Privacy-Preserving Trust Management Mechanisms from Private Matching Schemes

Cryptographic primitives are essential for constructing privacy-preserving communication mechanisms. There are situations in which two parties that do not know each other need to exchange sensitive information on the Internet. Trust management mechanisms make use of digital credentials and certificates in order to establish trust among these strangers. We address the problem of choosing which credentials are exchanged. During this process, each party should learn no information about the preferences of the other party other than strictly required for trust establishment. We present a method to reach an agreement on the credentials to be exchanged that preserves the privacy of the parties. Our method is based on secure two-party computation protocols for set intersection. Namely, it is constructed from private matching schemes.Comment: The material in this paper will be presented in part at the 8th DPM International Workshop on Data Privacy Management (DPM 2013

Multiparty Generation of an RSA Modulus

We present a new multiparty protocol for the distributed generation of biprime RSA moduli, with security against any subset of maliciously colluding parties assuming oblivious transfer and the hardness of factoring. Our protocol is highly modular, and its uppermost layer can be viewed as a template that generalizes the structure of prior works and leads to a simpler security proof. We introduce a combined sampling-and-sieving technique that eliminates both the inherent leakage in the approach of Frederiksen et al. (Crypto\u2718), and the dependence upon additively homomorphic encryption in the approach of Hazay et al. (JCrypt\u2719). We combine this technique with an efficient, privacy-free check to detect malicious behavior retroactively when a sampled candidate is not a biprime, and thereby overcome covert rejection-sampling attacks and achieve both asymptotic and concrete efficiency improvements over the previous state of the art

Efficient noninteractive certification of RSA moduli and beyond

In many applications, it is important to verify that an RSA public key (N; e) speci es a permutation over the entire space ZN, in order to prevent attacks due to adversarially-generated public keys. We design and implement a simple and e cient noninteractive zero-knowledge protocol (in the random oracle model) for this task. Applications concerned about adversarial key generation can just append our proof to the RSA public key without any other modi cations to existing code or cryptographic libraries. Users need only perform a one-time veri cation of the proof to ensure that raising to the power e is a permutation of the integers modulo N. For typical parameter settings, the proof consists of nine integers modulo N; generating the proof and verifying it both require about nine modular exponentiations. We extend our results beyond RSA keys and also provide e cient noninteractive zero- knowledge proofs for other properties of N, which can be used to certify that N is suitable for the Paillier cryptosystem, is a product of two primes, or is a Blum integer. As compared to the recent work of Auerbach and Poettering (PKC 2018), who provide two-message protocols for similar languages, our protocols are more e cient and do not require interaction, which enables a broader class of applications.https://eprint.iacr.org/2018/057First author draf

Combining Private Set-Intersection with Secure Two-Party Computation

Private Set-Intersection (PSI) is one of the most popular and practically relevant secure two-party computation (2PC) tasks. Therefore, designing special-purpose PSI protocols (which are more efficient than generic 2PC solutions) is a very active line of research. In particular, a recent line of work has proposed PSI protocols based on oblivious transfer (OT) which, thanks to recent advances in OT-extension techniques, is nowadays a very cheap cryptographic building block. Unfortunately, these protocols cannot be plugged into larger 2PC applications since in these protocols one party (by design) learns the output of the intersection. Therefore, it is not possible to perform secure post-processing of the output of the PSI protocol. In this paper we propose a novel and efficient OT-based PSI protocol that produces an encrypted output that can therefore be later used as an input to other 2PC protocols. In particular, the protocol can be used in combination with all common approaches to 2PC including garbled circuits, secret sharing and homomorphic encryption. Thus, our protocol can be combined with the right 2PC techniques to achieve more efficient protocols for computations of the form $z=f(X\cap Y)$ for arbitrary functions $f$

Round-optimal Black-box Commit-and-prove with Succinct Communication

We give a four-round black-box construction of a commit-and-prove protocol with succinct communication. Our construction is WI and has constant soundness error, and it can be upgraded into a one that is ZK and has negligible soundness error by relying on a round-preserving transformation of Khurana et al. (TCC 2018). Our construction is obtained by combining the MPC-in-the-head technique of Ishai et al. (SICOMP 2009) with the two-round succinct argument of Kalai et al. (STOC 2014), and the main technical novelty lies in the analysis of the soundness---we show that, although the succinct argument of Kalai et al. does not necessarily provide soundness for NP statements, it can be used in the MPC-in-the-head technique for proving the consistency of committed MPC views. Our construction is based on sub-exponentially hard collision-resistant hash functions, two-round PIRs, and two-round OTs

Secure multiparty PageRank algorithm for collaborative fraud detection

Collaboration between financial institutions helps to improve detection of fraud. However, exchange of relevant data between these institutions is often not possible due to privacy constraints and data confidentiality. An important example of relevant data for fraud detection is given by a transaction graph, where the nodes represent bank accounts and the links consist of the transactions between these accounts. Previous works show that features derived from such graphs, like PageRank, can be used to improve fraud detection. However, each institution can only see a part of the whole transaction graph, corresponding to the accounts of its own customers. In this research a new method is described, making use of secure multiparty computation (MPC) techniques, allowing multiple parties to jointly compute the PageRank values of their combined transaction graphs securely, while guaranteeing that each party only learns the PageRank values of its own accounts and nothing about the other transaction graphs. In our experiments this method is applied to graphs containing up to tens of thousands of nodes. The execution time scales linearly with the number of nodes, and the method is highly parallelizable. Secure multiparty PageRank is feasible in a realistic setting with millions of nodes per party by extrapolating the results from our experiments

Non-malleable codes for space-bounded tampering

Non-malleable codes—introduced by Dziembowski, Pietrzak and Wichs at ICS 2010—are key-less coding schemes in which mauling attempts to an encoding of a given message, w.r.t. some class of tampering adversaries, result in a decoded value that is either identical or unrelated to the original message. Such codes are very useful for protecting arbitrary cryptographic primitives against tampering attacks against the memory. Clearly, non-malleability is hopeless if the class of tampering adversaries includes the decoding and encoding algorithm. To circumvent this obstacle, the majority of past research focused on designing non-malleable codes for various tampering classes, albeit assuming that the adversary is unable to decode. Nonetheless, in many concrete settings, this assumption is not realistic

SWiM: Secure Wildcard Pattern Matching From OT Extension

Suppose a server holds a long text string and a receiver holds a short pattern string. Secure pattern matching allows the receiver to learn the locations in the long text where the pattern appears, while leaking nothing else to either party besides the length of their inputs. In this work we consider secure wildcard pattern matching WPM, where the receiver\u27s pattern is allowed to contain wildcards that match to any character. We present SWiM, a simple and fast protocol for WPM that is heavily based on oblivious transfer (OT) extension. As such, the protocol requires only a small constant number of public-key operations and otherwise uses only very fast symmetric-key primitives. SWiM is secure against semi-honest adversaries. We implemented a prototype of our protocol to demonstrate its practicality. We can perform WPM on a DNA text (4-character alphabet) of length $10^5$ and pattern of length $10^3$ in just over 2 seconds, which is over two orders of magnitude faster than the state-of-the-art scheme of Baron et al. (SCN 2012)

Insured MPC: Efficient Secure Computation with Financial Penalties

Fairness in Secure Multiparty Computation (MPC) is known to be impossible to achieve in the presence of a dishonest majority. Previous works have proposed combining MPC protocols with Cryptocurrencies in order to financially punish aborting adversaries, providing an incentive for parties to honestly follow the protocol. This approach also yields privacy-preserving Smart Contracts, where private inputs can be processed with MPC in order to determine the distribution of funds given to the contract. The focus of existing work is on proving that this approach is possible and unfortunately they present monolithic and mostly inefficient constructions. In this work, we put forth the first modular construction of ``Insured MPC\u27\u27, where either the output of the private computation (which describes how to distribute funds) is fairly delivered or a proof that a set of parties has misbehaved is produced, allowing for financial punishments. Moreover, both the output and the proof of cheating are publicly verifiable, allowing third parties to independently validate an execution. We present a highly efficient compiler that uses any MPC protocol with certain properties together with a standard (non-private) Smart Contract and a publicly verifiable homomorphic commitment scheme to implement Insured MPC. As an intermediate step, we propose the first construction of a publicly verifiable homomorphic commitment scheme achieving composability guarantees and concrete efficiency. Our results are proven in the Global Universal Composability framework using a Global Random Oracle as the setup assumption. From a theoretical perspective, our general results provide the first characterization of sufficient properties that MPC protocols must achieve in order to be efficiently combined with Cryptocurrencies, as well as insights into publicly verifiable protocols. On the other hand, our constructions have highly efficient concrete instantiations, allowing for fast implementations

Scalable Multi-Party Private Set-Intersection

In this work we study the problem of private set-intersection in the multi-party setting and design two protocols with the following improvements compared to prior work. First, our protocols are designed in the so-called star network topology, where a designated party communicates with everyone else, and take a new approach of leveraging the 2PC protocol of [FreedmanNP04]. This approach minimizes the usage of a broadcast channel, where our semi-honest protocol does not make any use of such a channel and all communication is via point-to-point channels. In addition, the communication complexity of our protocols scales with the number of parties. More concretely, (1) our first semi-honest secure protocol implies communication complexity that is linear in the input sizes, namely $O((\sum_{i=1}^n m_i)\cdot\kappa)$ bits of communication where $\kappa$ is the security parameter and $m_i$ is the size of $P_i$\u27s input set, whereas overall computational overhead is quadratic in the input sizes only for a designated party, and linear for the rest. We further reduce this overhead by employing two types of hashing schemes. (2) Our second protocol is proven secure in the malicious setting. This protocol induces communication complexity O((n^2 + nm_\maxx + nm_\minn\log m_\maxx)\kappa) bits of communication where m_\minn (resp. m_\maxx) is the minimum (resp. maximum) over all input sets sizes and $n$ is the number of parties