Android Protected Confirmation (APC) is an authentication protocol designed by Google. It leverages the extra security of the Trusted Execution Environment (TEE) to secure transactions even in the presence of a compromised OS. The intended security guarantee for APC is that if a transaction has been signed under APC, then the user must have previously given its explicit consent, even if an attacker has gained root access to the victim’s Android OS. In this paper, we present a security analysis of APC in the Universal Composability (UC) framework. We uncover two attacks on the design of the protocol which allow a root adversary to issue transactions without the user consenting to them. We provide an attack implementation on a Google Pixel phone, and propose light-weight fixes. Finally, we specify the ideal UC functionality capturing the intended security guarantees for APC, and prove that the fixed protocol UC-realizes it.

Simultaneous broadcast (SBC) protocols, introduced in [Chor et al., FOCS 1985], constitute a special class of broadcast channels which, besides consistency, guarantee that all senders broadcast their messages independently of the messages broadcast by other parties. SBC has proved extremely useful in the design of various distributed computing constructions (e.g., multiparty computation, coin flipping, electronic voting, fair bidding). As with any communication channel, it is crucial that SBC security is composable, i.e., it is preserved under concurrent protocol executions. The work of [Hevia, SCN 2006] proposes a formal treatment of SBC in the state-of-the-art Universal Composability (UC) framework [Canetti, FOCS 2001] and a construction secure assuming an honest majority.

Recent advances indicate that quantum computers will soon be reality. Motivated by this ever more realistic threat for existing classical cryptographic protocols, researchers have developed several schemes to resist "quantum attacks". In particular, for electronic voting, several e-voting schemes relying on properties of quantum mechanics have been proposed. However, each of these proposals comes with a different and often not well-articulated corruption model, has different objectives, and is accompanied by security claims which are never formalized and are at best justified only against specific attacks. To address this, we propose the first formal security definitions for quantum e-voting protocols. With these at hand, we systematize and evaluate the security of previously-proposed quantum e-voting protocols; we examine the claims of these works concerning privacy, correctness and verifiability, and if they are correctly attributed to the proposed protocols. In all non-trivial cases, we identify specific quantum attacks that violate these properties. We argue that the cause of these failures lies in the absence of formal security models and references to the existing cryptographic literature.

In the context of statistical databases, the release of accurate statistical information about the collected data often puts at risk the privacy of the individual contributors. The goal of differential privacy is to maximise the utility of a query while protecting the individual records in the database. A natural way to achieve differential privacy is to add statistical noise to the result of the query. In this context, a mechanism for releasing statistical information is thus a trade-off between utility and privacy. In order to balance these two "conflicting" requirements, privacy preserving mechanisms calibrate the added noise to the so-called sensitivity of the query, and thus a precise estimate of the sensitivity of the query is necessary to determine the amplitude of the noise to be added. In this paper, we initiate a systematic study of sensitivity of counting queries over relational databases. We first observe that the sensitivity of a Relational Algebra query with counting is not computable in general, and that while the sensitivity of Conjunctive Queries with counting is computable, it becomes unbounded as soon as the query includes a join. We then consider restricted classes of databases (databases with constraints), and study the problem of computing the sensitivity of a query given such constraints. We are able to establish bounds on the sensitivity of counting conjunctive queries over constrained databases. The kind of constraints studied here are: functional dependencies and cardinality dependencies. The latter is a natural generalisation of functional dependencies that allows us to provide tight bounds on the sensitivity of counting conjunctive queries.

We present StatVerif, which is an extension of the ProVerif process calculus with constructs for explicit state, in order to be able to reason about protocols that manipulate global state. Global state is required by protocols used in hardware devices (such as smart cards and the trusted platform m odule), as well as by protocols involving databases that store persistent information. We provide the operational semantics of StatVerif. We extend the ProVerif compiler to a compiler for StatVerif, which takes processes written in the extended process language and produces Horn clauses. Our compilation is carefully engineered to avoid many false attacks. We prove the correctness of the StatVerif compiler. We illustrate our method on two examples: a small hardware security device and a contract signing protocol. We are able to prove their desired properties automatically.

Cloud computing means entrusting data to information systems that are managed by external parties on remote servers, in the “cloud”, raising new privacy and confidentiality concerns. We propose a general technique for designing cloud services that allows the cloud to see only encrypted data, while still facilitating some data-dependent computations. The technique is based on key translations and mixes in web browsers. We focus on a particular kind of software-as-a-service, namely, services that support applications, evaluations and decisions. Such services include job application management, public tender management (e.g., for civil construction), and conference management. We identify the specific security and privacy risks that existing systems pose. We propose a protocol that addresses them, and forms the basis of a system that offers strong security and privacy guarantees. We express the protocol and its properties in the language of ProVerif, and prove that it does provide the intended properties. We describe an implementation of a particular instance of the protocol called ConfiChair, which is geared to the evaluation of papers submitted to conferences.

Mobile telephony equipment is daily carried by billions of subscribers everywhere they go. Avoiding linkability of subscribers by third parties, and protecting the privacy of those subscribers is one of the goals of mobile telecommunication protocols. We use formal methods to model and analyse the security properties of 3G protocols. We expose two novel threats to the user privacy in 3G telephony systems, which make it possible to trace and identify mobile telephony subscribers, and we demonstrate the feasibility of a low cost implementation of these attacks. We propose fixes to these privacy issues, which also take into account and solve other privacy attacks known from the literature. We successfully prove that our privacy-friendly fixes satisfy the desired unlinkability and anonymity properties using the automatic verification tool ProVerif.

Formal methods have proved their usefulness for analysing the security of protocols. In this setting, privacy-type security properties (e.g. vote-privacy, anonymity, unlink ability) that play an important role in many modern applications are formalised using a notion of equivalence. In this paper, we study the notion of trace equivalence and we show how to establish such an equivalence relation in a modular way. It is well-known that composition works well when the processes do not share secrets. However, there is no result allowing us to compose processes that rely on some shared secrets such as long term keys. We show that composition works even when the processes share secrets provided that they satisfy some reasonable conditions. Our composition result allows us to prove various equivalence-based properties in a modular way, and works in a quite general setting. In particular, we consider arbitrary cryptographic primitives and processes that use non-trivial else branches. As an example, we consider the ICAO e-passport standard, and we show how the privacy guarantees of the whole application can be derived from the privacy guarantees of its sub-protocols.

We present StatVerif, which is an extension the ProVerif process calculus with constructs for explicit state, in order to be able to reason about protocols that manipulate global state. Global state is required by protocols used in hardware devices (such as smart cards and the TPM), as well as by protocols involving databases that store persistent information. We provide the operational semantics of StatVerif. We extend the ProVerif compiler to a compiler for StatVerif: it takes processes written in the extended process language, and produces Horn clauses. Our compilation is carefully engineered to avoid many false attacks. We prove the correctness of the StatVerif compiler. We illustrate our method on two examples: a small hardware security device, and a contract signing protocol. We are able to prove their desired properties automatically.

An attacker that can identify messages as coming from the same source, can use this information to build up a picture of targets' behaviour, and so, threaten their privacy. In response to this danger, unlinkable protocols aim to make it impossible for a third party to identify two runs of a protocol as coming from the same device. We present a framework for analysing unlinkability and anonymity in the applied pi calculus. We show that unlinkability and anonymity are complementary properties; one does not imply the other. Using our framework we show that the French RFID e-passport preserves anonymity but it is linkable therefore anyone carrying a French e-passport can be physically traced.