ID-based group-oriented cryptosystem and its digital signature scheme

Inspired from Tsujii and Itoh's system, we propose an ID-based group-oriented cryptosystem and its digital signature scheme. In the proposed cryptosystem, any entity for decrypting or signing a message is dedicated to ''group'', instead of ''individual''. A group, consisting of a set of individuals, could be specified as high-authorized, low-authorized or threshold-authorized according to the predefined authorization strategy. Any sender can encrypt a message addressed to a group with only knowing the group's identity and the corresponding public information, so that individuals of the group should cooperate to recover the message by following the authorization strategy. For signing a message, individuals of the group should cooperate to generate a group signature by following the authorization strategy, so that any verifier can validate the group signature with only requiring the group's identity and the corresponding public information. The sender of a secret message or the verifier of a group signature does not necessarily know the authorization strategy within that group in advance.