Abstract. The Mondex Electronic Purse system  is an outstanding example of formal refinement techniques applied to a genuine industrial scale application, and is notable for being the first verification to achieve ITSEC level E6 certification. A formal abstract model including security properties, and a formal concrete model of the system design were developed, and a complex formal refinement was then hand-proved between them in Z. Despite this success, certain requirements issues were set beyond the scope of the formal development, or handled in an unnatural manner, in order to establish the refinement relation. Retrenchment is reviewed in a form suitable for integration with Z refinement, and is used to address one such issue in detail: the finiteness of the transaction sequence number in the purse funds transfer protocol. A retrenchment is constructed from the lowest level model of the Purse system to a model in which sequence numbers are finite, using a suitable elaboration of the Z promotion  technique. We overview the lifting of that retrenchment to the abstraction level of the higher models of the Purse system. The retrenchment-enhanced formal development is proposed as a methodological pattern for the verification of an application of this kind. The concessions of the various retrenchments formally capture the dissonance between the unbounded sequence number idealisation and the bounded reality. Reasoning about when the concession can become valid influences the actual choice of sequence number bound.
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.