If the data collected within a sensor network is valuable or should be kept confidential then security measures should protect the access to this data. We focus on user authentication, a central problem when trying to build access control mechanisms for sensor networks. We first sketch some security issues in the context of user authentication in sensor networks. We then introduce the notion of n-authentication, a special form of authentication which is more adequate to sensor networks than previous forms of authentication. We finally present and analyze a protocol for n-authentication.