Mobile clients in context-aware systems benefit from the indirect addressing of users via their context (contextcast), such as addressing messages to all users in downtown Toronto whose age is below 35. There is, however, almost no support for a temporal decoupling in such a contextcast system, i.e., the addressing of users that were or will be in a certain context in the past or future, respectively. This could for instance be used to distribute the minutes of a meeting to all people who attended the meeting in room 1.138, 3 days ago, between 1 and 3 pm.

To enable a context-aware communication system to address messages with temporal relations, especially those contexts in the past, the system needs to manage information about user context histories. This poses the risk that the system can be abused to profile users, which would most probably hinder acceptance. Therefore, privacy aspects need to be considered in the core design of such a system. We present an extension to our earlier work, which allows a temporal decoupling of messages and users and requires very little additional overhead to manage historic context information. The solution includes mechanisms to efficiently disseminate messages to both users with past and future contexts, while effectively preventing user profiling through the use of virtual identities.