Verifying a bounded number of sessions and its complexity

Abstract

We investigate the complexity of the protocol insecurity pr oblem for a finite number of sessions (fixed number of interleaved runs). We show that this problem is NP-complete with respect to a Dolev-Yao model of i ntruders. The result does not assume a limit on the size of messages and supports as ymetric and nonatomic symmetric encryption keys. We also prove that in orde r to build an attack with a fixed number of sessions the intruder needs only to forg e messages of linear size, provided that they are represented as DAGs.