Process algebraic techniques for distributed systems are increasingly being targeted at identifying abstractions adequate both for high-level programming and specification, and for security analysis and verification. Drawing on our earlier work in (Bugliesi and Focardi, 2008) , we investigate the expressive power of a core set of security and network abstractions that provide high-level primitives for the specifications of the honest principals in a network, while at the same time enabling an analysis of the network-level adversarial attacks that may be mounted by an intruder. We analyze various bisimulation equivalences for security, arising from endowing the intruder with (i) different adversarial capabilities and (ii) increasingly powerful control on the interaction among the distributed principals of a network. By comparing the relative strength of the bisimulation equivalences, we obtain a direct measure of the discriminating power of the intruder, hence of the expressiveness of the corresponding intruder model. Achieving security in distributed systems is challenging and often creates a tension between two conflicting design requirements. On the one side, security requires formal, often low-level, specifications of the safeguards built against the threats to which the systems are exposed. On the other side, programming needs techniques and reasoning methods that abstract away from low-level details to focus instead on the expected functional properties. In the literature on process calculi, this tension has generated a range of approaches, with two extremes. At one end, lie specifications based on the pi calculus (Milner et al., 1992) . These assume very abstract mechanisms to secure communications by relying on private channels. While elegant and concise, the security guarantees conveyed by these mechanisms are often hard to realize in practice (Abadi, 1998) . At the other end, we find specifications that draw on explicit cryptographic primitives as in the spi calculus (Abadi and Gordon, 1999) or in the applied-pi calculus (Abadi and Fournet, 2001) . While this approach is very successful in providing a formal basis for the analysis of †
The presence of a spy in our model is reminiscent of the network abstraction of REF .
Channel Abstractions for Network Security
UNDER CONSIDERATION FOR PUBLICATION IN MATH. STRUCT. IN COMP. SCIENCE
Computer Science