Fully Adaptive Decentralized MA-ABE: Simplified, Optimized, ASP Supported
Nikhil Vanjani
VideoAbstract:
Access control is a foundational problem in computer security and it requires that only authorized users can access specific data. Attribute-Based Encryption (ABE) addresses this challenge by binding ciphertexts to access policies and decryption keys to user attributes. However, traditional ABE relies on a single trusted authority holding a master secret key: a central point of failure that limits real-world deployment. Multi-Authority ABE (MA-ABE) overcomes this limitation by distributing trust among multiple authorities managing disjoint sets of attributes. But prior MA-ABE schemes either were proven secure in presence of static corruption of authorities or relied on complex "dual-subsystem" proof techniques that made them inefficient.
In this talk, I will present a streamlined security analysis showing -- perhaps surprisingly -- that the classic Lewko–Waters MA-ABE scheme (EUROCRYPT 2011) already achieves full adaptive security, provided its design is carefully reinterpreted and, more crucially, its security proof is re-orchestrated to conclude with an information-theoretic hybrid in place of the original target-group-based computational step. By dispensing with dual subsystems and target-group-based assumptions, we achieve a significantly simpler and tighter security proof along with a more lightweight implementation. Our construction reduces ciphertext size by 33%, shrinks user secret keys by 66%, and requires 50% fewer pairing operations during decryption -- all while retaining full decentralization and collusion resistance.
If time permits, I will also briefly discuss how these proof techniques extend to construct the first MA-ABE scheme for arithmetic span programs, capturing a richer class of access policies.
This is joint work with Pratish Datta (NTT Research) and Junichi Tomida (NTT Research).
Bio:
Nikhil Vanjani is a final-year Ph.D. candidate at Carnegie Mellon University, advised by Elaine Shi. His research lies at the intersection of applied and theoretical cryptography, with a focus on developing new functional encryption techniques and its applications to decentralized systems. He received his B.Tech from IIT Kanpur and M.S. from CMU, where his thesis "Multi-Input Inner Product Encryption: Function-Hiding Instantiations without Random Oracles" won the department's Best Masters Thesis Award.