Verification of Multi-Party Ping-Pong Protocols via Program Transformation

The paper describes a verification technique based on program transformation with unfolding that allows to find short attacks on multi-party ping-pong protocols in the Dolev-Yao intruder model. Protocols are modelled by prefix grammars, and questions of model optimization and complexity are considered.

Global optimization and complexity trade-offs

It is sometimes assumed in research on language complexity that if languages tended towards some optimal level of global complexity, so that all languages would be roughly equally complex, then local complexity trade-offs should be a general principle in language. Drawing evidence from computer simulations I show that in equally complex systems the proportion of trade-offs (significant negative correlations) is higher than in random systems but far from being a general principle in language. In addition, it may be impossible to determine whether a certain correlation-set comes from random systems or equally complex systems. Based on these results a correlational approach on a handful of typological variables cannot be used to validate, or even falsify, the assumption that all languages are equally complex and, therefore, complexity trade-offs should be kept separate from that assumption. The typological distribution of complexity, drawn from the World Atlas of Language Structures, is further shown to differ from both random systems and equally complex systems.