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20 March 2017
ePrint Report New Limits for AES Known-Key Distinguishers Lorenzo Grassi, Christian Rechberger
Known-key distinguishers have been introduced to better understand the security of block ciphers in situations where the key can not be considered to be secret.

AES is often considered as a target of such analyses, simply because AES or its building blocks are used in many settings that go beyond classical encryption. The most recent known-key model of Gilbert (proposed at Asiacrypt 2014) allows to consider two 4-round distinguishers combined in an inside-out fashion (8 core rounds), and to extend it by one round in each direction (two extension rounds). The resulting 10-round distinguisher has a time complexity of $2^{64}$. In that work, arguments were put forward suggesting that two extension rounds seems to be the limit in the known-key model, and that likely only a distinguisher that exploits the balance property can be extended in such way.

In this paper we disprove both these conjectures and arrive at the following results. We firstly show that the technique proposed by Gilbert can also be used to extend a known-key distinguisher based on truncated differential trails. This allows to improve all the known-key distinguishers currently present in literature for AES up to 10 rounds of AES. In particular, we are able to set up a 9-round known-key distinguisher for AES with a time complexity of $2^{23}$ and a 10-round known-key distinguisher with a time complexity of $2^{50}$. Secondly we are also able to show that more than two extension rounds are possible. As a result of this, we describe the first known-key distinguishers on 12 rounds of AES, by extending an 8-round known-key distinguisher by two rounds in each direction (four extension rounds). The time complexity is $2^{82}$.

We conclude with a discussion on why it seems not feasible to set up similar distinguishers on 14 rounds exploiting the same strategy.
ePrint Report Towards Easy Key Enumeration Changhai Ou, Degang Sun, Zhu Wang, Xinping Zhou, Juan Ai
Key enumeration solutions are post-processing schemes for the output sequences of side channel distinguishers, the application of which are prevented by very large key candidate space and computation power requirements. The attacker may spend several days or months to enumerate a huge key candidate space (i.e. $2^{40}$). In this paper, we aim at pre-processing and reducing the key candidate space by deleting impossible key candidates before enumeration. A new distinguisher named Group Collision Attack (GCA) is given. Moreover, we introduce key verification into key recovery and a new divide and conquer strategy named Key Grouping Enumeration (KGE) is proposed. KGE divides the huge key space into several groups and uses GCA to delete impossible key combinations and output possible ones in each group. KGE then recombines the remaining key candidates in each group using verification. The number of remaining key candidates becomes much more smaller through these two impossible key candidate deletion steps with a small amount of computation. Thus, the attacker can use KGE as a pre-processing tool of key enumeration and enumerate the key more easily and fast in a much smaller candidate space.
We conduct a reduction-based security analysis of the Extensible Authentication Protocol (EAP), a widely used three-party authentication framework. EAP is often found in enterprise networks where it allows a client and an authenticator to establish a shared key with the help of a mutually trusted server. Considered as a three-party authenticated key exchange protocol, we show that the general EAP construction achieves a security notion we call 3P-AKE$^w$. %under the assumption that it employs \emph{channel binding}. %Channel binding ensures that the session key derived by the client and the authenticator is cryptographically bound to them. The 3P-AKE$^w$ security notion captures the idea of \emph{weak forward secrecy} and is a simplified three-party version of the well-known eCK model in the two-pass variant. Our analysis is modular and reflects the compositional nature of EAP.

Additionally, we show that the security of EAP can easily be upgraded to provide \emph{full} forward secrecy simply by adding a subsequent key-confirmation step between the client and the authenticator. In practice this key-confirmation step is often carried out in the form of a 2P-AKE protocol which uses EAP to bootstrap its authentication. A concrete example is the extremely common IEEE~802.11 protocol used in WLANs. In enterprise settings EAP is often used in conjunction with IEEE~802.11 in order to allow the wireless client to authenticate itself to a wireless access point (the authenticator) through some centrally administrated server. Building on our modular results for EAP, we get as our second major result the first reduction-based security result for IEEE~802.11 combined with EAP.
Masking with random values is an effective countermeasure against side-channel attacks. For cryptographic algorithms combining arithmetic and Boolean masking, it is necessary to switch from arithmetic to Boolean masking and vice versa. Following a recent approach by Hutter and Tunstall, we describe a high-order Boolean to arithmetic conversion algorithm whose complexity is independent of the register size k. Our new algorithm is proven secure in the Ishai, Sahai and Wagner (ISW) framework for private circuits. In practice, for small orders, our new countermeasure is one order of magnitude faster than previous work. We also describe a 3rd-order attack against the 3rd-order Hutter-Tunstall algorithm, and a constant, 4th-order attack against the t-th order Hutter-Tunstall algorithms, for any t>=4.
ePrint Report A Lattice-Based Universal Thresholdizer for Cryptographic Systems Dan Boneh, Rosario Gennaro, Steven Goldfeder, Sam Kim
We develop a general approach to thresholdizing a large class of (non-threshold) cryptographic schemes. We show how to add threshold functionality to CCA-secure public-key encryption (PKE), signature schemes, pseudorandom functions, and others primitives. To do so, we introduce a general tool, called a universal thresholdizer, from which many threshold systems are possible. The tool builds upon a lattice-based fully-homomorphic encryption (FHE) system. Applying the tool to a (non-threshold) lattice-based signature, gives the first single-round threshold signature from the learning with errors problem (LWE). Applying the tool to a (non-threshold) lattice-base CCA-secure PKE, gives a single-round lattice-based threshold CCA-secure PKE.
Recent works (Lin, EUROCRYPT'16, ePrint'16; Lin and Vaikunthanathan, FOCS'16; Ananth and Sahai, EUROCRYPT'17) establish a tight connection between constructions of indistinguishability obfuscation from $L$-linear maps and pseudo-random generators (PRGs) with output locality $L$. This approach appears however not to be suitable to obtain instantiations from bilinear maps, as no polynomial-stretch PRG with locality lower than 5 exists.

This paper presents new candidate constructions of indistinguishability obfuscation from (i) $L$-linear maps for any $L \ge 2$, and (ii) PRGs with block-wise locality $L$. A PRG has block-wise locality $L$ if every output bit depends on at most $L$ (disjoint) input blocks, each consisting of up to $\log \lambda$ input bits. In particular, we give:

A construction of a general-purpose indistinguishability obfuscator from $L$-linear maps and a subexponentially-secure PRG with block-wise locality $L$ and polynomial stretch.

A construction of general-purpose functional encryption from $L$-linear maps and any slightly super-polynomially secure PRG with block-wise locality $L$ and polynomial stretch.

All our constructions are based on the SXDH assumption on $L$-linear maps and subexponential Learning With Errors (LWE) assumption. In the special case of $L = 2$, our constructions can be alternatively based on bilinear maps with the Matrix Diffie-Hellman assumption and the 3-party Decision Diffie Hellman assumption, without assuming LWE. Concurrently, we initiate the study of candidate PRGs with block-wise locality $L\ge 2$ based on Goldreich's local functions, and their security. In particular, lower bounds on the locality of PRGs do not apply to block-wise locality for any $L \ge 2$, and the security of instantiations with block-wise locality $L \ge 3$ is backed by similar validation as constructions with (conventional) locality $5$. We complement this with hardness amplification techniques that weaken the pseudorandomness requirement on our candidates to qualitatively weaker requirements.
ePrint Report Proof of Luck: an Efficient Blockchain Consensus Protocol Mitar Milutinovic, Warren He, Howard Wu, Maxinder Kanwal
In the paper, we present designs for multiple blockchain consensus primitives and a novel blockchain system, all based on the use of trusted execution environments (TEEs), such as Intel SGX-enabled CPUs. First, we show how using TEEs for existing proof of work schemes can make mining equitably distributed by preventing the use of ASICs. Next, we extend the design with proof of time and proof of ownership consensus primitives to make mining energy- and time-efficient. Further improving on these designs, we present a blockchain using a proof of luck consensus protocol. Our proof of luck blockchain uses a TEE platform's random number generation to choose a consensus leader, which offers low-latency transaction validation, deterministic confirmation time, negligible energy consumption, and equitably distributed mining. Lastly, we discuss a potential protection against up to a constant number of compromised TEEs.
ePrint Report IPcore implementation susceptibility: A case study of Low latency ciphers Dillibabu Shanmugam, Ravikumar Selvam, Suganya Annadurai
Security evaluation of third-party cryptographic IP (Intellectual Property) cores is often ignored due to several reasons including, lack of awareness about its adversity, lack of trust validation methodology otherwise view security as a byproduct. Particularly, the validation of low latency cipher IP core on Internet of Things (IoT) devices is crucial as they may otherwise become vulnerable for information theft. In this paper, we share an (Un)intentional way of cipher implementation as IP core(hard) become susceptible against side channel attack and show how the susceptible implementation can be experimentally exploited to reveal secret key in FPGA using power analysis. In this paper our contributions are: First, we present Look-Up Table (LUT) based unrolled implementation of PRINCE block cipher with place and route constraints in FPGA. Second, using power analysis attack we recover 128-bit key of PRINCE with complexity of 2^9. Finally, we conclude the paper with the experimental results.
ePrint Report Efficient Multivariate Ring Signature Schemes Mohamed Saied Emam Mohamed, Albrecht Petzoldt
Multivariate Cryptography is one of the main candidates for creating post-quantum cryptosystems. Especially in the area of digital signatures, there exist many practical and secure multivariate schemes. However, there is a lack of more advanced schemes, such as schemes for oblivious transfer and signature schemes with special properties. While, in the last years, a number of multivariate ring signature schemes have been proposed, all of these have weaknesses in terms of security or efficiency. In this paper we propose a simple and efficient technique to extend arbitrary multivariate signature schemes to ring signature schemes and illustrate it using the example of Rainbow. The resulting scheme provides perfect anonymity for the signer (as member of a group), as well as shorter ring signatures than all previously proposed post-quantum ring signature schemes.
ePrint Report An Analysis of FV Parameters Impact Towards its Hardware Acceleration Joël Cathébras, Alexandre Carbon, Renaud Sirdey, Nicolas Ventroux
The development of cloud computing services is restrained by privacy concerns. Centralized medical services for instance, require a guarantee of confidentiality when using outsourced computation platforms. Fully Homomorphic Encryption is an intuitive solution to address such issue, but until 2009, existing schemes were only able to evaluate a reduced number of operations (Partially Homomorphic Encryption). In 2009, C. Gentry proposed a blueprint to construct FHE schemes from SHE schemes. However, it was not practical due to the huge data size overhead and the exponential noise growth of the initial SHE. Since then, major improvements have been made over SHE schemes and their noise management, and resulting schemes, like BGV and FV, allow to foresee small applications.

Besides scheme improvements, new practical approaches were proposed to bring homomorphic encryption closer to practice. The $IV$-based stream cipher trans-ciphering approach brought by Canteaut et al. in 2015 reduces the on-line latency of the trans-ciphering process to a simple homomorphic addition. The homomorphic evaluation of stream ciphers, that produces the trans-ciphering keystream, could be computed in an off-line phase, resulting in an almost transparent trans-ciphering process from the user point of view. This approach combined with hardware accelerations could bring homomorphic encryption closer to practice.

This paper deals the choice of FV parameters for efficient implementation of this scheme in the light of related works' common approaches. At first sight, using large polynomial degree to reduce the coefficients size seemed to be advantageous, but further observations contradict it. Large polynomial degrees imply larger ciphertexts and more complex implementations, but smaller ones imply more primes to find for CRT polynomial representation. The result of this preliminary work for the choice of an adequate hardware target motivates the choice of small degree polynomials rather than small coefficients for the FV scheme.
Cross-VM attacks have emerged as a major threat on commercial clouds. These attacks commonly exploit hardware level leakages on shared physical servers. A co-located machine can readily feel the presence of a co-located instance with a heavy computational load through performance degradation due to contention on shared resources. Shared cache architectures such as the last level cache (LLC) have become a popular leakage source to mount cross-VM attack. By exploiting LLC leakages, researchers have already shown that it is possible to recover fine grain information such as cryptographic keys from popular software libraries. This makes it essential to verify implementations that handle sensitive data across the many versions and numerous target platforms, a task too complicated, error prone and costly to be handled by human beings.

Here we propose a machine learning based technique to classify applications according to their cache access profiles. We show that with minimal and simple manual processing steps feature vectors can be used to train models using support vector machines to classify the applications with a high degree of success. The profiling and training steps are completely automated and do not require any inspection or study of the code to be classified. In native execution, we achieve a successful classification rate as high as 98\% (L1 cache) and 78\% (LLC) over 40 benchmark applications in the Phoronix suite with mild training. In the cross-VM setting on the noisy Amazon EC2 the success rate drops to 60\% for a suite of 25 applications. With this initial study we demonstrate that it is possible to train meaningful models to successfully predict applications running in co-located instances.
ePrint Report Model-counting Approaches For Nonlinear Numerical Constraints Mateus Borges, Quoc-Sang Phan, Antonio Filieri, Corina S. P\u{a}s\u{a}reanu
Model counting is of central importance in quantitative reasoning about systems. Examples include computing the probability that a system successfully accomplishes its task without errors, and measuring the number of bits leaked by a system to an adversary in Shannon entropy. Most previous work in those areas demonstrated their analysis on programs with linear constraints, in which cases model counting is polynomial time. Model counting for nonlinear constraints is notoriously hard, and thus programs with nonlinear constraints are not well-studied. This paper surveys state-of-the-art techniques and tools for model counting with respect to SMT constraints, modulo the bitvector theory, since this theory is decidable, and it can express nonlinear constraints that arise from the analysis of computer programs. We integrate these techniques within the Symbolic Pathfinder platform and evaluate them on difficult nonlinear constraints generated from the analysis of cryptographic functions.
14 March 2017
ePrint Report Key Recovery: Inert and Public Colin Boyd, Xavier Boyen, Christopher Carr, Thomas Haines
We propose a public key infrastructure framework, inspired by modern distributed cryptocurrencies, that allows for tunable key escrow, where the availability of key escrow is only provided under strict conditions and enforced through cryptographic measures. We argue that any key escrow scheme designed for the global scale must be both inert --- requiring considerable effort to recover a key --- and public --- everybody should be aware of all key recovery attempts. To this end, one of the contributions of this work is an abstract design of a proofof-work scheme that demonstrates the ability to recover a private key for some generic public key scheme. Our framework represents a new direction for key escrow, seeking an acceptable compromise between the demands for control of cryptography on the Internet and the fundamental rights of privacy, which we seek to align by drawing parallels to the physical world.
ePrint Report Full accounting for verifiable outsourcing Riad S. Wahby, Ye Ji, Andrew J. Blumberg, abhi shelat, Justin Thaler, Michael Walfish, Thomas Wies
Systems for verifiable outsourcing incur costs for a prover, a verifier, and precomputation; outsourcing makes sense when these costs are cheaper than not outsourcing. Yet, prover costs are generally ignored. The only exception is Verifiable ASICs (VA), wherein the prover is a custom chip; however, the only prior VA system ignores the cost of precomputation.

This paper describes a new VA system, called Giraffe; charges Giraffe for all three costs; and identifies regimes where outsourcing is worthwhile. Giraffe’s base is an interactive proof geared to data parallel computation. Giraffe makes this protocol asymptotically optimal for the prover, which is of independent interest. Giraffe also develops a design template that produces hardware designs automatically for a wide range of parameters, introduces hardware primitives molded to the protocol’s data flows, and incorporates program analyses that expand applicability. Giraffe wins even when outsourcing several tens of sub-computations, scales to 500x larger computations than prior work, and can profitably outsource parts of programs that are not worthwhile to outsource in full.
ePrint Report Forkable Strings are Rare Alexander Russell, Cristopher Moore, Aggelos Kiayias, Saad Quader
A fundamental combinatorial notion related to the dynamics of the Ouroboros proof-of-stake blockchain protocol (https://eprint.iacr.org/2016/889) is that of a forkable string. The original description and analysis of the protocol established that the probability that a string of length $n$ is forkable, when drawn from a binomial distribution with parameter $(1 - \epsilon)/2$, is $\exp(-\Omega(\sqrt{n}))$. In this note we improve this estimate to $\exp(-\Omega(n))$.
11 March 2017
Succinct non-interactive arguments (SNARGs) enable verifying NP computations with substantially lower complexity than that required for classical NP verification. In this work, we first construct a lattice-based SNARG candidate with quasi-optimal succinctness (where the argument size is quasilinear in the security parameter). Further extension of our methods yields the first SNARG (from any assumption) that is quasi-optimal in terms of both prover overhead (polylogarithmic in the security parameter) as well as succinctness. Moreover, because our constructions are lattice-based, they plausibly resist quantum attacks. Central to our construction is a new notion of linear-only vector encryption which is a generalization of the notion of linear-only encryption introduced by Bitansky et al. (TCC 2013). We conjecture that variants of Regev encryption satisfy our new linear-only definition. Then, together with new information-theoretic approaches for building statistically-sound linear PCPs over small finite fields, we obtain the first quasi-optimal SNARGs.

We then show a surprising connection between our new lattice-based SNARGs and the concrete efficiency of program obfuscation. All existing obfuscation candidates currently rely on multilinear maps. Among the constructions that make black-box use of the multilinear map, obfuscating a circuit of even moderate depth (say, 100) requires a multilinear map with multilinearity degree in excess of 2^100. In this work, we show that an ideal obfuscation of both the decryption function in a fully homomorphic encryption scheme and a variant of the verification algorithm of our new lattice-based SNARG yields a general-purpose obfuscator for all circuits. Finally, we give some concrete estimates needed to obfuscate this "obfuscation-complete" primitive. We estimate that at 80-bits of security, a (black-box) multilinear map with ≈2^12 levels of multilinearity suffices. This is over 2^80 times more efficient than existing candidates, and thus, represents an important milestone towards implementable program obfuscation for all circuits.
Secure and highly efficient authenticated encryption (AE) algorithms which achieve data confidentiality and authenticity in the symmetric-key setting have existed for well over a decade. By all conventional measures, AES-OCB seems to be the AE algorithm of choice on any platform with AES-NI: it has a proof showing it is secure assuming AES is, and it is one of the fastest out of all such algorithms. However, algorithms such as AES-GCM and ChaCha20+Poly1305 have seen more widespread adoption, even though they will likely never outperform AES-OCB on platforms with AES-NI. Given the fact that changing algorithms is a long and costly process, some have set out to maximize the security that can be achieved with the already deployed algorithms, without sacrificing efficiency: ChaCha20+Poly1305 already improves over GCM in how it authenticates, GCM-SIV uses GCM's underlying components to provide nonce misuse resistance, and TLS1.3 introduces a randomized nonce in order to improve GCM's multi-user security. We continue this line of work by looking more closely at GCM and ChaCha20+Poly1305 to see what robustness they already provide over algorithms such as OCB, and whether minor variants of the algorithms can be used for applications where defense in depth is critical. We formalize and illustrate how GCM and ChaCha20+Poly1305 offer varying degrees of resilience to nonce misuse, as they can recover quickly from repeated nonces, as opposed to OCB, which loses all security. More surprisingly, by introducing minor tweaks such as an additional XOR, we can create a GCM variant which provides security even when unverified plaintext is released.
The public nature of the blockchain has been shown to be a severe threat for the privacy of Bitcoin users. Even worse, since funds can be tracked and tainted, no two coins are equal, and fungibility, a fundamental property required in every currency, is at risk. With these threats in mind, several privacy-enhancing technologies have been proposed to improve transaction privacy in Bitcoin. However, they either require a deep redesign of the currency, breaking many currently deployed features, or they address only specific privacy issues and consequently provide only very limited guarantees when deployed separately.

The goal of this work is to overcome this trade-off. Building on CoinJoin, we design ValueShuffle, the first coin mixing protocol compatible with Confidential Transactions, a proposed enhancement to the Bitcoin protocol to hide payment values in the blockchain. ValueShuffle ensures the anonymity of mixing participants as well as the confidentiality of their payment values even against other possibly malicious mixing participants. By combining CoinJoin with Confidential Transactions and additionally Stealth Addresses, ValueShuffle provides comprehensive privacy (payer anonymity, payee anonymity, and payment value privacy) without breaking with fundamental design principles or features of the current Bitcoin system. Assuming that Confidential Transactions will be integrated in the Bitcoin protocol, ValueShuffle makes it possible to mix funds of different value as well as to mix and spend funds in the same transaction, which overcomes the two main limitations of previous coin mixing protocols.
Cryptographic agility is the ability to switch to larger cryptographic parameters or different algorithms in the case of security doubts. This very desirable property of cryptographic systems is inherently difficult to achieve in cryptocurrencies due to their permanent state in the blockchain: for example, if it turns out that the employed signature scheme is insecure, a switch to a different scheme can only protect the outputs of future transactions but cannot fix transaction outputs already recorded in the blockchain, exposing owners of the corresponding money to risk of theft. This situation is even worse with Confidential Transactions, a recent privacy-enhancing proposal to hide transacted monetary amounts in homomorphic commitments. If an attacker manages to break the computational binding property of a commitment, he can create money out of thin air, jeopardizing the security of the entire currency. The obvious solution is to use statistically or perfectly binding commitment schemes but they come with performance drawbacks due to the need for less efficient range proofs.

In this paper, our aim is to overcome this dilemma. We introduce switch commitments, which constitute a cryptographic middle ground between computationally binding and statistically binding commitments. The key property of this novel primitive is the possibility to switch existing commitments, e.g., recorded in the blockchain, from computational bindingness to statistical bindingness if doubts in the underlying hardness assumption arise. This switch trades off efficiency for security. We provide a practical and simple construction of switch commitments by proving that ElGamal commitments with a restricted message space are secure switch commitments.
We design a new McEliece-like rank metric based encryption scheme from Gabidulin codes. We explain why it is not affected by the invariant subspace attacks also known as Overbeck's attacks. The idea of the design mixes two existing approaches designing rank metric based encryption schemes. For a given security our public-keys are more compact than for the same security in the Hamming metric based settings.
In this article, a new oblivious transfer (OT) protocol, secure in the presence of erasure-free one-sided active adaptive adversaries is presented. The new bit OT protocol achieves better communication complexity than the existing bit OT protocol in this setting. The new bit OT protocol requires fewer number of public key encryption operations than the existing bit OT protocol in this setting. As a building block, a new two-party lossy threshold homomorphic public key cryptosystem is designed. It is secure in the same adversary model. It is of independent interest.
We develop foundations and several constructions for security protocols that can automatically detect, without false positives, if a secret (such as a key or password) has been misused. Such constructions can be used, e.g., to automatically shut down compromised services, or to automatically revoke misused secrets to minimize the effects of compromise. Our threat model includes malicious agents, (temporarily or permanently) compromised agents, and clones.

Previous works have studied domain-specific partial solutions to this problem. For example, Google's Certificate Transparency aims to provide infrastructure to detect the misuse of a certificate authority's signing key, logs have been used for detecting endpoint compromise, and protocols have been proposed to detect cloned RFID/smart cards. Contrary to these existing approaches, for which the designs are interwoven with domain-specific considerations and which usually do not enable fully automatic response (i.e., they need human assessment), our approach shows where automatic action is possible. Our results unify, provide design rationales, and suggest improvements for the existing domain-specific solutions.

Based on our analysis, we construct several mechanisms for the detection of misuse. Our mechanisms enable automatic response, such as revoking keys or shutting down services, thereby substantially limiting the impact of a compromise.

In several case studies, we show how our mechanisms can be used to substantially increase the security guarantees of a wide range of systems, such as web logins, payment systems, or electronic door locks. For example, we propose and formally verify an improved version of Cloudflare's Keyless SSL protocol that enables key misuse detection.
8 March 2017
Wee (TCC'14) and Attrapadung (Eurocrypt'14) introduced predicate and pair encodings, respectively, as a simple way to construct and analyze attribute-based encryption schemes, or more generally predicate encryption. However, many schemes do not satisfy the simple information theoretic property proposed in those works, and thus require much more complicated analysis. In this paper, we propose a new simple property for pair encodings called symbolic security. Proofs that pair encodings satisfy this property are concise and easy to verify. We show that this property is inherently tied to the security of predicate encryption schemes by arguing that any scheme which is not trivially broken must satisfy it. Then we use this property to discuss several ways to convert between pair encodings to obtain encryption schemes with different properties like small ciphertexts or keys. Finally, we show that any pair encoding satisfying our new property can be used to construct a fully secure predicate encryption scheme. The resulting schemes are secure under a new q-type assumption which we show follows from several of the assumptions used to construct such schemes in previous work.
ePrint Report TwinsCoin: A Cryptocurrency via Proof-of-Work and Proof-of-Stake Alexander Chepurnoy, Tuyet Duong, Lei Fan, Hong-Sheng Zhou
We design and implement TwinsCoin, the first cryptocurrency based on a provably secure and scalable public blockchain design using both proof-of-work and proof-of-stake mechanisms. Different from the proof-of-work based Bitcoin, our construction uses two types of resources, computing power and coins~(i.e., stake). The blockchain in our system is more robust than that in a pure proof-of-work based system; even if the adversary controls the majority of mining power, we can still have the chance to secure the system by relying on honest stake. In contrast, Bitcoin blockchain will be insecure if the adversary controls more than 50\% of mining power. Our design follows a recent provably secure proof-of-work/proof-of-stake hybrid blockchain by Duong et al.~(ePrint 2016). In order to make our construction practical, we enhance Duong et al.'s design. In particular, we introduce a new strategy for difficulty adjustment in the hybrid blockchain and provide an analysis of it. We also show how to construct a light client for proof-of-stake cryptocurrencies and evaluate the proposal practically.

We implement our new design. Our implementation uses a recent modular development framework for blockchains, called Scorex. It allows us to change only certain parts of an application leaving other codebase intact. In addition to the blockchain implementation, a testnet is deployed. Source code is publicly available.
We propose a nonce misuse-resistant message authentication scheme called EHE (Encrypt-Hash-Encrypt). In EHE, a message-dependent polynomial is evaluated at the point which is an encrypted nonce. The resulting polynomial hash value is encrypted again and becomes an authentication tag. We prove the prf-security of the EHE scheme and extend it to two authenticated encryption modes which follow the "encrypt-then-authenticate" paradigm.
Despite their incentive structure flaws, mining pools account for more than 95% of Bitcoin's computation power. This paper introduces an attack against mining pools in which a malicious party pays pool members to withhold their solutions from their pool operator. We show that an adversary with a tiny amount of computing power and capital can execute this attack. Smart contracts enforce the malicious party's payments, and therefore miners need neither trust the attacker's intentions nor his ability to pay. Assuming pool members are rational, an adversary with a single mining ASIC can, in theory, destroy all big mining pools without losing any money (and even make some profit).
Several Multi-Prover Interactive Proofs (MIPs) found in the literature contain proofs of soundness that are lacking. This was first observed by Cr\'epeau, Salvail, Simard and Tapp who defined a notion of {Prover isolation} to partly address the issue. Furthermore, some existing Zero-Knowledge MIPs suffer from a catastrophic flaw: they outright allow the Provers to communicate via the Verifier. Consequently, their soundness claims are now seriously in doubt, if not plain wrong. This paper outlines the lack of isolation and numerous other issues found in the (ZK)MIP literature. A follow-up paper will resolve most of these issues in detail.
ePrint Report Efficient and Secure Outsourcing of Genomic Data Storage João Sá Sousa, Cédric Lefebvre, Zhicong Huang, Jean Louis Raisaro, Carlos Aguilar, Marc-Olivier Killijian, Jean-Pierre Hubaux
loud computing is becoming the preferred solution for efficiently dealing with the increasing amount of genomic data. Yet, outsourcing storage and processing of sensitive data, such as genomic data, comes with important concerns related to privacy and security. This calls for new sophisticated techniques that ensure data protection from untrusted cloud providers and still enables researchers to obtain useful information. We present a novel privacy-preserving algorithm for fully outsourcing the storage of large genomic data files to a public cloud and enable researchers to efficiently search for variants of interest. To preserve data and query confidentiality from possible leakage, our solution exploits optimal encoding for genomic variants and combines it with homomorphic encryption and private information retrieval. The proposed algorithm is implemented in C++ and evaluated on real data as part of the 2016 iDash genome privacy-protection challenge. Results show that our solution outperforms the state-of-the-art and enables researchers to search over millions of encrypted variants in a few seconds. As opposed to prior beliefs that sophisticated privacy-enhancing technologies (PETs) are unpractical for real operational settings, our solution demonstrates that, in the case of genomic data, PETs can represent very efficient enablers.
ePrint Report Towards Shared Ownership in the Cloud Hubert Ritzdorf, Claudio Soriente, Ghassan O. Karame, Srdjan Marinovic, Damian Gruber, Srdjan Capkun
Cloud storage platforms promise a convenient way for users to share files and engage in collaborations, yet they require all files to have a single owner who unilaterally makes access control decisions. Existing clouds are, thus, agnostic to the notion of shared ownership. This can be a significant limitation in many collaborations because, for example, one owner can delete files and revoke access without consulting the other collaborators.

In this paper, we first formally define a notion of shared ownership within a file access control model. We then propose two possible instantiations of our proposed shared ownership model. Our first solution, called Commune, relies on secure file dispersal and collusion-resistant secret sharing to ensure that all access grants in the cloud require the support of an agreed threshold of owners. As such, Commune can be used in existing clouds without modifications to the platforms. Our second solution, dubbed Comrade, leverages the blockchain technology in order to reach consensus on access control decision. Unlike Commune, Comrade requires that the cloud is able to translate access control decisions that reach consensus in the blockchain into storage access control rules, thus requiring minor modifications to existing clouds. We analyze the security of our proposals and compare/evaluate their performance through implementation integrated with Amazon S3.
LEGO-style cut-and-choose is known for its asymptotic efficiency in realizing actively-secure computations. The dominant cost of LEGO protocols is due to wire-soldering — the key technique enabling to put independently generated garbled gates together in a bucket to realize a logical gate. Existing wire-soldering constructions rely on homomorphic commitments and their security requires the majority of the garbled gates in every bucket to be correct.

In this paper, we propose an efficient construction of LEGO protocols that does not use homomorphic commitments but is able to guarantee security as long as at least one of the garbled gate in each bucket is correct. Additionally, the faulty gate detection rate in our protocol doubles that of the state-of-the-art LEGO constructions. We have implemented our protocol and our experiments on several benchmark applications show that the performance of our approach is highly competitive in comparison with existing implementations.

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