In ubiquitous computing, the main security challenges arise from network heterogeneity and from a dynamic population of nomadic users and limited devices. For these environments, security infrastructures based on traditional PKIs present a number of major drawbacks: limited scalability and reconfigurability, static vision of trust, and high administrative costs. Besides, these infrastructures are not well adapted to authorisation, and lack delegation capabilities. In this paper, we present an enhanced PKI for ubiquitous networks which solves a number of these issues by providing flexible authentication and authorisation services in disconnected environments