Cryptographic Enforcement of Information Flow Policies Without Public Information

The enforcement of access control policies using cryptographic primitives has been studied for over 30 years. When symmetric cryptographic primitives are used, each protected resource is encrypted and only authorized users are given the decryption key. Hence, users may require many keys. In most schemes in the literature, keys are derived from a single key explicitly assigned to the user and publicly available information. Recent work has challenged this design by developing schemes that do not require public information, the trade-off being that a user may require more than one key. However, these new schemes, which require a chain partition of the partially ordered set on which the access control policy is based, generally require more keys than necessary. Moreover, no algorithm is known for determining the best chain partition to use. In this paper we define the notion of a tree-based cryptographic enforcement scheme, which, like chain-based schemes, requires no public information but simultaneously has lower storage requirements. We formally establish that the strong security properties of recent chain-based schemes are preserved by tree-based schemes, and provide an efficient construction for deriving a tree-based enforcement scheme from a given policy that minimizes the number of keys required.