Multi-receiver identity-based key encapsulation with shortened ciphertext

Abstract

This paper describes two identity based encryption (IBE) protocols in the multi-receiver setting. The first protocol is secure in the selective-ID model while the second protocol is secure in the full model. The proofs do not depend on the random oracle heuristic. The main interesting feature of both protocols is that the ciphertext size is |S|/N, where S is the intended set of receivers and N is a parameter of the protocol. To the best of our knowledge, in the multi-receiver IBE setting, these are the first protocols to achieve sub-linear ciphertext sizes. There are three previous protocols for this problem – two using the random oracle heuristic and one without. We make a detailed comparison to these protocols and highlight the advantages of the new constructions.