Traditional security protocols are mainly concerned with authentication and key establishment and rely on predistributed keys and properties of cryptographic operators. In contrast, new application areas are emerging that establish and rely on properties of the physical world. Examples include protocols for secure localization, distance bounding, and secure time synchronization. We present a formal model for modeling and reasoning about such physical security protocols. Our model extends standard, inductive, trace-based, symbolic approaches with a formalization of physical properties of the environment, namely communication, location, and time. In particular, communication is subject to physical constraints, for example, message transmission takes time determined by the communication medium used and the distance between nodes. All agents, including intruders, are subject to these constraints and this results in a distributed intruder with restricted, but more realistic, communication capabilities than those of the standard Dolev-Yao intruder. We have formalized our model in Isabelle/HOL and have used it to verify protocols for authenticated ranging, distance bounding, broadcast authentication based on delayed key disclosure, and time synchronization.

Security protocol design is a creative discipline where the solution space depends on the problem to be solved and the cryptographic operators available. In this paper, we examine the general question of when two agents can create a shared secret. Namely, given an equational theory describing the cryptographic operators available, is there a protocol that allows the agents to establish a shared secret? We examine this question in several settings. First, we provide necessary and sufficient conditions for secret establishment using subterm convergent theories. This directly yields a decision procedure for this problem. As a consequence, we obtain impossibility results for symmetric encryption and signature schemes. Second, we use algebraic methods to prove impossibility results for two important theories that are not subterm convergent: XOR and abelian groups. Finally, we develop a general combination result that enables modular impossibility proofs. For example, the results for symmetric encryption and XOR can be combined to obtain impossibility for the joint theory.

We present a formal model for modeling and reasoning about security protocols. Our model extends standard, inductive, trace-based, symbolic approaches with a formalization of physical properties of the environment, namely communication, location, and time. In particular, communication is subject to physical constraints, for example, message transmission takes time determined by the communication medium used and the distance traveled. All agents, including intruders, are subject to these constraints and this results in a distributed intruder with restricted, but more realistic, communication capabilities than those of the standard Dolev-Yao intruder. We have formalized our model in Isabelle/HOL and used it to verify protocols for authenticated ranging, distance bounding, and broadcast authentication based on delayed key disclosure.