All-But-Many Encryption - A New Framework for Fully-Equipped UC Commitments

We present a general framework for constructing non-interactive universally composable (UC) commitment schemes that are secure against adaptive adversaries in the non-erasure model under a re-usable common reference string. Previously, such “fully-equipped” UC commitment schemes have been known only in [CF01,CLOS02], with strict expansion factor O(κ); meaning that to commit λ bits, communication strictly requires O(λκ) bits, where κ denotes the security parameter. Efficient construction of a fully-equipped UC commitment scheme is a long-standing open problem. We introduce new abstraction, called all-but-many encryption (ABME), and prove that it captures fully-equipped UC commitment schemes. We propose the first fully-equipped UC commitment scheme with optimal expansion factor O(1) from our ABME scheme related to the DCR assumption. We also provide an all-but-many lossy trapdoor function (ABM-LTF) [Hof12] from our DCR-based ABME scheme, with a better lossy rate than [Hof12].