Diffie-Hellman without Difficulty (Extended Version)

An excellent way for a protocol to obtain shared keys is Diffie-Hellman. For the automated verification of security protocols, the use of Diffie-Hellman poses a certain amount of difficulty, because it requires algebraic reasoning. Several tools work in the free algebra and even for tools that do support Diffie-Hellman, the algebraic reasoning becomes a bottleneck. We provide a new relative-soundness result: for a large class of protocols, significantly restricting the abilities of the intruder is without loss of attacks. We also show the soundness of a very restrictive encoding of Diffie-Hellman proposed by Millen and how to obtain a problem that can be answered in the free algebra without increasing its size upon encoding. This enables the efficient use of free-algebra verification tools for Diffie-Hellman based protocols and significantly reduces search-spaces for tools that do support algebraic reasoning.