Constructing fair-exchange protocols for cloud computing

The existing fair-exchange protocols can't be run in cloud servers,because the user's data aren't stored in local computers,but be escrowed to 'clound'.An anonymous conditional proxy re-encryption scheme was constructed and its security was proved in random oracle model based on modified decisional bilinear Diffie-Hellman problem.At the same time,a fair-exchange protocol based on their scheme was designed,it can be employed when two users exchange their data information in cloud.