Handling Infeasible Speciications of Cryptographic Protocols

In the veriication of cryptographic protocols along the approach of the logic for authentication by Burrows, Abadi, and Needham, it is possible to write a speciica-tion which does not faithfully represent the real world situation. Such a speciication, though impossible or unreasonable to implement, can go undetected and be veriied to be correct. It can also lead to logical statements that do not preserve causality, which in turn can have undesirable consequences. Such a speciication, called an infeasible speciication here, can be subtle and hard to locate. This note shows how the logic of cryptographic protocols by Gong, Needham, and Yahalom can be enhanced with a notion of eligibility to preserve causality of beliefs and detect infeasible speciications. It is conceivable that this technique can be adopted in other similar logics.