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Architecture and Protocol

This architecture uses reliable multicast and the group key management protocol described in section 4 as the main building block. Under group key management protocol, Trent is the trusted server who is responsible for admitting and expelling members, while updating fresh group keys during the process. With a fresh group key (K.Group), the protocol is quite simple:

1. Alice creates the hash of the message H(M) and encrypts together with her message using K.Group. If authentication is necessary (and hence her identity is revealed), Alice puts her signature of H(M) before the encryption. Then she multicasts the encrypted message to the group:

   Alice $\Rightarrow$ Group: {M, (H(M) or {H(M)}K.Alice.priv)}K.Group

2. Bob, a member of the group, decrypts the message, verifies that the hash matches the message and that Alice's signature is correct (if available).