A Certificateless Ordered Sequential Aggregate Signature Scheme Secure against Super Adversaries

Certificateless cryptosystem is a hybrid scheme of traditional PKI and ID-based scheme and has positive aspects of both of PKI and ID-based cryptosystem, i.e. solving key escrow problem and cer-tificate management problem simultaneously. Cryptographic schemes constructed in such a hybrid setting, generally called certificateless setting, retain these positive aspects and have been extensively studied recently. To the best of our knowledge, an ordered sequential aggregate signature (OSAS) scheme, which is a signature scheme verifying both the validity of a document and a signing order of a group of signers, has never been proposed in the certificateless setting. Therefore we propose an OSAS scheme in a certificateless setting called certificateless ordered sequential aggregate signature (CLOSAS) scheme. Our proposed scheme has advantages in its communication cost and the security proof. In particular, its signature size is fixed with respect to the number of signers, and the security is proven in the random oracle model against super adversaries that are the strongest adversary in certificateless signature scheme. Our scheme resists KGC’s malicious activities associated with key escrow and forgery of signatures as long as both of each user and KGC involve directly in a key generation