Recent works have shown the usefulness of network and application layer protocols that connect low power sensor nodes directly to multiple applications and users on the Internet. We propose a security solution for this scenario. While previous works have provided security support for various communication patterns in sensor networks, such as among nodes, from nodes to a base station, and from users to nodes, the security of communication from sensor nodes to multiple users has not been sufficiently addressed. Specifically, we explore this design space and develop a security solution, named Secure-TWS, for efficient authentication of data sent by a resource constrained sensor node to multiple users, using digital signatures. We investigate the resource overheads in communication and computation for two most suitable signature schemes the Elliptic Curve Digital Algorithm (ECDSA) and the Boneh-Lynn-Shacham (BLS) short signature schemes. We implement these schemes on two popular sensor node architectures (based onMSP430 and ARM processors with 802.15.4 radios) and experimentally characterize relevant trade-offs.