Storytelling Security: User-Intention Based Traffic Sanitization

Malicious software (malware) with decentralized communication infrastructure, such as peer-to-peer botnets, is difficult to detect. In this paper, we describe a traffic-sanitization method for identifying malware-triggered outbound connections from a personal computer. Our solution correlates user activities with the content of outbound traffic. Our key observation is that user-initiated outbound traffic typically has corresponding human inputs, i.e., keystroke or mouse clicks. Our analysis on the causal relations between user inputs and packet payload enables the efficient enforcement of the inter-packet dependency at the application level. We formalize our approach within the framework of protocol-state machine. We define new application-level traffic-sanitization policies that enforce the inter-packet dependencies. The dependency is derived from the transitions among protocol states that involve both user actions and network events. We refer to our methodology as storytelling security. We demonstrate a concrete realization of our methodology in the context of peer-to-peer file-sharing application, describe its use in blocking traffic of P2P bots on a host. We implement and evaluate our prototype in Windows operating system in both online and offline deployment settings. Our experimental evaluation along with case studies of real-world P2P applications demonstrates the feasibility of verifying the inter-packet dependencies. Our deep packet inspection incurs overhead on the outbound network flow. Our solution can also be used as an offline collect-and-analyze tool

On the semantic security of functional encryption schemes

Functional encryption (FE) is a powerful cryptographic primitive that generalizes many asymmetric encryption systems proposed in recent years. Syntax and security definitions for FE were proposed by Boneh, Sahai, and Waters (BSW) (TCC 2011) and independently by O’Neill (ePrint 2010/556). In this paper we revisit these definitions, identify several shortcomings in them, and propose a new definitional approach that overcomes these limitations. Our definitions display good compositionality properties and allow us to obtain new feasibility and impossibility results for adaptive token-extraction attack scenarios that shed further light on the potential reach of general FE for practical applications.ENIAC Joint UndertakingFundação para a Ciência e a Tecnologia (FCT

A privacy-preserving fuzzy interest matching protocol for friends finding in social networks

Nowadays, it is very popular to make friends, share photographs, and exchange news throughout social networks. Social networks widely expand the area of people’s social connections and make communication much smoother than ever before. In a social network, there are many social groups established based on common interests among persons, such as learning group, family group, and reading group. People often describe their profiles when registering as a user in a social network. Then social networks can organize these users into groups of friends according to their profiles. However, an important issue must be considered, namely many users’ sensitive profiles could have been leaked out during this process. Therefore, it is reasonable to design a privacy-preserving friends-finding protocol in social network. Toward this goal, we design a fuzzy interest matching protocol based on private set intersection. Concretely, two candidate users can first organize their profiles into sets, then use Bloom filters to generate new data structures, and finally find the intersection sets to decide whether being friends or not in the social network. The protocol is shown to be secure in the malicious model and can be useful for practical purposes.Peer ReviewedPostprint (author's final draft

Entangled cloud storage

Entangled cloud storage (Aspnes et al., ESORICS 2004) enables a set of clients to “entangle” their files into a single clew to be stored by a (potentially malicious) cloud provider. The entanglement makes it impossible to modify or delete significant part of the clew without affecting all files encoded in the clew. A clew keeps the files in it private but still lets each client recover his own data by interacting with the cloud provider; no cooperation from other clients is needed. At the same time, the cloud provider is discouraged from altering or overwriting any significant part of the clew as this will imply that none of the clients can recover their files. We put forward the first simulation-based security definition for entangled cloud storage, in the framework of universal composability (Canetti, 2001). We then construct a protocol satisfying our security definition, relying on an entangled encoding scheme based on privacy-preserving polynomial interpolation; entangled encodings were originally proposed by Aspnes et al. as useful tools for the purpose of data entanglement. As a contribution of independent interest we revisit the security notions for entangled encodings, putting forward stronger definitions than previous work (that for instance did not consider collusion between clients and the cloud provider). Protocols for entangled cloud storage find application in the cloud setting, where clients store their files on a remote server and need to be ensured that the cloud provider will not modify or delete their data illegitimately. Current solutions, e.g., based on Provable Data Possession and Proof of Retrievability, require the server to be challenged regularly to provide evidence that the clients’ files are stored at a given time. Entangled cloud storage provides an alternative approach where any single client operates implicitly on behalf of all others, i.e., as long as one client's files are intact, the entire remote database continues to be safe and unblemishe

POPE: Partial Order Preserving Encoding

Recently there has been much interest in performing search queries over encrypted data to enable functionality while protecting sensitive data. One particularly efficient mechanism for executing such queries is order-preserving encryption/encoding (OPE) which results in ciphertexts that preserve the relative order of the underlying plaintexts thus allowing range and comparison queries to be performed directly on ciphertexts. In this paper, we propose an alternative approach to range queries over encrypted data that is optimized to support insert-heavy workloads as are common in "big data" applications while still maintaining search functionality and achieving stronger security. Specifically, we propose a new primitive called partial order preserving encoding (POPE) that achieves ideal OPE security with frequency hiding and also leaves a sizable fraction of the data pairwise incomparable. Using only O(1) persistent and $O(n^\epsilon)$ non-persistent client storage for $0<\epsilon<1$, our POPE scheme provides extremely fast batch insertion consisting of a single round, and efficient search with O(1) amortized cost for up to $O(n^{1-\epsilon})$ search queries. This improved security and performance makes our scheme better suited for today's insert-heavy databases.Comment: Appears in ACM CCS 2016 Proceeding

Towards a secure and efficient search over encrypted cloud data

Includes bibliographical references.2016 Summer.Cloud computing enables new types of services where the computational and network resources are available online through the Internet. One of the most popular services of cloud computing is data outsourcing. For reasons of cost and convenience, public as well as private organizations can now outsource their large amounts of data to the cloud and enjoy the benefits of remote storage and management. At the same time, confidentiality of remotely stored data on untrusted cloud server is a big concern. In order to reduce these concerns, sensitive data, such as, personal health records, emails, income tax and financial reports, are usually outsourced in encrypted form using well-known cryptographic techniques. Although encrypted data storage protects remote data from unauthorized access, it complicates some basic, yet essential data utilization services such as plaintext keyword search. A simple solution of downloading the data, decrypting and searching locally is clearly inefficient since storing data in the cloud is meaningless unless it can be easily searched and utilized. Thus, cloud services should enable efficient search on encrypted data to provide the benefits of a first-class cloud computing environment. This dissertation is concerned with developing novel searchable encryption techniques that allow the cloud server to perform multi-keyword ranked search as well as substring search incorporating position information. We present results that we have accomplished in this area, including a comprehensive evaluation of existing solutions and searchable encryption schemes for ranked search and substring position search

Secret Communication over Broadcast Erasure Channels with State-feedback

We consider a 1-to-$K$ communication scenario, where a source transmits private messages to $K$ receivers through a broadcast erasure channel, and the receivers feed back strictly causally and publicly their channel states after each transmission. We explore the achievable rate region when we require that the message to each receiver remains secret - in the information theoretical sense - from all the other receivers. We characterize the capacity of secure communication in all the cases where the capacity of the 1-to-$K$ communication scenario without the requirement of security is known. As a special case, we characterize the secret-message capacity of a single receiver point-to-point erasure channel with public state-feedback in the presence of a passive eavesdropper. We find that in all cases where we have an exact characterization, we can achieve the capacity by using linear complexity two-phase schemes: in the first phase we create appropriate secret keys, and in the second phase we use them to encrypt each message. We find that the amount of key we need is smaller than the size of the message, and equal to the amount of encrypted message the potential eavesdroppers jointly collect. Moreover, we prove that a dishonest receiver that provides deceptive feedback cannot diminish the rate experienced by the honest receivers. We also develop a converse proof which reflects the two-phase structure of our achievability scheme. As a side result, our technique leads to a new outer bound proof for the non-secure communication problem

Universally Composable Verifiable Random Oracles

Random Oracles werden häufig in der Kryptographie eingesetzt um sehr effiziente Instanziierungen mächtiger kryptographischer Primitive zu konstruieren. Jedoch ist diese Praxis im Allgemeinen nicht zulässig wie verschiedene Nicht-Instanziierungs-Ergebnisse für Random Oracles mittels lokal berechenbarer Familien von Funktionen durch Halevi et al. (JACM ’04) zeigt. Die Random Oracle Modell kann sicher eingesetzt werden, indem Random Oracles nicht mit einer lokal berechenbaren Hashfunktion, sondern stattdessen mit einem interaktiven Protokoll instanziiert werden. In der realen Welt könnte solch ein interaktives Protokoll beispielsweise aus einem vertrauenswürdigen Server, welcher über das Internet erreichbar ist, bestehen. Dieser Server würde sodann eine der bekannten Techniken wie lazy sampling oder das Auswerten einer Pseudo-Zufälligen Funktion verwenden, um die Funktionalität eines Random Oracle bereitzustellen. Ein klarer Nachteil dieses Ansatzes ist die große Menge an Interaktion, die bei jeder Berechnung, die eine Auswertung des Random Oracle beinhaltet, nötig ist. Wir wollen diese Interaktion auf ein Minimum reduzieren. Um obiges Unmöglichkeitsresultat zu umgehen, muss die Auswertung des Random Oracle auf einer frischen Eingabe Interaktion der auswertenden Partei mit einer anderen Partei beinhalten. Dies ist jedoch nicht der einzige Verwendungszweck von Random Oracles, der häufig in kryptographischen Protokollen auftritt. Bei einem weiteren solchen Zweck wertet zunächst eine Partei A das Orakel auf einer Eingabe aus und erhält einen Hashwert. Im Anschluss sendet A Eingabe und Ausgabe (im Kontext eines Protokolls) an eine zweite Partei B und möchte B davon überzeugen, dass das Random Oracle korrekt ausgewertet wurde. Eine einfache Möglichkeit dies zu prüfen besteht darin, dass B selbst eine Auswertung des Random Oracle auf der erhaltenen Eingabe tätigt und die beiden Ausgaben vergleicht. In unserem Kontext benötigt dies jedoch erneut Interaktion. Der Wunsch diesen zweiten Verwendungszweck nicht-interaktiv zu machen führt uns zum Begriff eines Verifiable Random Oracle (VRO) als Erweiterung eines Random Oracle. Abstrakt besteht ein VRO aus zwei Orakeln. Das erste Orakel verhält sich wie ein Random Oracle dessen Ausgabe um einen Korrektheitsbeweis erweitert wurde. Mit Hilfe dieses Beweises kann das zweite Orakel dazu verwendet werden öffentlich die korrekte Auswertung des Random Oracle zu verifizieren. Obwohl diese Orakel-basierte Formulierung nicht notwendigerweise nicht-interaktive Verifikation besitzt, so erlaubt jedoch die Einführung expliziter Korrektheitsbeweise dies. In dieser Masterarbeit formalisieren wir zunächst den Begriff eines VRO im Universal Composability Framework von Canetti (FOCS ’01). Danach wenden wir VROs auf zwei kryptographische Anwendungen an, die in ihrer ursprünglichen Formulierung das Random Oracle Modell verwenden, und zeigen, das deren Sicherheitseigenschaften erhalten bleiben. Um zu zeigen, dass unsere Definition realisierbar ist, konstruieren wir mehrere Protokolle, die die ideale VRO Funktionalität realisieren. Diese reichen von Protokollen für eine einzelne vertrauenswürdige Partei bis hin zu verteilten Protokollen, die eine gewisse Menge an böswilliger Korruption erlauben. Wir vergleichen weiterhin VROs mit ähnlichen existierenden Primitiven