Access controls for oblivious and anonymous systems

The use of privacy-enhancing cryptographic protocols, such as anonymous credentials and oblivious transfer, could have a detrimental effect on the ability of providers to effectively implement access controls on their content. In this article, we propose a stateful anonymous credential system that allows the provider to implement nontrivial, real-world access controls on oblivious protocols conducted with anonymous users. Our system models the behavior of users as a state machine and embeds that state within an anonymous credential to restrict access to resources based on the state information. The use of state machine models of user behavior allows the provider to restrict the users' actions according to a wide variety of access control models without learning anything about the users' identities or actions. Our system is secure in the standard model under basic assumptions and, after an initial setup phase, each transaction requires only constant time. As a concrete example, we show how to implement the Brewer--Nash (Chinese Wall) and Bell-La Padula (Multilevel Security) access control models within our credential system. Furthermore, we combine our credential system with an adaptive oblivious transfer scheme to create a privacy-friendly oblivious database with strong access controls.