ACCESS-CONTROL AND SIGNATURES VIA QUORUM SECRET SHARING

We suggest a method of controlling the access to a secure database via quorum systems. A quorum system is a collection of sets (quorums) eve ry two of which have a nonempty intersection. Quorum systems have been used for a number of applications in the area of distributed systems. We propose a separation between access servers, which are protected a nd trustworthy, but may be outdated, and the data servers, which may a ll be compromised. The main paradigm is that only the servers in a com plete quorum can collectively grant (or revoke) access permission. The method we suggest ensures that, after authorization is revoked, a che ating user Alice will not be able to access the data even if many acce ss servers still consider her authorized and even if the complete raw database is available to her. The method has a low overhead in terms o f communication and computation. It can also be converted into a distr ibuted system for issuing secure signatures. An important building blo ck in our method is the use of secret sharing schemes that realize the access structures of quorum systems. We provide several efficient con structions of such schemes which may be of interest in their own right .