Cryptographic Approach for Delegation and Authorization in Cloud Computing

1. Di Ma NSF Workshop on Security for Cloud Computing Mar. 15 ~ Mar. 16, 2012 Arlington, VA Cryptographic Approach for Delegation and Authorization in Cloud Computing

2. Two Areas to Look At • Fine-grained access control (or authorization) • Complex access policy to support fine-grained authorization • Delegation from owner to cloud: to achieve fine-grained temporal access control • Delegation from user to cloud: to support mobile device access • Computation over encrypted/authenticated data Relationship with other panel talks: New data sharing platform: • Multi-owner and multi-user at large scale • Data sharing through cloud • Untrusted but resourceful cloud Leverage resourceful cloud delegation and authorization Deal with untrusted server and user Aim for end-to-end security adversary models end-to-end security

3. Attribute-Based Encryption (ABE) for Fine-Grained Access Control of Encrypted Data in Clouds • Cipher-policy attribute-based encryption • Encryptors can specify access policyas a booleanformmularccesstree structure where • Intermediate nodes: AND, OR gates • Leaf nodes: attributes expressed as strings • Access privileges (access keys): list of attributes • Access policy is embedded into the ciphertext and authorized user is allowed to access (decrypt) the data based on her attributes • End-to-end authorization • Owner defines access policy, not the server • Access policy is enforced by the encryption algorithm, not the server • Scalability • Complexity is dependent on #attributes; independent of #users

4. Issue 1: Secure Comparison for Complex Policy Enforcement • Current ABE systems lack an efficient mechanismto support a complete set of comparison relations (<,>,≤,≥ ) in policy specification • focus on string match (i.e., =) • E.g., Name = “Bob”; Rank = “3” • have limited (inefficient) support for integer comparison • Bit-wise integer comparison • Access (authorization) policy can be complex and attribute can be multi-dimensional • Various comparison relations: (<, >, ≤, ≥, between, contains, overlap, disjoint,…) • Range attribute (or interval): [8:00AM, 5:00PM] (attribute: one can access data in regular office hours) • Multi-dimensional attribute: • (policy: data can be accessed when current location is within the service area) • Efficient secure comparison mechanism is needed to express complex policy required by fine-grained access control • How to support various cryptographic comparison? • How to support multi-dimensional attribute?

5. Issue 2: Encryption Delegation for Fine-grained Temporal Control • Time is an important access control parameter • The corresponding access policy changes when time flies • In March, access policy [Jan, Jun] implicitly becomes [Mar, Jun] • Time attribute can expire • Efficient encryption delegation mechanism is needed • to achieve fine-grained temporal access control • or (more generally), how to transform ciphertext with a more restrictive policy

6. Issue 3: Decryption Delegation for Mobile Device Access • Cloud computing provides services accessible anytime, anywhere from any networked devices • A large portion of cloud services is anticipated to be accessed through mobile devices which • are comparably resource constrained • may access real-time cloud services • Efficient decryption delegation mechanism is needed • to shift (majority) decryption from mobile user to cloud • to reduce user-side computation

7. Computation over Encrypted/Authenticated Data • Try to summarize the state-of-the-art • Very recent new concepts • Motivation: • More and more data processing will be done in the clouds due to data and service outsourcing • However, for security and privacy concerns, data is in encrypted and/or authenticated form • Homomorphic encryption for computation over encrypted data • Allows derivation of computation result in the encrypted form without decryption • Homomorphicsignature for computation over authenticated data • Allows derivation of a valid signature for the computation result without private signing key(s) • Models: single-key vs. multi-key • Single-key: when data are encrypted/signed using the same key (k1=k2) • Multi-key: when data are encrypted/signed using different keys (k1 != k2)

8. The state-of-the-art Multiple-key Single-key • Traditional homomorphic encryption schemes belong to this category • Concept has existed for 30 years • Efficient semi-homomorphic schemes exist • Fully-homomorphicencryption schemes are not practical • Traditional homomorphic encryption schemes belong to this category • Concept has existed for 30 years • Efficient semi-homomorphic schemes exist • Fully-homomorphicencryption schemes are not practical • Initially explored, formal privacy model is introduced in 2011 • Support SUM over messages of very small size • Initially explored, formal privacy model is introduced in 2011 • Support SUM over messages of very small size Homomorphicencryption for encrypteddata Elaine Shi, T-H. Hubert Chan, Eleanor Rieffel, Richard Chow, Dawn Song. Privacy-Preserving Aggregation of Time-Series Data. In NDSS, Feb. 6~9, 2011. • No solution (that supports end-to-end authentication of computation result) is available yet • Related work: secure aggregation in sensor networks • “commit and re-check” involving multiple rounds of interaction (no end-to-end security) • Initially explored, formal security and privacy models are just introduced recently • Support computations: Quoting substring, subset predicate, average • Initially explored, formal security and privacy models are just introduced recently • Support computations: Quoting substring, subset predicate, average • No solution that supports end-to-end authentication of computation result is available yet • Related work: secure aggregation in sensor networks • “commit and re-check” involving multiple rounds of interaction (no end-to-end security) Homomorphic signature for authenticateddata Jae Hyun Ahn , Dan Boneh, Jan Camenisch, Susan Hohenberger,abhishelat ,and Brent Waters. Computing over Authenticated Data. In TCC, Mar. 19~21, 2012.

9. Summary • Fine-grained access control (or authorization) • Secure comparison for complex policy enforcement • Encryption delegation from owner to cloud: to enforce fine-grained temporal access control • Decryption delegation from user to cloud: to support mobile device access • Computation over encrypted/authenticated data • Homomorphic encryption in single-/multi-key models • Homomorphic signature in single-/multi-key models