Fair and Sound Secret Sharing from Homomorphic Time-Lock Puzzles

Achieving fairness and soundness in non-simultaneous rational secret sharing schemes has proved to be challenging. On the one hand, soundness can be ensured by providing side information related to the secret as a check, but on the other, this can be used by deviant players to compromise fairness. To overcome this, the idea of incorporating a time delay was suggested in the literature: in particular, time-delay encryption based on memory-bound functions has been put forth as a solution. In this paper, we propose a different approach to achieve such delay, namely using homomorphic time-lock puzzles (HTLPs), introduced at CRYPTO 2019, and construct a fair and sound rational secret sharing scheme in the non-simultaneous setting from HTLPs. HTLPs are used to embed sub-shares of the secret for a predetermined time. This allows to restore fairness of the secret reconstruction phase, despite players having access to information related to the secret which is required to ensure soundness of the scheme. Key to our construction is the fact that the time-lock puzzles are homomorphic so that players can compactly evaluate sub-shares. Without this efficiency improvement, players would have to independently solve each puzzle sent from the other players to obtain a share of the secret, which would be computationally inefficient. We argue that achieving both fairness and soundness in a non-simultaneous scheme using a time delay based on CPU-bound functions rather than memory-bound functions is more cost effective and realistic in relation to the implementation of the construction

KALwEN: a new practical and interoperable key management scheme for body sensor networks

Key management is the pillar of a security architecture. Body sensor networks (BSNs) pose several challenges–some inherited from wireless sensor networks (WSNs), some unique to themselves–that require a new key management scheme to be tailor-made. The challenge is taken on, and the result is KALwEN, a new parameterized key management scheme that combines the best-suited cryptographic techniques in a seamless framework. KALwEN is user-friendly in the sense that it requires no expert knowledge of a user, and instead only requires a user to follow a simple set of instructions when bootstrapping or extending a network. One of KALwEN's key features is that it allows sensor devices from different manufacturers, which expectedly do not have any pre-shared secret, to establish secure communications with each other. KALwEN is decentralized, such that it does not rely on the availability of a local processing unit (LPU). KALwEN supports secure global broadcast, local broadcast, and local (neighbor-to-neighbor) unicast, while preserving past key secrecy and future key secrecy (FKS). The fact that the cryptographic protocols of KALwEN have been formally verified also makes a convincing case. With both formal verification and experimental evaluation, our results should appeal to theorists and practitioners alike

Privacy-Preserving Observation in Public Spaces

One method of privacy-preserving accounting or billing in cyber-physical systems, such as electronic toll collection or public transportation ticketing, is to have the user present an encrypted record of transactions and perform the accounting or billing computation securely on them. Honesty of the user is ensured by spot checking the record for some selected surveyed transactions. But how much privacy does that give the user, i.e. how many transactions need to be surveyed? It turns out that due to collusion in mass surveillance all transactions need to be observed, i.e. this method of spot checking provides no privacy at all. In this paper we present a cryptographic solution to the spot checking problem in cyber-physical systems. Users carry an authentication device that authenticates only based on fair random coins. The probability can be set high enough to allow for spot checking, but in all other cases privacy is perfectly preserved. We analyze our protocol for computational efficiency and show that it can be efficiently implemented even on plat- forms with limited computing resources, such as smart cards and smart phones

On-Device Recommender Systems: A Comprehensive Survey

Recommender systems have been widely deployed in various real-world applications to help users identify content of interest from massive amounts of information. Traditional recommender systems work by collecting user-item interaction data in a cloud-based data center and training a centralized model to perform the recommendation service. However, such cloud-based recommender systems (CloudRSs) inevitably suffer from excessive resource consumption, response latency, as well as privacy and security risks concerning both data and models. Recently, driven by the advances in storage, communication, and computation capabilities of edge devices, there has been a shift of focus from CloudRSs to on-device recommender systems (DeviceRSs), which leverage the capabilities of edge devices to minimize centralized data storage requirements, reduce the response latency caused by communication overheads, and enhance user privacy and security by localizing data processing and model training. Despite the rapid rise of DeviceRSs, there is a clear absence of timely literature reviews that systematically introduce, categorize and contrast these methods. To bridge this gap, we aim to provide a comprehensive survey of DeviceRSs, covering three main aspects: (1) the deployment and inference of DeviceRSs (2) the training and update of DeviceRSs (3) the security and privacy of DeviceRSs. Furthermore, we provide a fine-grained and systematic taxonomy of the methods involved in each aspect, followed by a discussion regarding challenges and future research directions. This is the first comprehensive survey on DeviceRSs that covers a spectrum of tasks to fit various needs. We believe this survey will help readers effectively grasp the current research status in this field, equip them with relevant technical foundations, and stimulate new research ideas for developing DeviceRSs

Crowdsourcing atop blockchains

Traditional crowdsourcing systems, such as Amazon\u27s Mechanical Turk (MTurk), though once acquiring great economic successes, have to fully rely on third-party platforms to serve between the requesters and the workers for basic utilities. These third-parties have to be fully trusted to assist payments, resolve disputes, protect data privacy, manage user authentications, maintain service online, etc. Nevertheless, tremendous real-world incidents indicate how elusive it is to completely trust these platforms in reality, and the reduction of such over-reliance becomes desirable. In contrast to the arguably vulnerable centralized approaches, a public blockchain is a distributed and transparent global consensus computer that is highly robust. The blockchain is usually managed and replicated by a large-scale peer-to-peer network collectively, thus being much more robust to be fully trusted for correctness and availability. It, therefore, becomes enticing to build novel crowdsourcing applications atop blockchains to reduce the over-trust on third-party platforms. However, this new fascinating technology also brings about new challenges, which were never that severe in the conventional centralized setting. The most serious issue is that the blockchain is usually maintained in the public Internet environment with a broader attack surface open to anyone. This not only causes serious privacy and security issues, but also allows the adversaries to exploit the attack surface to hamper more basic utilities. Worse still, most existing blockchains support only light on-chain computations, and the smart contract executed atop the decentralized consensus computer must be simple, which incurs serious feasibility problems. In reality, the privacy/security issue and the feasibility problem even restrain each other and create serious tensions to hinder the broader adoption of blockchain. The dissertation goes through the non-trivial challenges to realize secure yet still practical decentralization (for urgent crowdsourcing use-cases), and lay down the foundation for this line of research. In sum, it makes the next major contributions. First, it identifies the needed security requirements in decentralized knowledge crowdsourcing (e.g., data privacy), and initiates the research of private decentralized crowdsourcing. In particular, the confidentiality of solicited data is indispensable to prevent free-riders from pirating the others\u27 submissions, thus ensuring the quality of solicited knowledge. To this end, a generic private decentralized crowdsourcing framework is dedicatedly designed, analyzed, and implemented. Furthermore, this dissertation leverages concretely efficient cryptographic design to reduce the cost of the above generic framework. It focuses on decentralizing the special use-case of Amazon MTurk, and conducts multiple specific-purpose optimizations to remove needless generality to squeeze performance. The implementation atop Ethereum demonstrates a handling cost even lower than MTurk. In addition, it focuses on decentralized crowdsourcing of computing power for specific machine learning tasks. It lets a requester place deposits in the blockchain to recruit some workers for a designated (randomized) programs. If and only if these workers contribute their resources to compute correctly, they would earn well-deserved payments. For these goals, a simple yet still useful incentive mechanism is developed atop the blockchain to deter rational workers from cheating. Finally, the research initiates the first systematic study on crowdsourcing blockchains\u27 full nodes to assist superlight clients (e.g., mobile phones and IoT devices) to read the blockchain\u27s records. This dissertation presents a novel generic solution through the powerful lens of game-theoretic treatments, which solves the long-standing open problem of designing generic superlight clients for all blockchains

Security in mobile agent systems: an approach to protect mobile agents from malicious host attacks

Mobile agents are autonomous programs that roam the Internet from machine to machine under their own control on behalf of their users to perform specific pre-defined tasks. In addition to that, a mobile agent can suspend its execution at any point; transfer itself to another machine then resume execution at the new machine without any loss of state. Such a mobile model can perform many possible types of operations, and might carry critical data that has to be protected from possible attacks. The issue of agent security and specially agent protection from host attacks has been a hot topic and no fully comprehensive solution has been found so far. In this thesis, we examine the possible security attacks that hosts and agents suffer from. These attacks can take one of four possible forms: Attacks from host to host, from agents to hosts, from agents to agents (peer to peer) and finally from hosts to agents. Our main concern in this thesis is these attacks from a malicious host on an agent. These attacks can take many forms including rerouting, spying out code, spying out data, spying out control flow, manipulation of code, manipulation of data, manipulation of control flow, incorrect execution of code, masquerading and denial of execution. In an attempt to solve the problem of malicious host attacks on agents, many partial solutions were proposed. These solutions ranged across simple legal protection, hardware solutions, partitioning, replication and voting, components, self-authentication, and migration history. Other solutions also included using audit logs, read-only state, append only logs, encrypted algorithms, digital signatures, partial result authentication codes, and code mess-up, limited life time of code and data as well as time limited black box security. In this thesis, we present a three-tier solution. This solution is a combination of code mess up, encryption and time out. Choosing code mess-up as part of the solution was due to the several strengths of this method that is based on obfuscating the features of the code so that any attacker will find it very difficult to understand the original code. A new algorithm iii was developed in this thesis to implement code mess-up that uses the concept of variable disguising by altering the values of strings and numerical values. Several encryption algorithms were studied to choose the best algorithm to use in the development of the proposed solution. The algorithms studied included DES, LUCIFER, MADRYGA, NEWDES, FEAL, REDOC, LOKI, KHUFU & KHAFRE, IDEA and finally MMB. The algorithm used was the DES algorithm due to several important factors including its key length. Not any language can be used to implement mobile agents. Candidate languages should possess the portability characteristic and should be safe and secure enough to guarantee a protection for the mobile agent. In addition to that the language should be efficient in order to minimize the implementation overhead and the overhead of providing safety and security. Languages used to implement mobile agents include Java, Limbo, Telescript, and Safe TCL. The Java language was chosen as the programming language for this thesis due to its high security, platform independence, and multithreading. This is in addition to several powerful features that characterize the Java language as will be mentioned later on. Implementing a mobile agent requires the assistance of a mobile agent system that helps in launching the agent from one host to another. There are many existing agent launching systems like Telescript, Aglets, Tacoma, Agent TCL and Concordia. Concordia was chosen to be the implementation tool used to launch our mobile agent. It is a software framework for developing, running and administering mobile agents, and it proved to be very efficient, and effective. The results of our proposed solutions showed the strength of the proposed model in terms of fully protecting the mobile agent from possible malicious host attacks. The model could have several points of enhancements. These enhancements include changing the code mess-up algorithm to a more powerful one, using a different encryption technique, and implementing an agent re-charge mechanism to recharge the agent after it is timeout

Security and privacy issues in some special-puropse networks

This thesis is about providing security and privacy to new emergent applications which are based on special-purpose networks. More precisely, we study different aspects regarding security and privacy issues related to sensor networks, mobile ad hoc networks, vehicular ad hoc networks and social networks.Sensor networks consist of resource-constrained wireless devices with sensor capabilities. This emerging technology has a wide variety of applications related to event surveillance like emergency response, habitat monitoring or defense-related networks.Ad hoc networks are suited for use in situations where deploying an infrastructure is not cost effective or is not possible for any other reason. When the nodes of an ad hoc network are small mobile devices (e.g. cell phones or PDAs), such a network is called mobile ad hoc network. One of many possible uses of MANETs is to provide crisis management services applications, such as in disaster recovery, where the entire communication infrastructure is destroyed and reestablishing communication quickly is crucial. Another useful situation for MANETs is a scenario without fixed communication systems where there is the need for any kind of collaborative computing. Such situation can occur in both business and military environments.When the mobile nodes of a MANET are embedded in cars, such a network is called Vehicular Ad hoc Network (VANET). This kind of networks can be very useful to increase the road traffic safety and they will be deployed for real use in the forthcoming years. As a proof of that, eight important European vehicle manufacturers have founded the CAR 2 CAR Communication Consortium. This non-profit organisation is dedicated to the objective of further increasing traffic safety and efficiency by means of inter-vehicle communications.Social networks differ from the special-purpose networks commented above in that they are not physical networks. Social networks are applications that work through classic networks. They can be defined as a community of web users where each user can publish and share information and services. Social networks have become an object of study both in computer and social sciences, with even dedicated journals and conferences.The special-purpose networks described above provide a wide range of new services and applications. Even though they are expected to improve the society in several ways, these innovative networks and their related applications bring also security and privacy issues that must be addressed.This thesis solves some security and privacy issues related to such new applications and services. More specifically, it focuses on:·Secure information transmission in many-to-one scenarios with resource-constrained devices such as sensor networks.·Secure and private information sharing in MANETs.·Secure and private information spread in VANETs.·Private resource access in social networks.Results presented in this thesis include four contributions published in ISI JCR journals (IEEE Transactions on Vehicular Technology, Computer Networks (2) and Computer Communications) and two contributions published in two international conferences (Lecture Notes in Computer Science).Esta tesis trata diversos problemas de seguridad y privacidad que surgen al implantar en escenarios reales novedosas aplicaciones basadas en nuevos y emergentes modelos de red. Estos nuevos modelos de red difieren significativamente de las redes de computadores clásicas y son catalogadas como redes de propósito especial. Específicamente, en este trabajo se estudian diferentes aspectos relacionados con la seguridad de la información y la privacidad de los usuarios en redes de sensores, redes ad hoc móviles (MANETs), redes ad hoc vehiculares (VANETs) y redes sociales.Las redes de sensores están formadas por dispositivos inalámbricos muy limitados a nivel de recursos (capacidad de computación y batería) que detectan eventos o condiciones del entorno donde se instalan. Esta tecnología tiene una amplia variedad de aplicaciones entre las que destacan la detección de emergencias o la creación de perímetros de seguridad. Una MANET esta formada por nodos móviles conectados entre ellos mediante conexiones inalámbricas y de forma auto-organizada. Este tipo de redes se constituye sin la ayuda de infraestructuras, por ello son especialmente útiles en situaciones donde implantar una infraestructura es inviable por ser su coste demasiado elevado o por cualquier otra razón. Una de las muchas aplicaciones de las MANETs es proporcionar servicio en situaciones críticas (por ejemplo desastres naturales) donde la infraestructura de comunicaciones ha sido destruida y proporcionar conectividad rápidamente es crucial. Otra aplicación directa aparece en escenarios sin sistemas de comunicación fijos donde existe la necesidad de realizar algún tipo de computación colaborativa entre diversas máquinas. Esta situación se da tanto en ámbitos empresariales como militares.Cuando los nodos móviles de una MANET se asocian a vehículos (coches, camiones.), dicha red se denomina red ad hoc vehicular o VANET. Este tipo de redes pueden ser muy útiles para incrementar la seguridad vial y se espera su implantación para uso real en los próximos años. Como prueba de la gran importancia que tiene esta tecnología, los ocho fabricantes europeos más importantes han fundado la CAR 2 CAR Communication Consortium. Esta organización tiene como objetivo incrementar la seguridad y la eficiencia del tráfico mediante el uso de comunicaciones entre los vehículos.Las redes sociales se diferencian de las redes especiales descritas anteriormente en que éstas no son redes físicas. Las redes sociales son aplicaciones que funcionan a través de las redes de computadores clásicas. Una red de este tipo puede ser definida como una comunidad de usuarios web en donde dichos usuarios pueden publicar y compartir información y servicios. En la actualidad, las redes sociales han adquirido gran importancia ofreciendo un amplio abanico de posibilidades a sus usuarios: trabajar de forma colaborativa, compartir ficheros, búsqueda de nuevos amigos, etc.A continuación se resumen las aplicaciones en las que esta tesis se centra según el tipo de red asociada:·Transmisión segura de información en escenarios muchos-a-uno (múltiples emisores y un solo receptor) donde los dispositivos en uso poseen recursos muy limitados. Este escenario es el habitual en redes de sensores.·Distribución de información de forma segura y preservando la privacidad de los usuarios en redes ad hoc móviles.·Difusión de información (con el objeto de incrementar la seguridad vial) fidedigna preservando la privacidad de los usuarios en redes ad hoc vehiculares.·Acceso a recursos en redes sociales preservando la privacidad de los usuarios. Los resultados de la tesis incluyen cuatro publicaciones en revistas ISI JCR (IEEE Transactions on Vehicular Technology, Computer Networks (2) y Computer Communications) y dos publicaciones en congresos internacionales(Lecture Notes in Computer Science)