We propose an on-site voting system Themis, that aims at improving security when local authorities are not fully trusted. Voters vote thanks to voting sheets as well as smart cards that produce encrypted ballots. Electronic ballots are systematically audited, without compromising privacy. Moreover, the system includes a precise dispute resolution procedure identifying misbehaving parties in most cases.

Tamarin is a popular tool dedicated to the formal analysis of security protocols. One major strength of the tool is that it offers an interactive mode, allowing to go beyond what push-button tools can typically handle. Tamarin is for example able to verify complex protocols such as TLS, 5G, or RFID protocols. However, one of its drawback is its lack of automation. For many simple protocols, the user often needs to help Tamarin by writing specific lemmas, called “sources lemmas”, which requires some knowledge of the internal behaviour of the tool. In this paper, we propose a technique to automatically generate sources lemmas in Tamarin. Following the intuition of manually written sources lemmas, our lemmas try to keep track of the origin of a term by looking into emitted messages or facts. We prove formally that our lemmas indeed hold, for arbitrary protocols that make use of cryptographic primitives that can be modelled with a subterm convergent equational theory (modulo associativity and commutativity). We have implemented our approach within Tamarin. Our experiments show that, in most examples of the literature, we are now able to generate suitable sources lemmas automatically, in replacement of the hand-written lemmas. As a direct application, many simple protocols can now be analysed fully automatically, while they previously required user interaction.

In 1968, Liu described the problem of securing documents in a shared secret project. In an example, at least six out of eleven participating scientists need to be present to open the lock securing the secret documents. Shamir proposed a mathematical solution to this physical problem in 1979, by designing an efficient k-out-of- n secret sharing scheme based on Lagrange’s interpolation. Liu and Shamir also claimed that the minimal solution using physical locks is clearly impractical and exponential in the number of participants. In this paper we relax some implicit assumptions in their claim and propose an optimal physical solution to the problem of Liu that uses physical padlocks, but the number of padlocks is not greater than the number of participants. Then, we show that no device can do better for k-out-of- n threshold padlock systems as soon as [Formula: see text], which holds true in particular for Liu’s example. More generally, we derive bounds required to implement any threshold system and prove a lower bound of [Formula: see text] padlocks for any threshold larger than 2. For instance we propose an optimal scheme reaching that bound for 2-out-of- n threshold systems and requiring less than [Formula: see text] padlocks. We also discuss more complex access structures, a wrapping technique, and other sublinear realizations like an algorithm to generate 3-out-of- n systems with [Formula: see text] padlocks. Finally we give an algorithm building k-out-of- n threshold padlock systems with only [Formula: see text] padlocks. Apart from the physical world, our results also show that it is possible to implement secret sharing over small fields.

Verifiability is a key requirement for electronic voting. However, the use of cryptographic techniques to achieve it usually requires specialist knowledge to understand; hence voters cannot easily assess the validity of such arguments themselves. To address this, solutions have been proposed using simple tables and checks, which require only simple verification steps with almost no cryptography.

In cryptographic protocols, in particular RFID protocols, exclusive-or (XOR) operations are common. Due to the inherent complexity of faithful models of XOR, there is only limited tool support for the verification of cryptographic protocols using XOR. In this paper, we improve the Tamarin prover and its underlying theory to deal with an equational theory modeling XOR operations. The XOR theory can be combined with all equational theories previously supported, including user-defined equational theories. This makes Tamarin the first verification tool for cryptographic protocols in the symbolic model to support simultaneously this large set of equational theories, protocols with global mutable state, an unbounded number of sessions, and complex security properties including observational equivalence. We demonstrate the effectiveness of our approach by analyzing several protocols that rely on XOR, in particular multiple RFID-protocols, where we can identify attacks as well as provide proofs.

Mobile communication networks connect much of the world's population. The security of users' calls, SMSs, and mobile data depends on the guarantees provided by the Authenticated Key Exchange protocols used. For the next-generation network (5G), the 3GPP group has standardized the 5G AKA protocol for this purpose. We provide the first comprehensive formal model of a protocol from the AKA family: 5G AKA. We also extract precise requirements from the 3GPP standards defining 5G and we identify missing security goals. Using the security protocol verification tool Tamarin, we conduct a full, systematic, security evaluation of the model with respect to the 5G security goals. Our automated analysis identifies the minimal security assumptions required for each security goal and we find that some critical security goals are not met, except under additional assumptions missing from the standard. Finally, we make explicit recommendations with provably secure fixes for the attacks and weaknesses we found.

Exclusive-or (XOR) operations are common in cryptographic protocols, in particular in RFID protocols and electronic payment protocols. Although there are numerous applications, due to the inherent complexity of faithful models of XOR, there is only limited tool support for the verification of cryptographic protocols using XOR. The Tamarin prover is a state-of-the-art verification tool for cryptographic protocols in the symbolic model. In this paper, we improve the underlying theory and the tool to deal with an equational theory modeling XOR operations. The XOR theory can be freely combined with all equational theories previously supported, including user-defined equational theories. This makes Tamarin the first tool to support simultaneously this large set of equational theories, protocols with global mutable state, an unbounded number of sessions, and complex security properties including observational equivalence. We demonstrate the effectiveness of our approach by analyzing several protocols that rely on XOR, in particular multiple RFID-protocols, where we can identify attacks as well as provide proofs.

Touch screens have become ubiquitous in the past few years, like for instance in smartphones and tablets. These devices are often the entry door to numerous information systems, hence having a secure and practical authentication mechanism is crucial. In this paper, we examine the complexity of different authentication methods specifically designed for such devices. We study the widely spread technology to authenticate a user using a Personal Identifier Number code (PIN code). Entering the code is a critical moment where there are several possibilities for an attacker to discover the secret. We consider the three attack models: a Bruteforce Attack (BA) model, a Smudge Attack (SA) model, and an Observation Attack (OA) model where the attacker sees the user logging in on his device. The aim of the intruder is to learn the secret code. Our goal is to propose alternative methods to enter a PIN code. We compare such different methods in terms of security. Some methods require more intentional resources than other, this is why we performed a psychological study on the different methods to evaluate the users' perception of the different methods and their usage.

In this paper, we provide the first methodology for reasoning about livenessproperties of cryptographic protocols in a machine-assisted manner withoutimposing any artificial, finite bounds on the protocols and execution models. To this end, we design an extension of the SAPiC process calculus so that itsupports key concepts for stating and reasoning about liveness properties, along with a corresponding translation into the formalism of multiset rewritingthat the state-of-the-art theorem prover Tamarin relies upon. We prove thatthis translation is sound and complete and can thereby automatically generatesound Tamarin specifications and automate the protocol analysis. Second, we applied our methodology to two widely investigated fair exchangeprotocols - ASW and GJM - and to the Secure Conversation Protocol standardfor industrial control systems, deployed by major players such as Siemens, SAPand ABB. For the fair exchange protocols, we not only re-discovered knownattacks, but also uncovered novel attacks that previous analyses based onfinite models and a restricted number of sessions did not detect. We suggestfixed versions of these protocols for which we prove both fairness andtimeliness, yielding the first automated proofs for fair exchange protocolsthat rely on a general model without restricting the number of sessions andmessage size. For the Secure Conversation Protocol, we prove several strongsecurity properties that are vital for the safety of industrial systems, inparticular that all messages (e.g., commands) are eventually delivered inorder.

International audience

Many cryptographic security definitions can be naturally formulated as observational equivalence properties. However, existing automated tools for verifying the observational equivalence of cryptographic protocols are limited: they do not handle protocols with mutable state and an unbounded number of sessions. We propose a novel definition of observational equivalence for multiset rewriting systems. We then extend the Tamarin prover, based on multiset rewriting, to prove the observational equivalence of protocols with mutable state, an unbounded number of sessions, and equational theories such as Diffie-Hellman exponentiation. We demonstrate its effectiveness on case studies, including a stateful TPM protocol.

International audience

Auctions have a long history, having been recorded as early as 500 B.C. [Auction Theory, Academic Press, San Diego, USA, 2002]. Nowadays, electronic auctions have been a great success and are increasingly used in various applications, including high performance computing [Concurrency and Computatio n: Practice and Experience 14(13–15) (2002), 1507–1542]. Many cryptographic protocols have been proposed to address the various security requirements of these electronic transactions, in particular to ensure privacy. Brandt [International Journal of Information Security 5 (2006), 201–216] developed a protocol that computes the winner using homomorphic operations on a distributed ElGamal encryption of the bids. He claimed that it ensures full privacy of the bidders, i.e. no information apart from the winner and the winning price is leaked. We first show that this protocol – when using malleable interactive zero-knowledge proofs – is vulnerable to attacks by dishonest bidders. Such bidders can manipulate the publicly available data in a way that allows the seller to deduce all participants’ bids. We provide an efficient parallelized implementation of the protocol and the attack to show its practicality. Additionally we discuss some issues with verifiability as well as attacks on non-repudiation, fairness and the privacy of individual bidders exploiting authentication problems.

International audience

An electronic auction protocol will only be used by those who trust that it operates correctly. Therefore, e-auction protocols must be verifiable: seller, buyer and losing bidders must all be able to determine that the result was correct. We pose that the importance of verifiability for e-auctions necessitates a formal analysis. Consequently, we identify notions of verifiability for each stakeholder. We formalize these and then use the developed framework to study the verifiability of two examples, the protocols due to Curtis et al. and Brandt, identifying several issues.

Privacy is one of the main issues in electronic voting. We propose a family of symbolic privacy notions that allows to assess the level of privacy ensured by a voting protocol. Our definitions are applicable to protocols featuring multiple votes per voter and special attack scenarios such as vote-copying or forced abstention. Finally we employ our definitions on several existing voting protocols to show that our model allows to compare different types of protocols based on different techniques, and is suitable for automated verification using existing tools.