Authorization-based Group-oriented Secure Broadcasting System

This paper presents an authorization-based group-oriented secure broadcasting system based on the ID-based system proposed by Tsujii and Itoh. In the proposed system, any broadcasting secret in ciphertext form is addressed to “group” instead of “individual”. First of all, the broadcaster defines the message authorization strategy among the individuals in the destination group by considering the characteristics or restrictions of the secret, such as timeliness, integrity, or importance etc. Afterwards, the broadcaster broadcasts the secret in ciphertext form to the destination group such that the individuals in the destination group only can recover the secret by means of authentication, following a predefined message authorization strategy. In general, the broadcaster can define the message authorization strategy for a secure broadcast as one of the following: (1) restricted 1-out-of-n, so that only the specified individual in the destination group can recover it; (2) unrestricted 1-out-of-n, so that any individual in the destination group can recover it; (3) n-out-of-n, so that all individuals in the destination group should collaborate together to recover it; (4) restricted t-out-of-n, so that only the specified t individuals in the destination group should collaborate together to recover it; and (5) unrestricted t-out-of-n, so that any t individuals in the destination group can collaborate together to recover it. The security of the proposed system is based on the difficulty of computing discrete logarithms over GF(p). Also, some possible conspiracy attacks on the proposed system are analyzed.