An intruder model with message inspection for model checking security protocols

Abstract

Model checking security protocols is based on an intruder model that represents the eavesdropping or interception of the exchanged messages, while at the same time performs attack actions against the ongoing protocol session(s). Any attempt to enumerate all messages that can be deduced by the intruder and the possible actions in all protocol steps results in an enormous branching of the model's state-space. In current work, we introduce a new intruder model that can be exploited for state-space reduction, optionally in combination with known techniques, such as partial order and symmetry reduction. The proposed intruder modeling approach called Message Inspection (MI) is based on enhancing the intruder's knowledge with metadata for the exchanged messages. In a preliminary simulation run, the intruder tags the analyzed messages with protocol-specific values for a set of predefined parameters. This metadata is used to identify possible attack actions, for which it is a priori known that they cannot cause a security violation. The MI algorithm selects attack actions that can be discarded, from an open-ended base of primitive attack actions. Thus, model checking focuses only on attack actions that may disclose a security violation. The most interesting consequence is a non-negligible state-space pruning, but at the same time our approach also allows customizing the behavior of the intruder model, in order e.g. to make it appropriate for model checking problems that involve liveness. We provide experimental results obtained with the SPIN model checker, for the Needham–Schroeder security protocol.