Multicast communication is arguably the most promising paradigm to enable mass Internet transmission of live events, or any other content sent on the network from a single source to a large community of receivers. A tree model is normally used to represent this type of communication. Scalability problems arise when communication is many-to-one (leaf-toroot) rather than one-to-many (root-to-leaf): this is the case when the root collects data (sensor information, usage data, micropayments in real-time pay-per-view, etc.) from the leaves. The matter is further complicated if there are confidentiality and authenticity requirements on the leaf-to-root traffic. We present here a new method offering security and scalability in many-to-one communication which outperforms previous proposals in the literature: it is more general, it saves more bandwidth and it is computationally simpler. In particular, computation at the leaves is simple enough to be encapsulated in a smart card, which is att...