We describe our new results in developing and extending Automatic Protocol Generation (APG), an approach to automatically generate security protocols. We explore two-party mutual authentication and key agreement protocols, with a trusted third party (TTP) which shares a symmetric key with each of the two principals. During the process, we experienced the challenge of a gigantic protocol space. Facing this challenge, we develop more powerful reduction techniques for the protocol generator. We also develop new pruning theorems and probabilistic methods of picking goal orderings for the protocol screener, Athena, which greatly improve the efficiency and worst-case performance of Athena. In our first experiment, APG found new protocols for two-party mutual authentication with a TTP using symmetric keys. In our second experiment, APG also found new protocols for three different sets of security properties for two-party authentication and key agreement. Our new list of security properties for key agreement also uncovered an undocumented deficiency in the Yahalom protocol.
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