Flexible and secure communications in an identity-based coalition environment

In this paper we consider the problem of how entities operating under distinct roots of trust in a coalition environment can flexibly and securely communicate with one another. We consider the identity-based setting, with each entity being pre-configured with a private key from a particular trusted authority (TA), but where multiple, independent TAs are involved in the coalition. Our solution to the problem adapts the Boneh-Franklin identity-based encryption (IBE) scheme. It allows any entity to securely communicate with any other entity, even without knowing the TA with which the intended recipient is associated. To enable this, we assume that the TAs co-operate to distribute certain additional public information to all entities which allows entities to decrypt a ciphertext that was composed using the public parameters of one TA, using a private key issued by another. We include a security analysis of our new approach.