COMPUTER NETWORK RUNNING A DISTRIBUTED APPLICATION

A computer network is disclosed in which a group of computers co-operate to perform a distributed application. In order to ensure that only members of that group of computers are able to carry out certain operations, messages sent in the performance of the distributed application are checked by the recipient for the presence of a group membership token. The inclusion of a group membership token is controlled by one or more group membership handlers which intercept messages from local components and only include a group membership token with the message if they list the sending local component as being entitled to include the group membership token in the message. Furthermore, by operating the group membership token on a separate machine, or preferably a separate virtual machine from the local component, security is further improved. In the most preferred embodiments, the group token handler and/or the local component are hosted on virtual machines which provide virtualised cryptographic functionality.