This paper presents a comprehensive introduction to the ELAN rule-based programming language. We describe the main features of the language, the ELAN environment, and introduce bibliographic references to various papers addressing foundations, implementation and applications of ELAN. 1 Introduction The ELAN system [18] provides an environment for specifying and prototyping deduction systems in a language based on rules controlled by strategies. Its purpose is to support the design of theorem provers, logic programming languages, constraints solvers and decision procedures and to offer a modular framework for studying their combination. ELAN takes from functional programming the concept of abstract data types and the function evaluation principle based on rewriting. But rewriting is inherently non-deterministic since several rules can be applied at different positions in a same term, and in ELAN, a computation may have several results. This aspect is taken into account through choice...

. On a case study, we present a new approach for verifying cryptographic protocols, based on rewriting and on tree automata techniques. Protocols are operationally described using Term Rewriting Systems and the initial set of communication requests is described by a tree automaton. Starting from these two representations, we automatically compute an over-approximation of the set of exchanged messages (also recognized by a tree automaton). Then, proving classical properties like confidentiality or authentication can be done by automatically showing that the intersection between the approximation and a set of prohibited behaviors is the empty set. Furthermore, this method enjoys a simple and powerful way to describe intruder work, the ability to consider an unbounded number of parties, an unbounded number of interleaved sessions, and a theoretical property ensuring safeness of the approximation. Introduction In this paper, we present a new way of verifying cryptographic pro...

In this paper we introduce a static analysis technique for graph transformation systems. We present an algorithm which, given a graph transformation system and a start graph, produces a nite structure consisting of a hypergraph decorated with transitions (Petri graph) which can be seen as an approximation of the Winskel style unfolding of the graph transformation system. The fact that any reachable graph has an homomorphic image in the Petri graph and the additional causal information provided by transitions allow us to prove several interesting properties of the original system. As an application of the proposed technique we show how it can be used to verify the absence of deadlocks in an innite-state Dining Philosophers system.

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We explore the notion of alternating two-way tree automata modulo the theory of finitely many associative-commutative (AC) symbols, some of them with a unit (AC1). This was prompted by questions arising in cryptographic protocol verification, where the emptiness question for intersections of such automata is fundamental. We show that the use of conditional push clauses, or of alternation, leads to undecidability, already in the case of one AC or AC1 symbol, with only functions of arity zero. On the other hand, emptiness is decidable in the general case of many function symbols, including many AC or AC1 symbols, provided push clauses are unconditional and intersection clauses are final. To this end, extensive use of refinements of resolution is made.

. Starting from the regular tree language E of ground constructor-instances of any linear term, we build a nite tree automaton that recognizes the set of descendants R (E) of E for a constructor-based term rewrite system whose right-hand-sides fulll the following three restrictions : linearity, no nested function symbols, function arguments are variables or ground terms. Note that left-linearity is not assumed. We next present several applications. 1 Introduction Tree automata have already been applied to many areas of computer science, and in particular to rewriting techniques [2]. In comparison with more sophisticated renements, nite tree automata are obviously less powerful, but have plenty of good properties and lead to much simpler algorithms from a practical point of view. Because of potential applications to automated deduction and program validation, the problem of expressing by a nite tree automaton the transitive closure of a regular set E of ground terms with resp...

We present Timbuk -- a tree automata library -- which implements usual operations on tree automata as well as a completion algorithm used to compute an over-approximation of the set of descendants (E) for a regular set E and a term rewriting system R, possibly non linear and non terminating. On several examples of term rewriting systems representing programs and systems to verify, we show how to use Timbuk to construct their approximations and then prove unreachability properties of these systems.

This paper presents an approximation function developed for the verication of cryptographic protocols. The main properties of this approximation are that it can be build automatically and its computation is guaranteed to terminate unlike Genet and Klay's algorithm. This approximation has been used for the verication of the Needham-Schroeder, OtwayRees and Woo Lam protocols. To be more precise, the approximation allows us to check secrecy and authenticity properties of the protocols.

Abstract. This paper shows how to construct static analyzers using tree automata and rewriting techniques. Starting from a term rewriting system representing the operational semantics of the target programming language and given a program to analyze, we automatically construct an over-approximation of the set of reachable terms, i.e. of the program states that can be reached. The approach enables fast prototyping of static analyzers because modifying the analysis simply amounts to changing the set of rewrite rules defining the approximation. A salient feature of this approach is that the approximation is correct by construction and hence does not require an explicit correctness proof. To illustrate the framework proposed here on a realistic programming language we instantiate it with the Java Virtual Machine semantics and perform class analysis on Java bytecode programs. 1

A term rewriting system which effectively preserves recognizability (EPR-TRS) has good mathematical properties. In this paper, a new subclass of TRSs, layered transducing TRSs (LT-TRSs) is defined and its recognizability preserving property is discussed. The class of LT-TRSs contains some EPR-TRSs, e.g., {f(x) → f(g(x))} which do not belong to any of the known decidable subclasses of EPR-TRSs. Bottom-up linear tree transducer, which is a well-known computation model in the tree language theory, is a special case of LT-TRS. We present a sufficient condition for an LT-TRS to be an EPR-TRS. Also reachability and joinability are shown to be decidable for LT-TRSs. 1