An Information Theoretic Analysis of Rooted-Tree Based Secure Multicast Key Distribution Schemes

Several variations of rooted tree based solutions have been recently proposed for member revocation in multicast communications. In this paper, we show that by assigning probabilities for member revocations, the optimality, correctness, and the system requirements of some of these schemes can be systematically studied using information theoretic concepts. Specifically, we show that the optimal average number of keys per member in a rooted tree is related to the entropy of the member revocation event.Using our derivations, we show that (a) the key assignments in correspond to the maximum entropy solution, (b) and direct application of source coding will lead to member collusion (we present recently proposed solutions as examples of this) and a general criteria that admits member collusion. We also show the relationship between entropy of member revocation event and key length.Crypto '99 :: 19th Annual IACR Crypto Conference</i

SECURE, POLICY-BASED, MULTI-RECIPIENT DATA SHARING

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