Blind identity-based encryption and simulatable oblivious transfer

Abstract

In an identity-based encryption (IBE) scheme, there is a key extraction protocol where a user submits an identity string to a master authority who then returns the corresponding secret key for that identity. In this work, we describe how this protocol can be performed efficiently and in a blind fashion for several known IBE schemes; that is, a user can obtain a secret key for an identity without the master authority learning anything about this identity. We formalize this notion as blind IBE and discuss its many practical applications. In particular, we build upon the recent work of Camenisch, Neven, and shelat [12] to construct oblivious transfer (OT) schemes which achieve full simulatability for both sender and receiver. OT constructions with comparable efficiency prior to Camenisch et al. were proven secure in the weaker half-simulation model. Our OT schemes are constructed from the blind IBE schemes we propose, which require only static complexity assumptions (e.g., DBDH) whereas prior comparable schemes require dynamic assumptions (e.g., q-PDDH). © International Association for Cryptology Research 2007.