A new technique for compacting ciphertext in multi-channel broadcast encryption and attribute-based encryption
S. Canard, D. H. Phan, D. Pointcheval, and V. C. Trinh.

Standard Broadcast Encryption (BE) and Attribute-Based Encryption (ABE) aim at sending a content to a large arbitrary group of users at once. Regarding Broadcast Encryption, currently, the most efficient schemes provide constant-size headers, that encapsulate ephemeral session keys under which the payload is encrypted. However, in practice, and namely for pay-TV, providers have to send various contents to different groups of users. Headers are thus specific to each group, one for each channel: as a consequence, the global overhead is linear in the number of channels. Furthermore, when one wants to zap to and watch another channel, one has to get the new header and decrypt it to learn the new session key: either the headers are sent quite frequently or one has to store all the headers, even if one watches one channel only. Otherwise, the zapping time becomes unacceptably long. We consider the encapsulation of several ephemeral keys, for various groups and thus various channels, in one header only, and we call this new primitive Multi-Channel Broadcast Encryption or MCBE: one can hope for a much shorter global overhead and a much shorter zapping time since the decoder already has the information to decrypt any available channel at once. Regarding Attribute-Based Encryption, a scheme with constant-size ciphertext is still a challenging task. In this paper, we introduce a new technique of optimizing the ciphertext-size for both MCBE and ABE schemes.