Key establishment is a fundamental prerequisite for secure communication in wireless sensor networks. A new node joining the network needs to efficiently and autonomously set up secret keys with his communication partners without the use of a central infrastructure. Most cited current research papers focus on a probabilistic distribution of sets of keys from larger key pools to new nodes. This results in unnecessary expensive communication and memory consumption, growing linearly with the size of the network, and guarantees secure connections only with a certain probability. This work presents a novel approach for efficient and secure key establishment of nodes joining the network by utilizing the fact that communication in sensor networks follows a paradigm called aggregation. Keys are split into shares and forwarded using disjoint paths in the network. The approach is self-organizing and minimizes memory consumption as well as radio transmissions efficiently -- down to logarithmic behavior.