Fully Distributed Broadcast Encryption

Broadcast encryption schemes rely on a centralized authority to generate decryption keys for each user. It is observed that, when a broadcast encryption scheme is deployed for secret escrows, a dishonest dealer can read the escrowed secrets without leaving any witnesses. We present a new broadcast encryption paradigm referred to as fully distributed broadcast encryption (FDBE) without suffering from this vulnerability. In the new paradigm, there are multiple dealers, and by contacting a number of them equal to a threshold or more, any user can join the system; then the secrets can be encrypted to any subset of users and only the intended receivers can decrypt, while an attacker cannot get any information about the encrypted message even if the attacker controls all the users outside the receiver set and corrupts some dealers, provided that the number of corrupted dealers is less than a threshold. We realize the first fully distributed broadcast encryption scheme which is proven secure under the decision Bilinear Diffie-Hellman Exponentiation assumption in the standard model. A variant is also shown to achieve sub-linear complexity in terms of public key, decryption key and ciphertext, comparable to up-to-date regular broadcast encryption schemes without robustness and strong security against misbehaving dealers.