As sensor networks edge closer towards wide-spread deployment, security issues become a central concern. Sensor networks have been identified as being useful in a variety of domains to include the battlefield and perimeter defense. So far, much research has focused on making sensor networks feasible and useful, and has not concentrated on security. We present a suite of security building blocks optimized for resource constrained environments and wireless communication. SPINS has two secure building blocks: SNEP and ?TESLA SNEP provides the following important baseline security primitives: Data confidentiality, two-party data authentication, and data freshness. A particularly hard problem is to provide efficient broadcast authentication, which is an important mechanism for sensor networks. ?TESLA is a new protocol which provides authenticated broadcast for severely resource-constrained environments. We implemented the above protocols, and show that they are practical even on minimal hardware: the performance of the protocol suite easily matches the data rate of our network. Additionally, we demonstrate that the suite can be used for building higher level protocols.