• Incentives for surrogates: If users are to be persuaded to collaborate and share their resources with others, there needs to be motivation either through monetary or social incentives to do so. An interesting method is using common goals [12], but in the absence of common activities this will not prevail. In the case of monetary incentives, several questions need to be answered such as: how is credit represented in a mobile cloud? how will monetary transactions proceed in a secure method? how will the price of resources be decided? Using social incentives such as suggested in [71] also raises challenges such as preventing free riding and enforcing standards.