In multicast communication, a source transmits the same content to a set of receivers. Current protocols for multicast follow a tree communication model which makes them scalable. This allows the set of receivers to be arbitrarily large. A large set of receivers (leaves) poses scalability problems when the multicast source (root) needs to collect data from the receivers. The literature on this subject is rather scarce. For a reverse multicast communication system to be scalable, it is necessary that intermediate multicast routers in the tree collect messages from their child nodes, aggregate them and send them back to their parent node. In this way, the root finally obtains a single message containing all data. Scalability also requires that aggregation of messages does not result in a size growth of the aggregated message. We focus on this problem with the extra requirements that leaf-to-root traffic should stay confidential and authentic. The few existing solutions satisfying all ...