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We use expander graphs to provide efficient new constructions for two security applications: authentication of long digital streams over lossy networks and building scalable, robust overlay networks. Here is a summary of our contributions: (1) To authenticate long digital streams over lossy networks, we provide a construction with a provable lower bound on the ability to authenticate a packet - and that lower bound is independent of the size of the graph. To achieve this, we present an authentication expander graph with constant degree. (Previous work used authentication graphs but required graphs with degree linear in the number of vertices.) (2) To build efficient, robust, and scalable overlay networks, we provide a construction using undirected expander graphs with a provable lower bound on the ability of a broadcast message to successfully reach any receiver. This also gives us a new, more efficient solution to the decentralized certificate revocation problem.