Vehicular networks require secure communication, especially for safety applications. A public key infrastructure using a Combinatorial Certificate Scheme was implemented in the US Vehicle Infrastructure Integration (VII) Proof-ofConcept (PoC) trial to secure V2V communication and preserve vehicle privacy. This paper analyzes the privacy and scalability of the Combinatorial Certificate approach for a nationwide network of 200 million vehicles. It examines the tradeoffs between privacy, the ability to efficiently detect and remove bad actors, and the need to minimize the impact on innocent vehicles due to revocation and replacement of compromised shared certificates. Key findings include the level of vehicle anonymity that exists in situations of low vehicular density and the impact that certificate revocations have on innocent vehicles. A refinement to the Combinatorial Certificate Scheme is described that improves the innocent vehicle re-key quota lifetime by an order of magnitude.

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Abstract| Substantial interest has developed in recent years in building computer systems that support cooperative w ork among groups without the need for physical proximity. This paper examines some of the di cult data management issues in designing systems for computer-supported cooperative w ork CSCW. Speci cally, w e consider an example CSCW system to support large-scale team science over the Internet, Collaboratory Builder's Environment; we discuss the issues of managing shared data in such systems, reducing information overload, and providing group awareness and access control. We discuss several promising approaches to these issues. We point out where a signi cant gap remains in addressing the requirements of such systems and where designers have to make design tradeo s that can be di cult to evaluate. Finally, w e discuss several open issues for future work.

T he emergence of the Internet has enabled interactions between people working in the same field across the globe. This has resulted in a number of efforts to solve the problems of supporting group communication, and providing access to information sources that are geographically and administratively scattered. Collaboratories are one way to provide virtual shared workspaces to users. We define a collaboratory as a wide area distributed system supporting a rich set of services and tools that facilitate effective synchronous and asynchronous communication between two or more people who are not co-located. The Upper Atmospheric Research Collaboratory is an Internet-based system that gives space scientists a virtual shared workspace for conducting real-time experiments as well as various asynchronous collaborations from geographically dispersed facilities. 1 During the past two years, UARC has integrated a significant fraction of the worldwide observational systems devoted to space physics and aeronomy. (Figure 1 shows the sources available to the last data-collection campaign in April 1998.) Moreover, the system now supports real-time outputs from supercomputer-class theoretical models. This has enabled simultaneous comparison of experimental and theoretical data on a global scale. Such comparisons would not be possible without collaboratory technology. UARC has evolved into a scalable Web-based collaboratory over a period of six years. Since its inception in 1993, UARC has supported more than 10 scientific campaigns, each of which represented a concerted effort to collect real-time data simultaneously from instruments around the world for several days at a time. The first few campaigns ran on a distributed set of Next systems on the NSFNET and could support about a dozen active par

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