Policy privacy in cryptographic access control

Abstract

Cryptographic access control offers selective access
to encrypted data via a combination of key management and
functionality-rich cryptographic schemes, such as attribute-based
encryption. Using this approach, publicly available meta-data
may inadvertently leak information on the access policy that is
enforced by cryptography, which renders cryptographic access
control unusable in settings where this information is highly
sensitive.
We begin to address this problem by presenting rigorous
definitions for policy privacy in cryptographic access control.
For concreteness we set our results in the model of Role-Based
Access Control (RBAC), where we identify and formalize several
different flavors of privacy; however, our framework should serve
as inspiration for other models of access control. Based on our
insights we propose a new system which significantly improves on
the privacy properties of state-of-the-art constructions. Our design
is based on a novel type of privacy-preserving attribute-based
encryption, which we introduce and show how to instantiate.
We present our results in the context of a cryptographic
RBAC system by Ferrara et al. (CSF’13), which uses cryptography
to control read access to files, while write access is
still delegated to trusted monitors. We give an extension of the
construction that permits cryptographic control over write access.
Our construction assumes that key management uses out-of-band
channels between the policy enforcer and the users but eliminates
completely the need for monitoring read/write access to the data.

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