Fair Exchange with Guardian Angels

In this paper we propose a new probabilistic Fair Exchange Protocol which requires no central Trusted Third Party. Instead, it relies on a virtually distributed and decentralized Trusted Third Party which is formalized as a Guardian Angel: a kind of Observer e.g. a tamper proof security device. We thus introduce a network model with Pirates and Guardian Angels which is well suited for Ad Hoc networks. In this setting we reduce the Fair Exchange Problem to a Synchronization Problem in which honest parties need to eventually decide whether or not a protocol succeeded in a synchronous way through a hostile network which does not guaranty that sent messages will be eventually received. This problem can be of independent interest in order to add reliability of protocol termination in secure channels.