Abstract.For ubiquitous communication self-organising adhoc networks become more and more important. We consider mobile phones as an appropriate trusted gateway for external machines with low communication needs. A message-based approach is best in such a scenario with moving mobile phones and machines. We propose a security model for access control to the communication infrastructure that is also messagebased. To meet the requirements of ubiquitous communicating machines, all algorithms on the sender's side are based on symmetric cryptography resulting in low computation needs. A sophisticated symmetric key infrastructure for message authentication provides the necessary key management. The trustworthiness of the mobile phone is achieved by using the SIM as a secure storage and computing module. This makes it possible to use the mobile phone not only as a user terminal but also as a trusted infrastructure component of the mobile network.