Cryptographic Key Secrecy of the Strengthened Yahalom Protocol via a Symbolic Security Proof

Symbolic secrecy of exchanged keys is arguably one of the most important notions of secrecy shown with automated proof tools. It means that an adversary restricted to symbolic operations on terms can never get the entire key into its knowledge set. Cryptographic key secrecy essentially means computational indistinguishability between the real key and a random one, given the view of a much more general adversary. We provide the first proof of cryptographic key secrecy for the strengthened Yahalom protocol, which constitutes one of the most prominent key exchange protocols analyzed by means of automated proof tools. The proof holds in the presence of arbitrary active attacks provided that the protocol is implemented using standard provably secure cryptographic primitives. We exploit recent results on linking symbolic and cryptographic key secrecy in order to perform a symbolic proof of secrecy for the Yahalom protocol in a specific setting that allows us to derive the desired cryptographic key secrecy from the symbolic proof.

By: Michael Backes; Birgit Pfitzmann

Published in: RZ3601 in 2005


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