The invention belongs to the technical field of 
cloud computing, and discloses a cross-domain 
authentication and fair audit duplicate removal 
cloud storage system based on a block chain. The 
system comprises a cross-domain 
authentication module, a data 
encryption module, a 
ciphertext duplicate removal module, an integrity audit module and a 
server punishment module. In the 
authentication module, acloud user generates a 
certificate at a 
client in a self-signed mode, the 
certificate is stored in a block 
chain network after being verified by an 
authentication server, the user is verified to compare information such as the 
certificate and a block, and whether the certificate is valid or not is verified. According to the method, the problem of hierarchical query 
verification of a traditional PKI structure is avoided, the authentication frequency is greatly reduced, and the 
verification cost is reduced; By using an intelligent contract, a malicious 
server is punished to receive certain penalties and compensate users with impaired benefits under the condition that user data is damaged; And in the duplicate removal module, a data duplicate removal function is realized by using a convergence 
encryption technology, so that the 
cloud server is prevented from storing too much duplicate data, and the user and the 
cloud server are prevented from consuming a large amount of calculation expenditure and storage expenditure.