This paper presents practical protocols for authenticating the parameters that characterise a principal's context. Context features in what are known as mobile, ubiquitous, pervasive and nomadic computing systems. Location, in particular, is a highly significant contextual parameter. We present a model of context authentication based on the characteristics of communication channels. Then we present protocols for location authentication that are based on physical channel characteristics, including a protocol that we have implemented over HTTP for use from any Web browser. We conclude with a summary and discussion of the work.