Cryptographic protocols are essential to secure communication but remain difficult to design correctly, motivating the need for rigorous verification methods. This paper provides a brief overview of symbolic verification techniques, focusing on the evolution and impact of the tool ProVerif over the past 25 years. We discuss its core principles, key extensions for richer security properties, and recent work on handling algebraic theories such as exclusive-or (XOR). We conclude by highlighting ongoing challenges, including usability and interoperability between verification approaches.

Cryptographic protocols commonly use (random) nonces to guarantee security properties. Although it is known for a long time that nonces should benefit from clear security properties, modern standards regularly miss this fundamental requirement. The lack of clear recommendations leads to error-prone cryptographic implementations, especially vulnerabilities due to nonce reuse and nonce leakage. This paper introduces a method based on TAMARIN to identify with a systematic approach the nonce-related properties an implementation should guarantee to ensure the security of a cryptographic protocol. As a corollary, the method also determines the security impact of a nonce misuse. Our method also applies to other types of random values used in protocols, namely ephemeral keys, masks, and nonces used in randomized primitives. This approach is then extended to take into account the well-known weaknesses of some randomized primitives when nonces are reused. The paper finally applies the method to real-life cryptographic protocols, discovering so new vulnerabilities related to nonce misuses in Dragonfly, WPA3, and Bluetooth.

Cryptographers are interested in showing that facts hold with overwhelming probability, i.e., a probability that grows fast enough to 1 wrt. some security parameter. It is thus natural to consider a formal logic where terms and formulas are interpreted as random variables, with a notion of validity based on overwhelming truth. In such a setting, one can postulate e.g. an axiom stating that the hash of two distinct adversarial terms do not collide, even though there is actually a negligible probability that an attacker finds such a collision. This results in a logic that is both fully formal and allows easy reasoning. However, the non-standard semantics of the logic makes it non-trivial to use common automation techniques. In this work, we show that it is actually possible to use classical reasoning tools, and more specifically SMT solvers. We develop this approach in practice in the setting of the Squirrel proof assistant, designing efficient encodings that leverage standard theories. We present benchmarks comparing our approach to existing automated reasoning techniques in Squirrel, and show how the new SMT-based tactics enable much shorter proof scripts.

In this paper, we propose a way to automate proofs of cryptographic protocols in the computational setting. We focus on non-deducibility – a weak notion of secrecy – and we aim to use type systems. Techniques based on typing were mainly used in symbolic models, and we show how they can be adapted to the Ccsa framework to obtain computational guarantees. We consider for now a fixed set of primitives, namely symmetric and asymmetric encryption, as well as pairing (i.e. concate-nation). Our approach has the usual benefits of type systems: it is modular, allows the security analysis for an unbounded number of sessions, and could be extended to other primitives (e.g. hashing) without excessive difficulties. We successfully applied our framework on several protocols from the literature and the ISO/IEC 11770 standard.

Electronic voting promises the possibility of convenient and efficient systems for recording and tallying votes in an election. To be widely adopted, ensuring the security of the cryptographic protocols used in e-voting is of paramount importance. However, the security analysis of this type of protocol raises a number of challenges, and they are often out of reach of existing verification tools. In this paper, we study vote privacy , a central security property that should be satisfied by any e-voting system. More precisely, we propose the first formalisation of the recent BPRIV notion in the symbolic setting. To ease the formal security analysis of this notion, we propose a reduction result allowing one to bind the number of voters and ballots needed to mount an attack. We first consider the case where voters do not revote, and the ballot box is trusted. Then, we extend this reduction result, as well as our formalisation of BPRIV , to account for the case of re-voting and a dishonest ballot box. We apply our reduction results to a number of case studies including several versions of Helios, Belenios, JCJ/Civitas, and Prêt-à-Voter. For some of these protocols, thanks to our result, we are able to conduct the analysis relying on the automatic tool Proverif .

Unlinkability is a privacy property of crucial importance for several systems such as mobile phones or RFID chips. Analysing this security property is very complex, and highly error-prone. Therefore, formal verification with machine support is desirable. Unfortunately, existing techniques are not sufficient to directly apply verification tools to automatically prove unlinkability. In this paper, we overcome this limitation by defining a simple transformation that will exploit some specific features of ProVerif. This transformation, together with some generic axioms, allows the tool to successfully conclude on several case studies. We have implemented our approach, effectively obtaining direct proofs of unlinkability on several protocols that were, until now, out of reach of automatic verification tools.

Bana and Comon have proposed a logical approach to proving protocols in the computational model, which they call the Computationally Complete Symbolic Attacker (CCSA). The proof assistant Squirrel implements a verification technique that elaborates on this approach, building on a meta-logic over the CCSA base logic. In this paper, we show that this meta-logic can naturally be extended to handle protocols with mutable states (key updates, counters, etc.) and we extend Squirrel'S proof system to be able to express the complex proof arguments that are sometimes required for these protocols. Our theoretical contributions have been implemented in Squirrel and validated on a number of case studies, including a proof of the YubiKey and YubiHSM protocols.

Bounding the number of sessions is a long-standing problem in the context of security protocols. It is well known that even simple properties like secrecy are undecidable when an unbounded number of sessions is considered. Yet, attacks on existing protocols only require a few sessions. In this paper, we propose a sound algorithm that computes a sufficient set of scenarios that need to be considered to detect an attack. Our approach can be applied for both reachability and equivalence properties, for protocols with standard primitives that are type-compliant (unifiable messages have the same type). Moreover, when equivalence properties are considered, else branches are disallowed, and protocols are supposed to be simple (an attacker knows from which role and session a message comes from). Since this class remains undecidable, our algorithm may return an infinite set. However, our experiments show that on most basic protocols of the literature, our algorithm computes a small number of sessions (a dozen). As a consequence, tools for a bounded number of sessions like DeepSec can then be used to conclude that a protocol is secure for an unbounded number of sessions.

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.

The continuous adoption of Near Field Communication (NFC) tags offers many new applications whose security is essential (e.g., contactless payments). In order to prevent flaws and attacks, we develop in this article a framework allowing us to analyse the underlying security protocols, taking into account the location of the agents and the transmission delay when exchanging messages. We propose two reduction results to render automatic verification possible relying on the existing verification tool ProVerif . Our first result allows one to consider a unique topology to catch all possible attacks. The second result simplifies the security analysis when considering Terrorist fraud. Then, based on these results, we perform a comprehensive case study analysis (27 protocols), in which we obtain new proofs of security for some protocols and detect attacks on some others.

Given the central importance of designing secure protocols, providing solid mathematical foundations and computer-assisted methods to attest for their correctness is becoming crucial. Here, we elaborate on the formal approach introduced by Bana and Comon in [10], [11], which was originally designed to analyze protocols for a fixed number of sessions, and lacks support for proof mechanization.In this paper, we present a framework and an interactive prover allowing to mechanize proofs of security protocols for an arbitrary number of sessions in the computational model. More specifically, we develop a meta-logic as well as a proof system for deriving security properties. Proofs in our system only deal with high-level, symbolic representations of protocol executions, similar to proofs in the symbolic model, but providing security guarantees at the computational level. We have implemented our approach within a new interactive prover, the Squirrel prover, taking as input protocols specified in the applied pi-calculus, and we have performed a number of case studies covering a variety of primitives (hashes, encryption, signatures, Diffie-Hellman exponentiation) and security properties (authentication, strong secrecy, unlinkability).

Contactless systems, such as the EMV (Europay, Mastercard and Visa) payment protocol, are vulnerable to relay attacks. The typical countermeasure to this relies on distance bounding protocols, in which a reader estimates an upper bound on its physical distance from a card by doing round-trip time (RTT) measurements. However, these protocols are trivially broken in the presence of rogue readers. At Financial Crypto 2019, we proposed two novel EMV-based relay-resistant protocols: they integrate distance-bounding with the use of hardware roots of trust (HWRoT) in such a way that correct RTT-measurements can no longer be bypassed.

The rise of contactless and wireless devices such as mobile phones and RFID chips justifies significant concerns over privacy, and calls for communication protocols that ensure some form of unlinkability. Formally specifying this property is difficult and context-dependent, and analysing it is very complex; as is common with security protocols, several incorrect unlinkability claims can be found in the literature. Formal verification is therefore desirable, but current techniques are not sufficient to directly analyse unlinkability. In [21], two conditions have been identified that imply unlinkability and can be automatically verified. That work, however, only considers a restricted class of protocols. We adapt their formal definition as well as their proof method to the common setting of RFID authentication protocols, where readers access a central database of authorised users. Moreover, we also consider protocols where readers may update their database, and tags may also carry a mutable state. We propose sufficient conditions to ensure unlinkability, find new attacks, and obtain new proofs of unlinkability using Tamarin to establish our sufficient conditions.

Security properties of cryptographic protocols are typically expressed as reachability or equivalence properties. Secrecy and authentication are examples of reachability properties, while privacy properties such as untraceability, vote secrecy, or anonymity are generally expressed as behavioral equivalence in a process algebra that models security protocols. Our main contribution is to reduce the search space for attacks for reachability as well as equivalence properties. Specifically, we show that if there is an attack then there is one that is well-typed. Our result holds for a large class of typing systems, a family of equational theories that encompasses all standard primitives, and protocols without else branches. For many standard protocols, we deduce that it is sufficient to look for attacks that follow the format of the messages expected in an honest execution, therefore considerably reducing the search space.

In this paper, we consider the problem of verifying anonymity and unlinkability in the symbolic model, where protocols are represented as processes in a variant of the applied pi calculus, notably used in the [Formula: see text] tool. Existing tools and techniques do not allow to verify directly these properties, expressed as behavioral equivalences. We propose a different approach: we design two conditions on protocols which are sufficient to ensure anonymity and unlinkability, and which can then be effectively checked automatically using [Formula: see text]. Our two conditions correspond to two broad classes of attacks on unlinkability, i.e. data and control-flow leaks. This theoretical result is general enough that it applies to a wide class of protocols based on a variety of cryptographic primitives. In particular, using our tool, [Formula: see text], we provide the first formal security proofs of protocols such as BAC and PACE (e-passport), Hash-Lock (RFID authentication), etc. Our work has also lead to the discovery of new attacks, including one on the LAK protocol (RFID authentication) which was previously claimed to be unlinkable (in a weak sense).

The 13th ACM SIGSAC Workshop on Programming Languages and Analysis for Security (PLAS 2018) is co-located with the 25th ACM Conference on Computer and Communications Security (ACM CCS 2018). Over its now more than ten-year history, PLAS has provided a unique forum for researchers and practitioners to exchange ideas about programming language and program analysis techniques with the goal of improving the security of software systems. PLAS aims to provide a forum for exploring and evaluating ideas on using programming language and program analysis techniques to improve the security of software systems. Strongly encouraged are proposals of new, speculative ideas, evaluations of new or known techniques in practical settings, and discussions of emerging threats and important problems.

For many modern applications like e.g., contactless payment, and keyless systems, ensuring physical proximity is a security goal of paramount importance. Formal methods have proved their usefulness when analysing standard security protocols. However, existing results and tools do not apply to e.g., distance bounding protocols that aims to ensure physical proximity between two entities. This is due in particular to the fact that existing models do not represent in a faithful way the locations of the participants, and the fact that transmission of messages takes time. In this paper, we propose several reduction results: when looking for an attack, it is actually sufficient to consider a simple scenario involving at most four participants located at some specific locations. These reduction results allow one to use verification tools (e.g. ProVerif, Tamarin) developed for analysing more classical security properties. As an application, we analyse several distance bounding protocols, as well as a contactless payment protocol.

In symbolic verification of security protocols, process equivalences have recently been used extensively to model strong secrecy, anonymity and unlinkability properties. However, tool support for automated analysis of equivalence properties is limited compared to trace properties, e.g., modeling authentication and weak notions of secrecy. In this paper, we present a novel procedure for verifying equivalences on finite processes, i.e., without replication, for protocols that rely on various cryptographic primitives including exclusive or (xor). We have implemented our procedure in the tool AKISS, and successfully used it on several case studies that are outside the scope of existing tools, e.g., unlinkability on various RFID protocols, and resistance against guessing attacks on protocols that use xor.

Automatic tools based on symbolic models have been successful in analyzing security protocols. Such tools are particularly adapted for trace properties (e.g. secrecy or authentication), while they often fail to analyse equivalence properties.Equivalence properties can express a variety of security properties, including in particular privacy properties (vote privacy, anonymity, untraceability). Several decision procedures have already been proposed but the resulting tools are rather inefficient.In this paper, we propose a novel algorithm, based on graph planning and SAT-solving, which significantly improves the efficiency of the analysis of equivalence properties. The resulting implementation, SAT-Equiv, can analyze several sessions where most tools have to stop after one or two sessions.

Modern security protocols may involve humans in order to compare or copy short strings between different devices. Multi-factor authentication protocols, such as Google 2-factor or 3D-secure are typical examples of such protocols. However, such short strings may be subject to brute force attacks. In this paper we propose a symbolic model which includes attacker capabilities for both guessing short strings, and producing collisions when short strings result from an application of weak hash functions. We propose a new decision procedure for analysing (a bounded number of sessions of) protocols that rely on short strings. The procedure has been integrated in the AKISS tool and tested on protocols from the ISO/IEC 9798-6:2010 standard.

Many privacy-type properties of security protocols can be modelled using trace equivalence properties in suitable process algebras. It has been shown that such properties can be decided for interesting classes of finite processes (i.e., without replication) by means of symbolic execution and constraint solving. However, this does not suffice to obtain practical tools. Current prototypes suffer from a classical combinatorial explosion problem caused by the exploration of many interleavings in the behaviour of processes. M\"odersheim et al. have tackled this problem for reachability properties using partial order reduction techniques. We revisit their work, generalize it and adapt it for equivalence checking. We obtain an optimisation in the form of a reduced symbolic semantics that eliminates redundant interleavings on the fly. The obtained partial order reduction technique has been integrated in a tool called APTE. We conducted complete benchmarks showing dramatic improvements.

In this paper, we consider the problem of verifying anonymity and unlinkability in the symbolic model, where protocols are represented as processes in a variant of the applied pi calculus notably used in the ProVerif tool. Existing tools and techniques do not allow one to verify directly these properties, expressed as behavioral equivalences. We propose a different approach: we design two conditions on protocols which are sufficient to ensure anonymity and unlinkability, and which can then be effectively checked automatically using ProVerif. Our two conditions correspond to two broad classes of attacks on unlinkability, corresponding to data and control-flow leaks. This theoretical result is general enough to apply to a wide class of protocols. In particular, we apply our techniques to provide the first formal security proof of the BAC protocol (e-passport). Our work has also lead to the discovery of new attacks, including one on the LAK protocol (RFID authentication) which was previously claimed to be unlinkable (in a weak sense) and one on the PACE protocol (e-passport).

Security protocols are concurrent processes that communicate using cryptography with the aim of achieving various security properties. Recent work on their formal verification has brought procedures and tools for deciding trace equivalence properties (e.g. anonymity, unlinkability, vote secrecy) for a bounded number of sessions. However, these procedures are based on a naive symbolic exploration of all traces of the considered processes which, unsurprisingly, greatly limits the scalability and practical impact of the verification tools. In this paper, we mitigate this difficulty by developing partial order reduction techniques for the verification of security protocols. We provide reduced transition systems that optimally eliminate redundant traces, and which are adequate for model-checking trace equivalence properties of protocols by means of symbolic execution. We have implemented our reductions in the tool Apte, and demonstrated that it achieves the expected speedup on various protocols.

Privacy properties such as anonymity, unlink ability, or vote secrecy are typically expressed as equivalence properties. In this paper, we provide the first decidability result for trace equivalence of security protocols, for an unbounded number of sessions and unlimited fresh nonce's. Our class encompasses most symmetric key protocols of the literature, in their tagged variant.

Formal methods have been very successful in analyzing security protocols for reachability properties such as secrecy or authentication. In contrast, there are very few results for equivalence-based properties, crucial for studying, for example, privacy-like properties such as anonymity or vote secrecy. We study the problem of checking equivalence of security protocols for an unbounded number of sessions. Since replication leads very quickly to undecidability (even in the simple case of secrecy), we focus on a limited fragment of protocols (standard primitives but pairs, one variable per protocol’s rules) for which the secrecy preservation problem is known to be decidable. Surprisingly, this fragment turns out to be undecidable for equivalence. Then, restricting our attention to deterministic protocols, we propose the first decidability result for checking equivalence of protocols for an unbounded number of sessions. This result is obtained through a characterization of equivalence of protocols in terms of equality of languages of (generalized, real-time) deterministic pushdown automata. We further show that checking for equivalence of protocols is actually equivalent to checking for equivalence of generalized, real-time deterministic pushdown automata. Very recently, the algorithm for checking for equivalence of deterministic pushdown automata has been implemented. We have implemented our translation from protocols to pushdown automata, yielding the first tool that decides equivalence of (some class of) protocols, for an unbounded number of sessions. As an application, we have analyzed some protocols of the literature including a simplified version of the basic access control (BAC) protocol used in biometric passports.

Reasoning about the knowledge of an attacker is a necessary step in many formal analyses of security protocols. In the framework of the applied pi-calculus, as in similar languages based on equational logics, knowledge is typically expressed by two relations: deducibility and static equivalence. Several decision procedures have been proposed for these relations under a variety of equational theories. However, each theory has its particular algorithm, and none has been implemented so far. We provide a generic procedure for deducibility and static equivalence that takes as input any convergent rewrite system. We show that our algorithm covers most of the existing decision procedures for convergent theories. We also provide an efficient implementation and compare it briefly with the tools ProVerif and KiSs.

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 consider security properties of cryptographic protocols that can be modeled using the notion of trace equivalence. The notion of equivalence is crucial when specifying privacy-type properties, like anonymity, vote-privacy, and unlinkability.

We present a Horn-clause-based framework for analysing security protocols that use \emph{platform configuration registers} (PCRs), which are registers for maintaining state inside the Trusted Platform Module (TPM). In our model, the PCR state space is unbounded, and our experience shows that a na\"\i ve analysis using ProVerif or SPASS does not terminate. To address this, we extract a set of instances of the Horn clauses of our model, for which ProVerif does terminate on our examples. We prove the soundness of this extraction process: no attacks are lost, that is, any query derivable in the more general set of clauses is also derivable from the extracted instances. The effectiveness of our framework is demonstrated in two case studies: a simplified version of Microsoft Bit locker, and a digital envelope protocol that allows a user to choose whether to perform a decryption, or to verifiably renounce the ability to perform the decryption.

Formal, symbolic techniques are extremely useful for modelling and analysing security protocols. They improved our understanding of security protocols, allowed to discover flaws, and also provide support for protocol design. However, such analyses usually consider that the protocol is executed in isolation or assume a bounded number of protocol sessions. Hence, no security guarantee is provided when the protocol is executed in a more complex environment. In this paper, we study whether password protocols can be safely composed, even when a same password is reused. More precisely, we present a transformation which maps a password protocol that is secure for a single protocol session (a decidable problem) to a protocol that is secure for an unbounded number of sessions. Our result provides an effective strategy to design secure password protocols: (i) design a protocol intended to be secure for one protocol session; (ii) apply our transformation and obtain a protocol which is secure for an unbounded number of sessions. Our technique also applies to compose different password protocols allowing us to obtain both inter-protocol and inter-session composition.

Mobile ad hoc networks consist of mobile wireless devices which autonomously organize their infrastructure. In such networks, a central issue, ensured by routing protocols, is to find a route from one device to another. Those protocols use cryptographic mechanisms in order to prevent malicious nodes from compromising the discovered route. Our contribution is twofold. We first propose a calculus for modeling and reasoning about security protocols, including in particular secured routing protocols. Our calculus extends standard symbolic models to take into account the characteristics of routing protocols and to model wireless communication in a more accurate way. Our second main contribution is a decision procedure for analyzing routing protocols for any network topology. By using constraint solving techniques, we show that it is possible to automatically discover (in NPTIME) whether there exists a network topology that would allow malicious nodes to mount an attack against the protocol, for a bounded number of sessions. We also provide a decision procedure for detecting attacks in case the network topology is given a priori. We demonstrate the usage and usefulness of our approach by analyzing the protocol SRP applied to DSR.

We propose a symbolic semantics for the finite applied pi calculus. The applied pi calculus is a variant of the pi calculus with extensions for modelling cryptographic protocols. By treating inputs symbolically, our semantics avoids potentially infinite branching of execution trees due to inputs fr om the environment. Correctness is maintained by associating with each process a set of constraints on terms. We define a symbolic labelled bisimulation relation, which is shown to be sound but not complete with respect to standard bisimulation. We explore the lack of completeness and demonstrate that the symbolic bisimulation relation is sufficient for many practical examples. This work is an important step towards automation of observational equivalence for the finite applied pi calculus, e.g. for verification of anonymity or strong secrecy properties.

PKCS#11 defines an API for cryptographic devices that has been widely adopted in industry. However, it has been shown to be vulnerable to a variety of attacks that could, for example, compromise the sensitive keys stored on the device. In this paper, we set out a formal model of the operation of th e API, which differs from previous security API models notably in that it accounts for non-monotonic mutable global state. We give decidability results for our formalism, and describe an implementation of the resulting decision procedure using the model checker NuSMV. We report some new attacks and prove the safety of some configurations of the API in our model. We also analyse proprietary extensions proposed by nCipher (Thales) and Eracom (Safenet), designed to address the shortcomings of PKCS#11.

Formal methods have proved their usefulness for analyzing the security of protocols. Most existing results focus on trace properties like secrecy (expressed as a reachability property) or authentication. There are however several security properties, which cannot be defined (or cannot be naturally defined) as trace properties and require the notion of observational equivalence. Typical examples are anonymity, privacy related properties or statements closer to security properties used in cryptography. In this paper, we consider the applied pi calculus and we show that for determinate processes, observational equivalence actually coincides with trace equivalence, a notion simpler to reason with. We exhibit a large class of determinate processes, called simple processes, that capture most existing protocols and cryptographic primitives. Then, for simple processes without replication nor else branch,we reduce the decidability of trace equivalence to deciding an equivalence relation introduced by M. Baudet. Altogether, this yields the first decidability result of observational equivalence for a general class of equational theories.

We present a symbolic framework for refinement and composition of security protocols. The framework uses the notion of ideal functionalities. These are abstract systems which are secure by construction and which can be combined into larger systems. They can be separately refined in order to obtain concrete protocols implementing them. Our work builds on ideas from the ``trusted party paradigm'' used in computational cryptography models. The underlying language we use is the applied pi calculus which is a general language for specifying security protocols. In our framework we can express the different standard flavours of simulation-based security which happen to all coincide. We illustrate our framework on an authentication functionality which can be realized using the Needham-Schroeder-Lowe protocol. For this we need to define an ideal functionality for asymmetric encryption and its realization. We show a joint state result for this functionality which allows composition (even though the same key material is reused) using a tagging mechanism.

Electronic voting promises the possibility of a convenient, efficient and secure facility for recording and tallying votes in an election. Recently highlighted inadequacies of implemented systems have demonstrated the importance of formally verifying the underlying voting protocols. We study three privacy-type properties of electronic voting protocols: in increasing order of strength, they are vote-privacy, receipt-freeness and coercion-resistance. We use the applied pi calculus, a formalism well adapted to modelling such protocols, which has the advantages of being based on well-understood concepts. The privacy-type properties are expressed using observational equivalence and we show in accordance with intuition that coercion-resistance implies receipt-freeness, which implies vote-privacy. We illustrate our definitions on three electronic voting protocols from the literature. Ideally, these three properties should hold even if the election officials are corrupt. However, protocols that were designed to satisfy receipt-freeness or coercion-resistance may not do so in the presence of corrupt officials. Our model and definitions allow us to specify and easily change which authorities are supposed to be trustworthy.

We investigate the composition of protocols that share a common secret. This situation arises when users employ the same password on different services. More precisely we study whether resistance against guessing attacks composes when the same password is used. We model guessing attacks using a common definition based on static equivalence in a cryptographic process calculus close to the applied pi calculus. We show that resistance against guessing attacks composes in the presence of a passive attacker. However, composition does not preserve resistance against guessing attacks for an active attacker. We therefore propose a simple syntactic criterion under which we show this composition to hold. Finally, we present a protocol transformation that ensures this syntactic criterion and preserves resistance against guessing attacks.

PKCS#11 defines an API for cryptographic devices that has been widely adopted in industry. However, it has been shown to be vulnerable to a variety of attacks that could, for example, compromise the sensitive keys stored on the device. In this paper, we set out a formal model of the operation of the API, which differs from previous security API models notably in that it accounts for non-monotonic mutable global state. We give decidability results for our formalism, and describe an implementation of the resulting decision procedure using a model checker. We report some new attacks and prove the safety of some configurations of the API in our model.

Key conjuring is the process by which an attacker obtains an unknown, encrypted key by repeatedly calling a cryptographic API function with random values in place of keys. We propose a formalism for detecting computationally feasible key conjuring operations, incorporated into a Dolev-Yao style model of the security API. We show that security in the presence of key conjuring operations is decidable for a particular class of APIs, which includes the key management API of IBM's common cryptographic architecture (CCA).

In this paper we formally study important properties of electronic voting protocols. In particular we are interested in coercion-resistance and receipt-freeness. Intuitively, an election protocol is coercion-resistant if a voter A cannot prove to a potential coercer C that she voted in a particular way. We assume that A cooperates with C in an interactive fashion. Receipt-freeness is a weaker property, for which we assume that A and C cannot interact during the protocol: to break receipt-freeness, A later provides evidence (the receipt) of how she voted. While receipt-freeness can be expressed using observational equivalence from the applied pi calculus, we need to introduce a new relation to capture coercion-resistance. Our formalization of coercion-resistance and receipt-freeness are quite different. Nevertheless, we show in accordance with intuition that coercion-resistance implies receipt-freeness, which implies privacy, the basic anonymity property of voting protocols, as defined in previous work. Finally we illustrate the definitions on a simplified version of the Lee et al. voting protocol

Cryptographic protocols are successfully analyzed using formal methods. However, formal approaches usually consider the encryption schemes as black boxes and assume that an adversary cannot learn anything from an encrypted message except if he has the key. Such an assumption is too strong in general since some attacks exploit in a clever way the interaction between protocol rules and properties of cryptographic operators. Moreover, the executability of some protocols relies explicitly on some algebraic properties of cryptographic primitives such as commutative encryption. We give a list of some relevant algebraic properties of cryptographic operators, and for each of them, we provide examples of protocols or attacks using these properties. We also give an overview of the existing methods in formal approaches for analyzing cryptographic protocols.

International audience