Data Integrity Proofs in Cloud Storage

1. Data Integrity Proofs in Cloud Storage Author:Sravan Kumar R and Ashutosh Saxena. Source: TheThird International Conference onCommunication Systems and Networks (COMSNETS), 2011, Bangalore, India,pp. 1–4. Presenter: Tsuei-Hung Sun (孫翠鴻) Date:2011/3/4

2. Outline • Introduction • Motivation • Scheme • Performance Evaluation • Advantage vs. Drawback • Comment

3. Introduction(1/3) • Data outsourcing to cloud storage server • Data Authentication • Data Integrity • Proof of retrievability (POR) • Obtain and verify data is not modified. • Using a keyed hash function hK(F). • Prevent the cloud storage archives modifying the data without the consent of the data owner. H(.):hash function K: Secret Key F: File

4. Introduction(2/3) • Drawback of POR • It need high resource cost for the implementation. • It need to store secret key and hash value as many as file that stored at server. • It is a burdensome for server and some device.

5. Introduction(3/3) A. Juels and B. S. Kaliski, Jr., “Pors: proofs of retrievability for large files,” Proceedings of the 14th ACM conference on Computer and communications security, New York, USA, 2007, pp. 584–597. • Proof of retrievability for large files using “sentinels.” • Only a single key can be used. • Access only a small portion of the file F. • Setup phase: Randomly embeds sentinels among the data blocks. • Verification phase: Verifier check the integrity of the data file F by challenge prover specifies positions and asking return the associated sentinel values.

6. Motivation • The improve scheme need to insert sentinels and error correcting codes. • The improve scheme need to store all sentinels. • In the future, the owner of data maybe a small device (ex. PDA, mobile phone) • Goal • Deal with the problem of implementing POR. • Proof without the need to access the entire file or client to retrieving the entire file from the server. • Minimizing the local computation and bandwidth consumed at the client.

7. Scheme(1/4) • Assumption and Limit • Storage server might not be malicious. • The proof of data integrity protocol just checks the integrity of data. • Only apply to static storage of data. • The number of queries that can be asked by the client is fixed.

8. Scheme(2/4) • Setup phase • Let file F consist of n blocks and create metadata to append on it. • Let each of n data blocks have m bits in them. Fig. A data file F with 6 data blocks.

9. Scheme(3/4) • Setup phase (cont.) 1. Generation of meta-data 2.Encrypting the meta data 3. Appending of meta data k: The number of bits per data block which read as meta data. g: a function to generates a set of k bit positions. h: a function to generatesa k bit integer αi for each i. Mi: a encrypted mi by h.

10. Scheme(4/4) • Verification phase Verifier (Client) Archive (Cloud) Challenges g(i,j) Using g(i,j) to find the corresponding meta data. k+1 bits Using αi to decrypt Compare decrypted bits and send by cloud bits

11. Performance Evaluation • Storing only a single cryptographic key and two functions which using to generate a random sequence. • Only encrypting a part of file, so it can save computational time of the client. • Using XOR to instead hash function is more efficient. • Verification just need to find and send few bits of data to the client. • Network bandwidth is very less(k+1 bits for one proof).

12. Advantage vs. Drawback • Advantage • Reducing the computational and storage overhead of the client. • Minimizing the computational overhead of the cloud. • Reducing the network bandwidth consumption. • It is advantageous to thin clients. • Drawback • It not prevent the archive from modifying the data.

13. Comment • The decryption and comparison are not very clear to check that the response is correct or not. • It still need to store αi as many as number of files.