A method comprising the use of a bootstrapping protocol to define a security relationship between a first server and a second server, the first and second servers co-operating to provide a service to a user terminal. A bootstrapping protocol is used to generate a shared key for securing communication between the first server and the second server. The shared key is based on a context of the bootstrapping protocol, and the context is associated with a Subscriber Identity Module (SIM) associated with the user terminal and provides a base for the shared key. A method of the invention may, for example, be employed within a computing/service network such as a “cloud”, and in particular for communications between two servers in the cloud that are co-operating to provide a service to a user.