1. Trusted Data Sharing over Untrusted Cloud Storage Provider Gansen Zhao, Chunming Rong, Jin Li, Feng Zhang, and Yong Tang Cloud Computing Technology and Science (CloudCom), 2010 IEEE Second International Conference on

2. 2 Outline Introduction Security requirements Progressive elliptic curve encryption scheme Trusted sharing on untrusted cloud servers Security analysis Related work Conclusions

3. Introduction With cloud computing, data owners have only limited control over the IT infrastructure. Cloud service providers have excessive privileges. 3

4. Introduction The general idea of the proposal mechanism is to encrypt the data before storing on the cloud. On sharing the data, the encrypted data will be re-encrypted without being decrypted first. The re-encrypted data will then be cryptographically accessible to the authorize user only. 4

5. Security requirements 1.Data stored on the cloud should be confidential. 2.Sharing of the data can be achieved by the authorization by the data owner. 3.Permissions given by data owner cannot be transferred to others by the permission bearer. 5 Alice Bob Cloud Storage Provider Trudy

6. Security requirements The challenge of meeting the requirements is that secure data sharing needs to be achieved via an untrusted cloud storage provider. 6

7. Progressive elliptic curve encryption The PECE scheme allows a piece of data to be encrypted multiple times using different keys such that the final cipertext can be decrypted in a single run with a single key. The encryption and decryption are both based on Elliptic Curve Cryptography. 7

8. Notation Let m be a piece of data, U be a set of N users. For each, has the secret key k i. Let q be a random number agreed by all The encryption is performed in the order of For, it computes Where 8

9. Encrypt When all has participated in the encryption process, the final encrypted data is as follows. 9

10. Decrypt Let, then m e can be decrypted by a single operation as follows. 10

11. Trusted sharing on untrusted cloud servers 11

12. Notation k a : Alice private key (Alices key is not used) k a G : Alice public key k b : Bob private key k b G : Bob public key k c : Cloud Storage Provider private key k c G: Colud Storage Provider public key Assuming k c shared with Alice (the key kc can be a key that is dynamically generated by Alice and the Cloud Provider mutually) 12

13. Trusted sharing on untrusted cloud servers 13 Alice Bob Cloud Storage Provider Random number r, t Random number r c, r b

14. Algorithm Proof 14

15. Security Analysis Unauthorized Access to Data: 1.The attacker acquires a credential that can decrypt the data without the help of the Cloud Storage Provider. To acquire such a credential, the attacker will need the knowledge of rk c G + tG, or the knowledge of the three secrets of r, k c and t. As m, r, k c, and t are all kept in secret, the knowledge of rk c G+tG, or the knowledge of the three secrets of r, k c and t, are not possible. 15

16. Security Analysis 2.The attacker acquires a credential that can decrypt the data with the help of the Cloud Storage Provider. To acquire such a credential, the attacker must have the knowledge of r b, k b, or the knowledge of r b k b G. As r b is delivered to Bob in the form of r b G, it is not possible for the attacker to calculate r b from r b G. k b is a secret that is kept in private by Bob, hence the attacker could not acquire k b. 16

17. Security Analysis Information Disclosure During Sharing To acquire the clear data during the sharing, an attacker must either have the decryption key for m e, m c or m b. The above discussion proves that the attacker cannot decrypt m e or m b. To decrypt m c, the attacker needs the knowledge of r c k c G. As k c is the private secret kept by the Cloud Storage Provider, the attacker could be able to calculate r c k c G from r c G. 17 m c =m e + r c k c G + t c G

18. Attacker Use Case 18

19. Conclusions Limitations: This work assumes that the private key of the cloud provider is known to the data owner. This is a very strong assumption as no system administrators would want to share their systems keys with users, making it impractical to be deployed. The proposed algorithm and the protocol are less efficient than those protocols that requires only a single ECC encryption operation 19

20. Conclusions Contributions: 1.Identify the need for implementing trusted data sharing over untrusted cloud storage providers. 2.Propose a progressive encryption scheme based on elliptic curve encryption. 3.Devise a scheme for secure sharing on the cloud. 4.Perform a comprehensive security analysis of the proposed scheme and show that the scheme achieves trusted sharing over untrusted cloud servers. 20