How to Construct a Divisible Online/Offline Signature

The notion of Divisible Online/Offline Signature (DOS) was recently introduced by Gao et al. in CT-RSA 2009. In a DOS system, the idea is to perform the signing process in two phases. The first phase outputs offline tokens before the arrival of a message to be signed, where the offline tokens are regarded as a part of full signature, and the second phase is performed very efficiently after knowing the message. The DOS schemes are useful in many devices that have low bandwidth of the communication channel. Gao et al. also proposed a efficient DOS scheme, which is based on Boneh and Boyen(BB)´s short signature scheme. However, their scheme is approximately 480 bits in length. In this paper, we introduce a simple chameleon-hash-based transformation which changes traditional online/offline signature scheme into DOS scheme. With the Boneh, Lynn and Shacham (BLS)´s short signature as an example, we demonstrate a efficient DOS scheme which the length is only 320 bits, and prove its security in random oracles model.