Bounded Memory Protocols and Progressing Collaborative Systems

It is well-known that the Dolev-Yao adversary is a powerful adversary. Besides acting as the network, intercepting, sending, and composing messages, he can remember as much information as he needs. That is, his memory is unbounded. We recently proposed a weaker Dolev-Yao like adversary, which also acts as the network, but whose memory is bounded. We showed that this Bounded Memory Dolev-Yao adversary, when given enough memory, can carry out many existing protocol anomalies. In particular, the known anomalies arise for bounded memory protocols, where there is only a bounded number of concurrent sessions and the honest participants of the protocol cannot generate an unbounded number of facts nor an unbounded number of nonces. This led us to the question of whether it is possible to infer an upper-bound on the memory required by the Dolev-Yao adversary to carry out an anomaly from the memory restrictions of the bounded protocol. This paper answers this question negatively. The second contribution of this paper is the formalization of Progressing Collaborative Systems that may create fresh values, such as nonces. We also prove the NP-completeness of the reachability problem for Progressing Collaborative Systems that may cre-