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We propose a new axiomatisation of the alpha-equivalence relation for nominal terms, based on a primitive notion of fixed-point constraint. We show that the standard freshness relation between atoms and terms can be derived from the more primitive notion of permutation fixed-point, and use this result to prove the correctness of the new $\alpha$-equivalence axiomatisation. This gives rise to a new notion of nominal unification, where solutions for unification problems are pairs of a fixed-point context and a substitution. Although it may seem less natural than the standard notion of nominal unifier based on freshness constraints, the notion of unifier based on fixed-point constraints behaves better when equational theories are considered: for example, nominal unification remains finitary in the presence of commutativity, whereas it becomes infinitary when unifiers are expressed using freshness contexts. We provide a definition of $\alpha$-equivalence modulo equational theories that take into account A, C and AC theories. Based on this notion of equivalence, we show that C-unification is finitary and we provide a sound and complete C-unification algorithm, as a first step towards the development of nominal unification modulo AC and other equational theories with permutative properties.

Confluence is a critical property of computational systems which is related with determinism and non ambiguity and thus with other relevant computational attributes of functional specifications and rewriting system as termination and completion. Several criteria have been explored that guarantee confluence and their formalisations provide further interesting information. This work discusses topics and presents personal positions and views related with the formalisation of confluence properties in the Prototype Verification System PVS developed at our research group.

We present an algorithm to decide the intruder deduction problem (IDP) for a class of locally stable theories enriched with normal forms. Our result relies on a new and efficient algorithm to solve a restricted case of higher-order associative-commutative matching, obtained by combining the Distinct Occurrences of AC- matching algorithm and a standard algorithm to solve systems of linear Diophantine equations. A translation between natural deduction and sequent calculus allows us to use the same approach to decide the emphelementary deduction problem for locally stable theories. As an application, we model the theory of blind signatures and derive an algorithm to decide IDP in this context, extending previous decidability results.

Orthogonality is a discipline of programming that in a syntactic manner guarantees determinism of functional specifications. Essentially, orthogonality avoids, on the one side, the inherent ambiguity of non determinism, prohibiting the existence of different rules that specify the same function and that may apply simultaneously (non-ambiguity), and, on the other side, it eliminates the possibility of occurrence of repetitions of variables in the left-hand side of these rules (left linearity). In the theory of term rewriting systems (TRSs) determinism is captured by the well-known property of confluence, that basically states that whenever different computations or simplifications from a term are possible, the computed answers should coincide. Although the proofs are technically elaborated, confluence is well-known to be a consequence of orthogonality. Thus, orthogonality is an important mathematical discipline intrinsic to the specification of recursive functions that is naturally applied in functional programming and specification. Starting from a formalization of the theory of TRSs in the proof assistant PVS, this work describes how confluence of orthogonal TRSs has been formalized, based on axiomatizations of properties of rules, positions and substitutions involved in parallel steps of reduction, in this proof assistant. Proofs for some similar but restricted properties such as the property of confluence of non-ambiguous and (left and right) linear TRSs have been fully formalized.

In this work, we present an algebraic approach for modeling the two-party cascade protocol of Dolev-Yao and for fully formalizing its security in the specification language of the Prototype Verification System PVS. Although cascade protocols could be argued to be a very limited model, it should be stressed here that they are the basis of more sophisticated protocols of great applicability, such as those which allow treatment of multiparty, tuples, nonces, name-stamps, signatures, etc. In the current algebraic approach, steps of the protocol are modeled in a monoid freely generated by the cryptographic operators. Words in this monoid are specified as finite sequences and the whole protocol as a finite sequence of protocol steps, that are functions from pairs of users to sequences of cryptographic operators. In a previous work, assuming that for balanced protocols admissible words produced by a potential intruder should be balanced, a formalization of the characterization of security of this kind of protocols was given in PVS. In this work, the previously assumed property is also formalized, obtaining in this way a complete formalization which mathematically guarantees the security of these protocols. Despite such property being relatively easy to specify, obtaining a complete formalization requires a great amount of effort, because several algebraic properties, that are related to the preservation of the balancing property of the admissible language of the intruder, should be formalized in the granularity of the underlying data structure (of finite sequences used in the specification). Among these properties, the most complex are related to the notion of linkage property, which allows for a systematic analysis of words of the admissible language of a potential saboteur, showing how he/she is unable to isolate private keys of other users under the assumption of balanced protocols. The difficulties that arose in conducting this formalization are also presented in this work.

This work presents a formalization of the theorem of existence of most general unifiers in first-order signatures in the higher-order proof assistant PVS. The distinguishing feature of this formalization is that it remains close to the textbook proofs that are based on proving the correctness of the well-known Robinson's first-order unification algorithm. The formalization was applied inside a PVS development for term rewriting systems that provides a complete formalization of the Knuth-Bendix Critical Pair theorem, among other relevant theorems of the theory of rewriting. In addition, the formalization methodology has been proved of practical use in order to verify the correctness of unification algorithms in the style of the original Robinson's unification algorithm.

The lambda-calculus with de Bruijn indices assembles each alpha-class of lambda-terms in a unique term, using indices instead of variable names. Intersection types provide finitary type polymorphism and can characterise normalisable lambda-terms through the property that a term is normalisable if and only if it is typeable. To be closer to computations and to simplify the formalisation of the atomic operations involved in beta-contractions, several calculi of explicit substitution were developed mostly with de Bruijn indices. Versions of explicit substitutions calculi without types and with simple type systems are well investigated in contrast to versions with more elaborate type systems such as intersection types. In previous work, we introduced a de Bruijn version of the lambda-calculus with an intersection type system and proved that it preserves subject reduction, a basic property of type systems. In this paper a version with de Bruijn indices of an intersection type system originally introduced to characterise principal typings for beta-normal forms is presented. We present the characterisation in this new system and the corresponding versions for the type inference and the reconstruction of normal forms from principal typings algorithms. We briefly discuss the failure of the subject reduction property and some possible solutions for it.

This paper shows how a formalization for the theory of Abstract Reduction Systems (ARSs) in which noetherianity was specified by the notion of well-foundness over binary relations is used in order to prove results such as the well-known Newman's and Yokouchi's Lemmas. The former is known as the diamond lemma and the latter states a property of commutation between ARSs. The theory ars was specified in the Prototype Verification System (PVS) for which to the best of our knowledge there was no available theory for dealing with rewriting techniques in general before this development. In addition to proof techniques available in the higher-order specification language of PVS, the verification of these lemmas implies an elaborated use of natural and noetherian induction.