This paper formally defines recognition as a new security principle closely related to authentication. Low-power sensor networks with no pre-deployment information require the less authoritative security of recognition. We give general properties of recognition protocols based on the method of key disclosure. We examine previously proposed low-power protocols according to the environment and security model presented. Finally, we give measurements from an implementation of a recognition protocol called Zero Common-Knowledge and discuss how well this proof-of-concept satisfies the properties of the environment.