Abstract Machines for Communication Security

We use an existing formal software developement method called B in order to build and verify specica-tions of a communication channel, cryptographic functions and security properties. We show on an example how these basic specications may be combined in order to write abstract specications of cryptographic protocols and to verify their security. 1 Introduction Security evaluation criteria recommend that developers produce mathematical models of their systems and use these models in order to prove the security of their product. But developers are often reluctant to use formal methods because they have no evidence that this would actually increase the security of their products. They often consider that using mathematical notations produces very complex descriptions of real systems from which security proofs are very di-cult to obtain. Moreover, in general, formal methods cannot easily be integrated in usual development practices. This leads to the undesirable situation where the formal modeling work is treated separately from the development. Formal methods when used to study functional cor-rectness seem to have reached a higher level of maturity. This might help to change the relunctance to use formal methods. These methods are able to treat (to give a formal counterpart to) structuration concepts used to simplify the development of software as layer, service, module, interface. Furthermore, these methods are supported by tools that assist the user during the writing of specications and during the proofs. In this paper, we would like to show that it is possible to verify some aspects of security using available and general purpose technology. We were more particularly interested in the verication of the security of cryptographic protocols. We used, for that purpose , a formal software developement method called B [1]. This method contains a specication language that is state-oriented as Z [12] or FDM [8]. We also used B-Toolkit [2], a tool that supports every aspects of this method, in order to build the various abstract