In distributed systems users often need to share sensitive data with other users

based on the latter's ability to satisfy various policies. In many cases the data owner

may not even know the identities of the data recipients, but deems it crucial that they

are legitimate; i.e., satisfy the policy. Enabling such data sharing over the Internet

faces the challenge of (1) securely associating access policies with data and enforcing

them, and (2) protecting data as it traverses untrusted proxies and intermediate

repositories. Furthermore, it is desirable to achieve properties such as: (1) flexibility

of access policies; (2) privacy of sensitive access policies; (3) minimal reliance on

trusted third parties; and (4) efficiency of access policy enforcement. Often schemes

enabling controlled data sharing need to trade one property for another. In this

dissertation, we propose two complimentary policy-based data sharing schemes that

achieve different subsets of the above desired properties.

In the first part of this dissertation, we focus on CiphertextPolicy Attribute-

Based Encryption (CP-ABE) schemes that specify and enforce access policies

cryptographically and eliminate trusted mediators. We motivate the need for flexible

attribute organization within user keys for efficient support of many practical

applications. We then propose Ciphertext-Policy Attribute-Set Based Encryption

(CP-ASBE) which is the first CP-ABE scheme to (1) efficiently support naturally

occurring compound attributes, (2) support multiple numerical assignments for a

given attribute in a single key and (3) provide efficient key management. While the

CP-ASBE scheme minimizes reliance on trusted mediators, it can support neither

context-based policies nor policy privacy. In the second part of this dissertation,

we propose Policy Based Encryption System (PBES), which employs mediated decryption

and supports both context-based policies and policy privacy. Finally, we integrate the

proposed schemes into practical applications (i.e., CP-ASBE scheme with Attribute-Based

Messaging (ABM) and PBES scheme with a conditional data sharing application in the Power Grid) and demonstrate their usefulness in practice.