems and modern cryptography, and in contrast to prior works, can 1) provably defend against pollution attacks where some malicious sensors lie about their values to sway the final result, and 2) perform within the computation and bandwidth limitations of cyber-physical systems. PwoP is flexible and extensible, covering a variety of application scenarios. We demonstrate the practicality of our system using Raspberry Pi Zero W, and we show that PwoP is efficient in both failure-free and failure scenarios.

Aggregation of Gamma-Signatures and Applications to Bitcoin

Wed, 12/05/2018 - 12:26
Aggregate signature (AS) allows non-interactively condensing multiple individual signatures into a compact one. Besides the faster verification, it is useful to reduce storage and bandwidth, and is especially attractive for blockchain and cryptocurrency. In this work, we first demonstrate the subtlety of achieving AS from general groups, by a concrete attack that actually works against the natural implementations of AS based on almost all the variants of DSA and Schnorr’s. Then, we show that aggregate signature can be derived from the &#915;-signature scheme proposed by Yao, et al. To the best of our knowledge, this is the first aggregate signature scheme from general elliptic curves without bilinear maps (in particular, the secp256k1 curve used by Bitcoin). The security of aggregate &#915;-signature is proved based on a new assumption proposed and justified in this work, referred to as non-malleable discrete-logarithm (NMDL), which might be of independent interest and could find more cryptographic applications in the future. When applying the resultant aggregate &#915;-signature to Bitcoin, the storage volume of signatures reduces about 49.8%, and the signature verification time can evenreduce about 72%. Finally, we specify in detail the application of the proposed AS scheme to Bitcoin, with the goal of maximizing performance and compatibility. We adopt a Merkle-Patricia tree based implementation, and the resulting system is also more friendly to segregated witness and provides better protection against transaction malleability attacks.

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