Towards Correctness of Program Transformations Through Unification and Critical Pair Computation

Correctness of program transformations in extended lambda calculi with a contextual semantics is usually based on reasoning about the operational semantics which is a rewrite semantics. A successful approach to proving correctness is the combination of a context lemma with the computation of overlaps between program transformations and the reduction rules, and then of so-called complete sets of diagrams. The method is similar to the computation of critical pairs for the completion of term rewriting systems. We explore cases where the computation of these overlaps can be done in a first order way by variants of critical pair computation that use unification algorithms. As a case study we apply the method to a lambda calculus with recursive let-expressions and describe an effective unification algorithm to determine all overlaps of a set of transformations with all reduction rules. The unification algorithm employs many-sorted terms, the equational theory of left-commutativity modelling multi-sets, context variables of different kinds and a mechanism for compactly representing binding chains in recursive let-expressions.Comment: In Proceedings UNIF 2010, arXiv:1012.455

Pseudo-contractions as Gentle Repairs

Updating a knowledge base to remove an unwanted consequence is a challenging task. Some of the original sentences must be either deleted or weakened in such a way that the sentence to be removed is no longer entailed by the resulting set. On the other hand, it is desirable that the existing knowledge be preserved as much as possible, minimising the loss of information. Several approaches to this problem can be found in the literature. In particular, when the knowledge is represented by an ontology, two different families of frameworks have been developed in the literature in the past decades with numerous ideas in common but with little interaction between the communities: applications of AGM-like Belief Change and justification-based Ontology Repair. In this paper, we investigate the relationship between pseudo-contraction operations and gentle repairs. Both aim to avoid the complete deletion of sentences when replacing them with weaker versions is enough to prevent the entailment of the unwanted formula. We show the correspondence between concepts on both sides and investigate under which conditions they are equivalent. Furthermore, we propose a unified notation for the two approaches, which might contribute to the integration of the two areas

Asymmetric Unification: A New Unification Paradigm for Cryptographic Protocol Analysis

The final publication is available at Springer via http://dx.doi.org/10.1007/978-3-642-38574-2_16We present a new paradigm for unification arising out of a technique commonly used in cryptographic protocol analysis tools that employ unification modulo equational theories. This paradigm relies on: (i) a decomposition of an equational theory into (R,E) where R is confluent, terminating, and coherent modulo E, and (ii) on reducing unification problems to a set of problems s=?ts=?t under the constraint that t remains R/E-irreducible. We call this method asymmetric unification. We first present a general-purpose generic asymmetric unification algorithm. and then outline an approach for converting special-purpose conventional unification algorithms to asymmetric ones, demonstrating it for exclusive-or with uninterpreted function symbols. We demonstrate how asymmetric unification can improve performanceby running the algorithm on a set of benchmark problems. We also give results on the complexity and decidability of asymmetric unification.S. Escobar and S. Santiago were partially supported by EU (FEDER) and the Spanish MEC/MICINN under grant TIN 2010-21062-C02-02, and by Generalitat Valenciana PROMETEO2011/052. The following authors were partially supported by NSF: S. Escobar, J. Meseguer, and R. Sasse under CNS 09-04749 and CCF 09- 05584; D. Kapur under CNS 09-05222; C. Lynch, Z. Liu, and C. Meadows under CNS 09-05378, and P. Narendran and S. Erbatur under CNS 09-05286. Part of the S. Erbatur’s work was supported while with the Department of Computer Science, University at Albany, and part of R. Sasse’s work was supported while with the Department of Computer Science, University of Illinois at Urbana-Champaign.Erbatur, S.; Escobar Román, S.; Kapur, D.; Liu, Z.; Lynch, CA.; Meadows, C.; Meseguer, J.... (2013). Asymmetric Unification: A New Unification Paradigm for Cryptographic Protocol Analysis. En Automated Deduction – CADE-24. 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In: Ölveczky, P.C. (ed.) WRLA 2010. LNCS, vol. 6381, pp. 86–103. Springer, Heidelberg (2010)Erbatur, S., Escobar, S., Kapur, D., Liu, Z., Lynch, C., Meadows, C., Meseguer, J., Narendran, P., Santiago, S., Sasse, R.: Effective symbolic protocol analysis via equational irreducibility conditions. In: Foresti, S., Yung, M., Martinelli, F. (eds.) ESORICS 2012. LNCS, vol. 7459, pp. 73–90. Springer, Heidelberg (2012)Erbatur, S., Escobar, S., Kapur, D., Liu, Z., Lynch, C., Meadows, C., Meseguer, J., Narendran, P., Sasse, R.: Asymmetric unification: A new unification paradigm for cryptographic protocol analysis. In: UNIF 2011 (2011), https://sites.google.com/a/cs.uni.wroc.pl/unif-2011/programEscobar, S., Sasse, R., Meseguer, J.: Folding variant narrowing and optimal variant termination. J. Log. Algebr. Program. 81(7-8), 898–928 (2012)Harju, T., Karhumäki, J., Krob, D.: Remarks on generalized post correspondence problem. In: Puech, C., Reischuk, R. (eds.) STACS 1996. LNCS, vol. 1046, pp. 39–48. Springer, Heidelberg (1996)Hopcroft, J.E., Motwani, R., Ullman, J.D.: Introduction to automata theory, languages, and computation - international edition, 2nd edn. Addison-Wesley (2003)Jouannaud, J.-P., Kirchner, H.: Completion of a set of rules modulo a set of equations. SIAM J. Comput. 15(4), 1155–1194 (1986)Liu, Z., Lynch, C.: Efficient general unification for XOR with homomorphism. In: Bjørner, N., Sofronie-Stokkermans, V. (eds.) CADE 2011. LNCS, vol. 6803, pp. 407–421. Springer, Heidelberg (2011)Liu, Z.: Dealing Efficiently with Exclusive OR, Abelian Groups and Homomorphism in Cryptographic Protocol Analysis. PhD thesis, Clarkson University (2012), http://people.clarkson.edu/~clynch/papers/Dissertation_of_Zhiqiang_Liu.pdfLowe, G., Roscoe, A.W.R.: Using CSP to detect errors in the TMN protocol. IEEE Transactions on Software Engineering 23, 659–669 (1997)Meseguer, J.: Conditional rewriting logic as a united model of concurrency. Theor. Comput. 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The Role of Term Symmetry in E-Unification and E-Completion

A major portion of the work and time involved in completing an incomplete set of reductions using an E-completion procedure such as the one described by Knuth and Bendix [070] or its extension to associative-commutative equational theories as described by Peterson and Stickel [PS81] is spent calculating critical pairs and subsequently testing them for coherence. A pruning technique which removes from consideration those critical pairs that represent redundant or superfluous information, either before, during, or after their calculation, can therefore make a marked difference in the run time and efficiency of an E-completion procedure to which it is applied. The exploitation of term symmetry is one such pruning technique. The calculation of redundant critical pairs can be avoided by detecting the term symmetries that can occur between the subterms of the left-hand side of the major reduction being used, and later between the unifiers of these subterms with the left-hand side of the minor reduction. After calculation, and even after reduction to normal form, the observation of term symmetries can lead to significant savings. The results in this paper were achieved through the development and use of a flexible E-unification algorithm which is currently written to process pairs of terms which may contain any combination of Null-E, C (Commutative), AC (Associative-Commutative) and ACI (Associative-Commutative with Identity) operators. One characteristic of this E-unification algorithm that we have not observed in any other to date is the ability to process a pair of terms which have different ACI top-level operators. In addition, the algorithm is a modular design which is a variation of the Yelick model [Ye85], and is easily extended to process terms containing operators of additional equational theories by simply plugging in a unification module for the new theory

Broken symmetry and Yang-Mills theory

From its inception in statistical physics to its role in the construction and in the development of the asymmetric Yang-Mills phase in quantum field theory, the notion of spontaneous broken symmetry permeates contemporary physics. This is reviewed with particular emphasis on the conceptual issues.Comment: Latex 30 pages, 9 figures. Typo corrected. Contribution to "Fifty years of Yang Mills theory", editor G. 't Hooft, to be published by World Scientifi

Hierarchical combination of intruder theories

International audienceRecently automated deduction tools have proved to be very effective for detecting attacks on cryptographic protocols. These analysis can be improved, for finding more subtle weaknesses, by a more accurate modelling of operators employed by protocols. Several works have shown how to handle a single algebraic operator (associated with a fixed intruder theory) or how to combine several operators satisfying disjoint theories. However several interesting equational theories, such as exponentiation with an abelian group law for exponents remain out of the scope of these techniques. This has motivated us to introduce a new notion of hierarchical combination for non-disjoint intruder theories and to show decidability results for the deduction problem in these theories. We have also shown that under natural hypotheses hierarchical intruder constraints can be decided. This result applies to an exponentiation theory that appears to be more general than the one considered before