An ad-hoc network is a group of wireless terminals that possess the ability to automatically organise themselves into a radio network, in which each terminal can perform the duties of both end node as well as router. A network so formed has the particular characteristics of being dynamic by nature, being inherently incapable of relying on any pre-existing infrastructure or centralised administration system as well as being unique in that it uses Hertzian transmission with air as its medium. This ad hoc communication capacity, together with multi technology support and more classical cellular access to networks - or to the Internet - is the fundamental network architecture for what is nowadays called . This architecture is sometimes denoted hybrid networks. These are the characteristics that render hybrid networks particularly vulnerable to attack. The security models employed in other infrastructure based networks (wireless or otherwise) are inapplicable in ad hoc networks, due to their design based on the fundamental principle of a reliable, fixed trusted third party or an established addressing system. We propose, in this paper, a solution that addresses the specific needs of ad hoc networks. First, we describe the paradox presented by the contrasting needs for mobility as well as strong security in ad hoc / hybrid networks. Second we describe our proposed solution, a , which uses the concept of trust transmission to form a mobile version of an infrastructure: an . As the ambient community grow in terms of members, the trust seed becomes capable of distributing trust to more and more entities. However, the originality of our contribution resides in the proposition of a new key agreement protocol, conceived to distribute a secret between ad-hoc nodes without ever transmitting it via a non-secure channel, and without any prior configuration. We conclude the document by presenting some perspectives that will allow us to further our work and approach the concept of adaptive trust and ambiances.