Relating two standard notions of secrecy

Two styles of definitions are usually considered to express that a security protocol preserves the confidentiality of a data s. Reachability-based secrecy means that s should never be disclosed while equivalence-based secrecy states that two executions of a protocol with distinct instances for s should be indistinguishable to an attacker. Although the second formulation ensures a higher level of security and is closer to cryptographic notions of secrecy, decidability results and automatic tools have mainly focused on the first definition so far. This paper initiates a systematic investigation of the situations where syntactic secrecy entails strong secrecy. We show that in the passive case, reachability-based secrecy actually implies equivalence-based secrecy for digital signatures, symmetric and asymmetric encryption provided that the primitives are probabilistic. For active adversaries, we provide sufficient (and rather tight) conditions on the protocol for this implication to hold.Comment: 29 pages, published in LMC

Relating two standard notions of secrecy

Two styles of definitions are usually considered to express that a security protocol preserves the confidentiality of a data { t s}. Reach-ability-based secrecy means that { t s} should never be disclosed while equi-valence-based secrecy states that two executions of a protocol with distinct instances for { t s} should be indistinguishable to an attacker. Although the second formulation ensures a higher level of security and is closer to cryptographic notions of secrecy, decidability results and automatic tools have mainly focused on the first definition so far. This paper initiates a systematic investigation of situations where syntactic secrecy entails strong secrecy. We show that in the passive case, reachability-based secrecy actually implies equivalence-based secrecy for signatures, symmetric and asymmetric encryption provided that the primitives are probabilistic. For active adversaries in the case of symmetric encryption, we provide sufficient (and rather tight) conditions on the protocol for this implication to hold

A Type System for Privacy Properties (Technical Report)

Mature push button tools have emerged for checking trace properties (e.g. secrecy or authentication) of security protocols. The case of indistinguishability-based privacy properties (e.g. ballot privacy or anonymity) is more complex and constitutes an active research topic with several recent propositions of techniques and tools. We explore a novel approach based on type systems and provide a (sound) type system for proving equivalence of protocols, for a bounded or an unbounded number of sessions. The resulting prototype implementation has been tested on various protocols of the literature. It provides a significant speed-up (by orders of magnitude) compared to tools for a bounded number of sessions and complements in terms of expressiveness other state-of-the-art tools, such as ProVerif and Tamarin: e.g., we show that our analysis technique is the first one to handle a faithful encoding of the Helios e-voting protocol in the context of an untrusted ballot box

Psi-calculi: a framework for mobile processes with nominal data and logic

The framework of psi-calculi extends the pi-calculus with nominal datatypes for data structures and for logical assertions and conditions. These can be transmitted between processes and their names can be statically scoped as in the standard pi-calculus. Psi-calculi can capture the same phenomena as other proposed extensions of the pi-calculus such as the applied pi-calculus, the spi-calculus, the fusion calculus, the concurrent constraint pi-calculus, and calculi with polyadic communication channels or pattern matching. Psi-calculi can be even more general, for example by allowing structured channels, higher-order formalisms such as the lambda calculus for data structures, and predicate logic for assertions. We provide ample comparisons to related calculi and discuss a few significant applications. Our labelled operational semantics and definition of bisimulation is straightforward, without a structural congruence. We establish minimal requirements on the nominal data and logic in order to prove general algebraic properties of psi-calculi, all of which have been checked in the interactive theorem prover Isabelle. Expressiveness of psi-calculi significantly exceeds that of other formalisms, while the purity of the semantics is on par with the original pi-calculus.Comment: 44 page

Automated Reasoning for Equivalences in the Applied Pi Calculus with Barriers

International audienceObservational equivalence allows us to study important security properties such as anonymity. Unfortunately, the difficulty of proving observational equivalence hinders analysis. Blanchet, Abadi & Fournet simplify its proof by introducing a sufficient condition for observational equivalence , called diff-equivalence, which is a reachability condition that can be proved automatically by ProVerif. However, diff-equivalence is a very strong condition, which often does not hold even if observational equivalence does. In particular, when proving equivalence between processes that contain several parallel components, e.g., P | Q and P | Q , diff-equivalence requires that P is equivalent to P and Q is equivalent to Q. To relax this constraint, Delaune, Ryan & Smyth introduced the idea of swapping data between parallel processes P and Q at synchronisation points, without proving its soundness. We extend their work by formalising the semantics of synchronisation, formalising the definition of swapping, and proving its soundness. We also relax some restrictions they had on the processes to which swapping can be applied. Moreover, we have implemented our results in ProVerif. Hence, we extend the class of equivalences that can be proved automatically. We showcase our results by analysing privacy in election schemes by Fujioka, Okamoto & Ohta and Lee et al., and in the vehicular ad-hoc network by Freudiger et al

Modeling and Verifying Security Protocols with the Applied Pi Calculus and ProVerif

International audienceProVerif is an automatic symbolic protocol verifier. It supports a wide range of cryptographic primitives, defined by rewrite rules or by equations. It can prove various security properties: secrecy, authentication, and process equivalences, for an unbounded message space and an unbounded number of sessions. It takes as input a description of the protocol to verify in a dialect of the applied pi calculus, an extension of the pi calculus with cryptography. It automatically translates this protocol description into Horn clauses and determines whether the desired security properties hold by resolution on these clauses. This survey presents an overview of the research on ProVerif

Equivalences and calculi for formal verification of cryptographic protocols

Security protocols are essential to the proper functioning of any distributed system running over an insecure network but often have flaws that can be exploited even without breaking the cryptography. Formal cryptography, the assumption that the cryptographic primitives are flawless, facilitates the construction of formal models and verification tools. Such models are often based on process calculi, small formal languages for modelling communicating systems. The spi calculus, a process calculus for the modelling and formal verification of cryptographic protocols, is an extension of the pi calculus with cryptography. In the spi calculus, security properties can be formulated as equations on process terms, so no external formalism is needed. Moreover, the contextual nature of observational process equivalences takes into account any attacker/environment that can be expressed in the calculus. We set out to address the problem of automatic verification of observational equivalence in an extension of the spi calculus: A channel-passing calculus with a more general expression language. As a first step, we study existing non-contextual proof techniques for a particular canonical contextual equivalence. In contrast to standard process calculi, reasoning on cryptographic processes must take into account the partial knowledge of the environment about transmitted messages. In the setting of the spi calculus, several notions of environment-sensitive bisimulation has been developed to treat this environment knowledge. We exhibit distinguishing examples between several of these notions, including ones previously believed to coincide. We then give a general framework for comparison of environment-sensitive relations, based on a comparison of the corresponding kinds of environment and notions of environment consistency. Within this framework we perform an exhaustive comparison of the different bisimulations, where every possible relation that is not proven is disproven. For the second step, we consider the question of which expression languages are suitable. Extending the expression language to account for more sophisticated cryptographic primitives or other kinds of data terms quickly leads to decidability issues. Two important problems in this area are the knowledge problem and an indistinguishability problem called static equivalence. It is known that decidability of static equivalence implies decidability of knowledge in many cases; we exhibit an expression language where knowledge is decidable but static equivalence is not. We then define a class of constructor-destructor expression languages and prove that environment consistency over any such language directly corresponds to static equivalence in a particular extension thereof. We proceed to place some loose constraints on deterministic expression evaluation, and redefine the spi calculus in this more general setting. Once we have chosen an expression language, we encounter a third problem, which is inherent in the operational semantics of message-passing process calculi: The possibility to receive arbitrary messages gives rise to infinite branching on process input. To mitigate this problem, we define a symbolic semantics, where the substitution of received messages for input variables never takes place. Instead, input variables are only subject to logical constraints. We then use this symbolic semantics to define a symbolic bisimulation that is sound and complete with respect to its concrete counterpart, extending the possibilities for automated bisimulation checkers

Relating two standard notions of secrecy

Two styles of definitions are usually considered to express that a security protocol preserves the confidentiality of a data s. Reachability-based secrecy means that s should never be disclosed while equivalence-based secrecy states that two executions of a protocol with distinct instances for s should be indistinguishable to an attacker. Although the second formulation ensures a higher level of security and is closer to cryptographic notions of secrecy, decidability results and automatic tools have mainly focused on the first definition so far. This paper initiates a systematic investigation of the situations where syntactic secrecy entails strong secrecy. We show that in the passive case, reachability-based secrecy actually implies equivalence-based secrecy for digital signatures, symmetric and asymmetric encryption provided that the primitives are probabilistic. For active adversaries, we provide sufficient (and rather tight) conditions on the protocol for this implication to hold