Vehicular ad hoc networks (VANETs) are gaining growing interest from both industry and academia. The primitive objective of VANETs is to enhance the road safety for public transportation systems through dedicated short range communications (DSRC) protocol. In addition to the powerful radios and abundant spectrums, DSRC also paves the way for VANETs to support numerous emerging Internet-related applications. In this paper, we introduce a promising commercial application which allows each vehicle to purchase file/data through a roadside unit (RSU). Due to the high mobility of vehicles, the contact period between an RSU and a vehicle could be insufficient to download the complete file. Thus, in the proposed file purchasing systems, once a vehicle in the process of downloading a file leaves the transmission range of the RSU, its neighboring vehicles with a piece of the file can cooperatively help to complete the file transfer via vehicle-to-vehicle (V2V) communications. Such a commercial file purchasing system can obviously initiate a new application scenario; however, it cannot be put into practice unless the security issues, such as the user privacy, incentives for inter-vehicle cooperation, and the copyright protection for∼the file content, are well addressed. In order to deal with these security issues, we develop a secure framework for the file purchasing system in VANETs. Performance evaluation will be conducted to verify the proposed scheme. Copyright © 2008 John Wiley & Sons, Ltd.