Provable bounds for portable and flexible privacy-preserving access

In this work we address the problem of portable and flexible privacy-preserving access rights for large online data repositories. Privacy-preserving access control means that the service provider can neither learn what access rights a customer has nor link a request to access an item to a particular customer, thus maintaining privacy of both customer activity and customer access rights. Flexible access rights allow any customer to choose any subset of items from the repository and correspondingly be charged only for the items selected. And portability of access rights means that the rights themselves can be stored on small devices of limited storage space and computational capabilities, and therefore the rights must be enforced using the limited resources available.Our main results are solutions to the problem that utilize minimal perfect hash functions and order-preserving minimal perfect hash functions. None of them use expensive cryptography, all require very little space, and they are therefore suitable for computationally weak and space-limited devices such as smartcards, sensors, etc. Performance of the schemes is measured as the probability of false positives (i.e., the probability that access to an unpurchased item will be permitted) for a given storage space bound. Using our techniques, for a data repository of size n and subscription order of m ll n items, we achieve a probability of false positives of m-c using only O(cm) bits of storage space, where c is an adjustable parameter (a constant or otherwise) that can be set to provide the desired performance. This is the first time that such provable bounds are established for this problem, and we believe the techniques we use are of more general interest through the unusual use we make of perfect hashing.