A Survey on Secure Cloud Data Storage ZENG, Xi CAI, Peng

Outsource your data to the cloud Cloud data storage is a rising business model. 2 Mobile devices Individual users Enterprises

Cloud Data Storage A cost-saving business solution: Save cost for unused storage Save technical support for data backups Save electric power and maintenance costs for data centers As a cloud client, how do we provide security guarantees for our outsourced data? Privacy Data Availability Data Integrity Consistency Access Control Assured Deletion

Security Challenges Can we protect outsourced data from improperly accessed? Unauthorized users must not access our data We don’t want cloud providers to mine our data for their marketing purposes We need access control: Only authorized parties can access outsourced data

Security Challenges Can we reliably remove data from cloud? We don’t want backups to exist after pre-defined time e.g., to avoid future exposure due to data breach or error management of operators If an employee quits, we want to remove his/her data e.g., to avoid legal liability Cloud makes backup copies. We don’t know if all backup copies are reliably removed. We need assured deletion: Data becomes inaccessible upon requests of deletion

Secure and E ffi cient Access to Outsourced Data W. Wang, Z. Li, R. Owens, and B. Bhargava W. Wang, Z. Li, R. Owens, and B. Bhargava

Wang’s Approach Aims for designing an approach to achieve flexible access control and large-scale dynamic data management in a high secure and efficient way.

Wang’s Approach Data Access Procedure Owner-write-user-read Scenario Data can be updated only by the original owner Users read the information according to access rights

Wang’s Approach Key generation

Wang’s Approach Dynamics Handling User Access Right Eavesdropping Over-encryption Lazy revocation Outsourced Data Deletion Insertion and appending Updating Control block One-to-one mapping hierarchy Overhead Analysis Limited storage overhead

Wang’s Approach Advantages Low clients’ responsibilities Low storage overhead Block insertion, update, deletion and appending Disadvantages Requires support from the cloud side No multiple policies combination

FADE: a secure overlay cloud storage system with File Assured Deletion Yang Tang, Patrick P. C. Lee, John C. S. Lui, and Radia Perlman Yang Tang, Patrick P. C. Lee, John C. S. Lui, and Radia Perlman

FADE A new policy-based file assured deletion scheme that reliably deletes files of revoked file access policies Implement a working prototype of FADE atop Amazon S3 Evaluate the performance overhead of FADE atop Amazon S3

Policy-based File Assured Deletion Each file is associated with a data key and a file access policy Each policy is associated with a control key All control keys are maintained by a key manager When a policy is revoked, its respective control key will be removed from the key manager

15 Main idea: File protected with data key Data key protected with control key File data key control key is maintained by the key manager Policy-based File Assured Deletion

16 When a policy is revoked, the control key is removed. The encrypted data key and hence the encrypted file cannot be recovered The file is deleted, i.e., even a copy exists, it is encrypted and inaccessible by everyone File data key Cannot be recovered without Policy-based File Assured Deletion

17 Multiple Policies Conjunctive policies Satisfy all policies to recover file File P1P1 P2P2 P3P3  Disjunctive policies Satisfy only one policy to recover fileSatisfy only one policy to recover file File P1P1 P2P2 P3P3 {F} K {{K} S1 } S2 … } Sm S 1 e1, S 2 e2, …, S m em {F} K {K} S1,{K} S2,{K} Sm,S 1 e1, S 2 e2, …, S m em

18 System Entities Data owner: the entity that originates data to be stored on cloud Key manager: maintains policy-based control keys for encrypting data keys Cloud: third-party cloud provider (e.g., Amazon S3) that stores data

19 Architecture of FADE FADE decouples key management and data management Key manager can be flexibly deployed in another trusted third party, or deployed within data owner No implementation changes on cloud key manager … Data owner Cloud file (encrypted) metadata file FADE

20 File Upload PiPi (n i, e i ) P i, {K} Si, S i ei, {F} K CloudData ownerKey manager Send policy P i Return RSA public key for P i Send metadata & encrypted file to cloud Data owner randomly chooses (i) K for file F and (ii) S i for policy P i. Things sent to cloud P i = policy P i {K} Si = data key K encrypted with S i using symmetric key crypto S i ej = secret key S i encrypted with e i using public key crypto S i is used for policy renewal {F} K = file encrypted with data key K using symmetric key crypto Cache (n i, e i ) for future use

21 File Download P i, S i ei R ei SiRSiR P i, {K} Si, S i ei, {F} K CloudData ownerKey manager Send all back to data owner Decrypt with d i, and return Send blinded S i ei Data owner randomly picks a number R, and blinds S i ei with R ei It unblinds S i R, and recovers K and F Unblind S i R

22 Policy Renewal Main idea: S i re-encrypted into S i em {K} Si and {F} K remain unchanged on cloud P i, S i ei R ei, P m S i R, (n m, e m ) P i, S i ei CloudData ownerKey manager Send only P i and S i ei Decrypt with d i, and return Send blinded S i ei and new policy P m P m, S i em Unblind S i Reencrypt with e m

23 Experiments What is the performance overhead of FADE? e.g., metadata, cryptographic operations Performance overhead: Time File transmission time Metadata transmission time Time for cryptographic operations (e.g., AES, HMAC, key exchanges) Space Metadata

24 File Upload/Download Overhead of metadata is less if file size is large Time for cryptographic operations is small File uploadFile download

25 Conclusions FADE, an overlay cloud storage system with access control and assured deletion Cryptographic operations for policy-based file assured deletion Implement a FADE prototype atop Amazon S3 FADE is feasible in practice

Comparison Wang’s approachFADE Supported by existing cloud infrastructureNo, required new protocol supportYes Access control policiesYes Multiple policies combinationNoYes Assured deletionYes ImplementationYes OverheadLow Block updateYesNo Block insertion and appendingYesNo Clients’ responsibilitiesLowHigh

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