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This paper experimentally investigates the feasibility of crypto-free communications in resource-constrained wireless sensor networks. We exploit the spatial signature induced by the radio communications of a node on its neighboring nodes. We design a primitive that robustly and efficiently realizes this concept, even at the level of individual packets and when the network is relatively sparse. Using this primitive, we design a protocol that robustly and efficiently validates the authenticity of the source of messages: authentic messages incur no communication overhead whereas masqueraded communications are detected cooperatively by the neighboring nodes. The protocol enables lightweight collusion-resistant methods for broadcast authentication, unicast authentication, non-repudiation and integrity of communication. We have implemented our primitive and protocol, and quantified the high-level of accuracy of the protocol via testbed experiments with CC1000 radio-enabled motes.