In distributed systems, and especially in pervasive grid environments, the users usually face difficulties to obtain permissions to use the resources from the sites they visit. Either they have an account locally, or they can show a certificate obtained beforehand that gives them the access. Problems arise when they arrive in an unknown environment, which is likely to occur in a pervasive grid environment. In this paper, we propose a trust based model of authentication and access control that allows nomadic users to roam from site to site and to gain access to surrounding/remote resources wrt her status in her home site and to the local policy of the site where she is standing. This model is supported by a software architecture called Chameleon. The Chameleon permits users to access pervasive grid resources and to implement adhoc interactions through the local grid site.