Distributed Private-Key Generators for Identity-Based Cryptography

An identity-based encryption (IBE) scheme can greatly reduce the complexity of sending encrypted messages. However, an IBE scheme necessarily requires a private-key generator (PKG), which can create private keys for clients, and so can passively eavesdrop on all encrypted communications. Although a distributed PKG has been suggested as a way to mitigate this key escrow problem for Boneh and Franklin’s IBE scheme, the security of this distributed protocol has not been proven. Further, a distributed PKG has not been considered for any other IBE scheme.

In this paper, we design distributed PKG setup and private key extraction protocols for three important IBE schemes; namely, Boneh and Franklin’s BF-IBE, Sakai and Kasahara’s SK-IBE, and Boneh and Boyen’s

$\mbox{BB}_1$

-IBE. We give special attention to the applicability of our protocols to all possible types of bilinear pairings and prove their IND-ID-CCA security in the random oracle model against a Byzantine adversary. Finally, we also perform a comparative analysis of these protocols and present recommendations for their use.