Multicast is an efficient method to deliver multimedia content from a sender to a group of receivers and is gaining popular applications such as real time stock quotes, interactive games, video conference, live video broadcast, or video on demand. Authentication is one of the critical topics in securing multicast in an environment attractive to malicious attacks. Multicast authentication may provide the following security services: data integrity, data origin authentication, non-repudiation. All the three services can be supported by an asymmetric key technique called signature. In an ideal case, the sender generates a signature for each packet with its private key, which is called signing, and each receiver checks the validity of the signature with the senders public key, which is called verifying. If the verification succeeds, the receiver knows the packet is authentic. First, efficiency needs to be considered, especially for receivers. Compared with the multicast sender, which could be a powerful server, receivers can have different capabilities and resources. The receiver heterogeneity requires that the multicast authentication protocol be able to execute on not only powerful desktop computers but also resource-constrained mobile handsets. In particular, latency, computation, and communication overhead are major issues to be considered. Second, packet loss is inevitable. In the internet, congestion at routers is a major reason causing packet loss. An overloaded router drops buffered packets according to its preset control policy. This is not desirable for applications like realtime online streaming or stock quotes delivering. End users of online streaming will start to complain if they experience constant service interruptions due to packet loss, and missing critical stock quotes can cause severe capital loss of service subscribers.