Gradual Release: Unifying Declassification, Encryption and Key Release Policies

Information security has a challenge to address: enabling information-flow controls with expressive information release (or declassification) policies. Existing approaches tend to address some aspects of information release, exposing the other aspects for possible attacks. It is striking that these approaches fall into two mostly separate categories: revelation-based (as in information purchase, aggregate computation, moves in a game, etc.) and encryption-based declassification (as in sending encrypted secrets over an untrusted network, storing passwords, etc.). This paper introduces gradual release, a policy that unifies declassification, encryption, and key release policies. The authors model an attacker's knowledge by the sets of possible secret inputs as functions of publicly observable outputs.