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

Termination of rewriting under strategies: a generic approach

We propose a synthesis of three induction based algorithms, we already have given to prove termination of rewrite rule based programs, respectively for the innermost, the outermost and the local strategies. A generic inference principle is presented, based on an explicit induction on the termination property, which genetates ordering constraints, defining the induction relation. The generic inference principle is then instantiated to provide proof procedures for the three specific considered strategies

Automatic program analysis using Max-SMT

This thesis addresses the development of techniques to build fully-automatic tools for analyzing sequential programs written in imperative languages like C or C++. In order to do the reasoning about programs, the approach taken in this thesis follows the constraint-based method used in program analysis. The idea of the constraint-based method is to consider a template for candidate invariant properties, e.g., linear conjunctions of inequalities. These templates involve both program variables as well as parameters whose values are initially unknown and have to be determined so as to ensure invariance. To this end, the conditions on inductive invariants are expressed by means of constraints (hence the name of the approach) on the unknowns. Any solution to these constraints then yields an invariant. In particular, if linear inequalities are taken as target invariants, conditions can be transformed into arithmetic constraints over the unknowns by means of Farkas' Lemma. In the general case, a Satisfiability Modulo Theories (SMT) problem over non-linear arithmetic is obtained, for which effective SMT solvers exist. One of the novelties of this thesis is the presentation of an optimization version of the SMT problems generated by the constraint-based method in such a way that, even when they turn out to be unsatisfiable, some useful information can be obtained for refining the program analysis. In particular, we show in this work how our approach can be exploited for proving termination of sequential programs, disproving termination of non-deterministic programs, and do compositional safety verification. Besides, an extension of the constraint-based method to generate universally quantified array invariants is also presented. Since the development of practical methods is a priority in this thesis, all the techniques have been implemented and tested with examples coming from academic and industrial environments. The main contributions of this thesis are summarized as follows: 1. A new constraint-based method for the generation of universally quantified invariants of array programs. We also provide extensions of the approach for sorted arrays. 2. A novel Max-SMT-based technique for proving termination. Thanks to expressing the generation of a ranking function as a Max-SMT optimization problem where constraints are assigned different weights, quasi-ranking functions -functions that almost satisfy all conditions for ensuring well-foundedness- are produced in a lack of ranking functions. Moreover, Max-SMT makes it easy to combine the process of building the termination argument with the usually necessary task of generating supporting invariants. 3. A Max-SMT constraint-based approach for proving that programs do not terminate. The key notion of the approach is that of a quasi-invariant, which is a property such that if it holds at a location during execution once, then it continues to hold at that location from then onwards. Our technique considers for analysis strongly connected subgraphs of a program's control flow graph and thus produces more generic witnesses of non-termination than existing methods. Furthermore, it can handle programs with unbounded non-determinism. 4. An automated compositional program verification technique for safety properties based on quasi-invariants. For a given program part (e.g., a single loop) and a postcondition, we show how to, using a Max-SMT solver, an inductive invariant together with a precondition can be synthesized so that the precondition ensures the validity of the invariant and that the invariant implies the postcondition. From this, we build a bottom-up program verification framework that propagates preconditions of small program parts as postconditions for preceding program parts. The method recovers from failures to prove validity of a precondition, using the obtained intermediate results to restrict the search space for further proof attempts.Esta tesis se centra en el desarrollo de técnicas para construir herramientas altamente automatizadas que analicen programas secuenciales escritos en lenguajes imperativos como C o C++. Para realizar el razonamiento sobre los programas, la aproximación tomada en esta tesis se basa en un conocido método basado en restricciones utilizado en análisis de progamas. La idea de dicho método consiste en considerar plantillas que expresen propiedades invariantes candidatas, p.e., conjunciones de desigualdades lineales. Estas plantillas contienen tanto variables del programa como parámetros cuyos valores son inicialmente desconocidos y tienen que ser determinados para garantizar la invariancia. Para este fin, las condiciones sobre invariantes inductivos son expresadas mediante restricciones sobre los valores desconocidos. Cualquier solución a estas restricciones llevan a un invariante. En particular, si desigualdades lineales son los invariantes objetivo, las condiciones pueden ser transformadas en restricciones aritméticas sobre los valores desconocidos mediante el lema de Farkas. En el caso general, un problema de Satisfactibilidad Modulo Teorías (SMT) sobre aritmética no-lineal es obtenido, para el cual existen resolvedores eficientes. Una de las novedades de esta tesis es la presentación de una versión de optimización de los problemas SMT generados por el método tal que, incluso cuando son insatisfactibles, se puede obtener cierta información útil para refinar el análisis del programa. En particular, en este trabajo se muestra como la aproximación tomada puede usarse para probar terminación de programas, probar la no terminación de programas y realizar verificación por partes de la corrección de programas. Además, también se describe una extensión del método basado en restricciones para generar invariantes universalmente cuantificados sobre arrays. Debido a que el desarrollo de métodos prácticos es una prioridad en esta tesis, todas las técnicas han sido implementadas y probadas con ejemplos extraídos del entorno académico e industrial. Las principales contribuciones de esta tesis pueden resumirse en: 1. Un nuevo método basado en restricciones para la generación de invariantes universalmente cuantificados sobre arrays. También se explica extensiones del método para aplicarlo a arrays ordenados. 2. Un técnica novedosa basada en Max-SMT para probar terminación. Gracias a expresar la generación de funciones de ranking como problemas de optimización Max-SMT, donde a las restricciones se les asigna diferentes pesos, se generan cuasi-funciones de ranking, funciones que casi satisfacen todas las condiciones que garantizan la existencia de una relación bien fundada, en ausencia de funciones de ranking. Además, Max-SMT facilita la combinación del proceso de construcción de un argumento de terminación con la tarea habitualmente necesaria de generar invariantes de apoyo. 3. Un método basado en restricciones y Max-SMT para probar que un programa no termina. El concepto clave del método es el de cuasi-invariante, que es una propiedad tal que si se cumple una vez en un punto del programa durante la ejecución, entonces continúa cumpliendose en ese punto desde entonces en adelante. Nuestra técnica considera en su análisis subgrafos fuertemente conexos del grafo de control de flujo del programa y produce testigos de no terminación más genéricos que otros métodos existentes. Además, es capaz de tratar programas con no determinismo. 4. Una técnica automatizada de verificación por partes de propiedades de corrección de un programa basada en cuasi-invariantes. Dado una parte de un programa (p.e., un único bucle) con una postcondición, se muestra como, usando Max-SMT, puede sintetizarse un invariante inductivo junto a una precondición que garantiza la validez del invariante y que el invariante implica la postcondición. Apartir de esto, se describe una infraestructura de verificación de programas de abajo a arriba que propaga precondiciones

Advanced Features in Protocol Verification: Theory, Properties, and Efficiency in Maude-NPA

The area of formal analysis of cryptographic protocols has been an active one since the mid 80’s. The idea is to verify communication protocols that use encryption to guarantee secrecy and that use authentication of data to ensure security. Formal methods are used in protocol analysis to provide formal proofs of security, and to uncover bugs and security flaws that in some cases had remained unknown long after the original protocol publication, such as the case of the well known Needham-Schroeder Public Key (NSPK) protocol. In this thesis we tackle problems regarding the three main pillars of protocol verification: modelling capabilities, verifiable properties, and efficiency. This thesis is devoted to investigate advanced features in the analysis of cryptographic protocols tailored to the Maude-NPA tool. This tool is a model-checker for cryptographic protocol analysis that allows for the incorporation of different equational theories and operates in the unbounded session model without the use of data or control abstraction. An important contribution of this thesis is relative to theoretical aspects of protocol verification in Maude-NPA. First, we define a forwards operational semantics, using rewriting logic as the theoretical framework and the Maude programming language as tool support. This is the first time that a forwards rewriting-based semantics is given for Maude-NPA. Second, we also study the problem that arises in cryptographic protocol analysis when it is necessary to guarantee that certain terms generated during a state exploration are in normal form with respect to the protocol equational theory. We also study techniques to extend Maude-NPA capabilities to support the verification of a wider class of protocols and security properties. First, we present a framework to specify and verify sequential protocol compositions in which one or more child protocols make use of information obtained from running a parent protocol. Second, we present a theoretical framework to specify and verify protocol indistinguishability in Maude-NPA. This kind of properties aim to verify that an attacker cannot distinguish between two versions of a protocol: for example, one using one secret and one using another, as it happens in electronic voting protocols. Finally, this thesis contributes to improve the efficiency of protocol verification in Maude-NPA. We define several techniques which drastically reduce the state space, and can often yield a finite state space, so that whether the desired security property holds or not can in fact be decided automatically, in spite of the general undecidability of such problems.Santiago Pinazo, S. (2015). Advanced Features in Protocol Verification: Theory, Properties, and Efficiency in Maude-NPA [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/4852