A system and method for cryptographically securing data communications between a group of networked devices establishes and maintains an overlay network at the Application Layer, on top of a unicast routing service provided at the Internetworking Layer. The overlay network provides first, the routes that are used to deliver multicast datagrams and second, the cryptographic keys used to secure multicast datagrams. A common cryptographic key is established between all members of each group, and end-to-end encryption ensures that multicast datagrams can be accessed only by authorized group members. In other embodiments, keys are established between pairs of adjacent devices in the overlay network, and hop-by-hop encryption ensures that multicast datagrams can be accessed only by overlay network members.