Multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires it. Multicasting security is hard because of Open group membership, everyone gets same pack ets, Senders need not be members. We first present taxonomy of mu lticast scenarios on the Internet and point out relevant security concerns. Next we address two major security problems of multicast communication: source authentication, and key revocation. Maintaining authenticity in multicast protocols is a much more complex problem than for unicast, in particular known solutions are prohibitively inefficient in many cases. We present a solution that is reasonable for a range of scenarios. Our approach can be regarded as a midpoi nt between traditional Message Authentication Codes and digital signatures. We also present an improved solution to the key revocation problem.