This issue of Mathematical Structures in Computer Science contains a selection of papers presented at the 17th International Workshop on Expressiveness in Concurrency (EXPRESS'10), a satellite event of CONCUR'10, held on August 30th in Paris, France.

In this work we address evolving systems, which are basically collaborative and distributed systems building up a larger scale system of system (SoS). These systems are able to adapt the current architecture to some changes in the environment. Constituent systems of a SoS, which represent the basic elements of our modeling approach, operate with different degrees of freedom and as a result the self-adaptation and cooperation between a set of constituent systems is driven by local needs. Based on our former work [11], we propose a well-defined modelling approach for SoS capturing both static and dynamic aspects. The aim is to address on the one hand the required flexibility to adapt the systems during run-time, and on the other hand to guarantee that the SoS reacts still in a safe manner. For this, we will use the contract paradigm for both the specification of legal configurations of the SoS, and to specify the dynamicity model, describing how the SoS architecture can change during run-time. Further, we depict how to adapt a system level analysis technique in order to check the dynamicity model against the invariants of the SoS. With this, we are able to determine, whether the SoS can reach some critical configurations. This enables us to modify the dynamicity model in an adequate manner.

This special issue of Mathematical Structures in Computer Science contains a selection of papers presented at three satellite events of CONCUR'09, which was held between 31 August and 5 September 2009 in Bologna (Italy). Specifically, it contains three papers from the 16th International Workshop on Expressiveness in Concurrency (EXPRESS'09), one paper from the 2nd Interaction and Concurrency Experience (ICE'09) and two papers from the 6th Workshop on Structural Operational Semantics (SOS'09).

In this paper we focus on tackling the insecurity problem of security protocols in the presence of an unbounded number of data such as nonces or session keys. First, we pinpoint four open problems in this category. The first two problems concern protocols with natural restrictions that any "realistic' protocol should satisfy while the second two concern protocols with disequality constraints. For protocols with disequality constraints we will prove: (1) Insecurity is decidable in NEXPTIME when bounding the size of messages and not requiring data to be freshly generated. (2) Insecurity is NEXPTIME-complete when bounding the size of messages and the number of freshly generated data used in honest sessions. This shows that unbounded data can be tackled in settings which do not trivially reduce to the case of bounded data. The second result is in contrast with a recently published proof, which appears to prove the same problem undecidable. We will point out why this proof cannot be considered to be valid.