International Journal of Computer Applications (0975 – 8887) Volume 81 – No 18, November 2013 1 Developing Third Party Auditing Scheme for Secure Cloud Storage Service Manasi Doshi Swapnaja Hiray PG student of Department of Associate Professor of Department of Computer Engineering,SCOE, Computer Engineering,SCOE, Vadgaon, Pune-411048 Vadgaon, Pune-411048 ABSTRACT Cloud computing is the use of computing of sources that are delivered as a service over a network. Cloud enables users to store their data, but data is stored at remote location. A major characteristic of the cloud services is that user’s data are usually processed remotely in unknown machines that users do not operate. So, basic need is to provide security to cloud server. To achieve this, perform flexible distributed storage, utilizing the homomorphism token and distributed erasure- coded data. Also allows for strong cloud storage correctness and simultaneously achieves fast data error localization. Keywords Cloud computing, security, distributed storage, error localization 1. INTRODUCTION In cloud computing we can share our data and application at common place. This uses internet and share resources to provide services. Security is important issue because cloud having many benefits so, it have many users. This paper focuses towards security to cloud. It is based on distributed storage on 3 machines. It uses homomorphic token for checking integrity of data. This helps user low cost communication and computational cost. The auditing result ensures strong cloud storage correctness as well as simultaneously achieves fast data error localization, i.e., the identification of misbehaving server. It allows client to perform secure and efficient dynamic operations on outsourced data, including block modification, deletion, and append [1]. 2. RELATED WORK In this section we first review related works addressing security in cloud. Security issue is very important in cloud there are many techniques available so here is review of all these. Data security is the major challenge in the cloud computing as user’s data reside in the servers which are remotely situated and far away from the end-users. These data may include confidential data (financial data, health records), personal information which may be disclosed to competitors or publicly. So security emerges as the highest priority issue [2]. In [3] Third party auditor for verification, they describes three network entities i.e. client which is user, cloud storage server which is handled by cloud service provider and Third party auditor which is verifier.TPA having public key, it is act with only trusted server, they are not focuses on data privacy. In [4] it defines 2 basic schemes. Scheme 1 : User computes the MAC of every file block. Transfers the file blocks & codes to cloud and shares the key with TPA. During the Audit phase, the TPA requests from the cloud server a number of randomly selected blocks and their corresponding MACs to verify the correctness of the data file. Drawback of this scheme is TPA can see cloud data. Scheme 2: In Setup phase, User uses s keys and computes the MAC for blocks and user shares the keys and MACs with TPA. During Audit, TPA gives a key (one of the s keys) to CSP and requests MACs for the blocks. TPA compares with the MACs at the TPA. Improvement from Scheme 1: TPA doesn’t see the data, preserves privacy. Drawback: a key can be used once, Schemes 1 & 2 are good for static data (data doesn’t change at the cloud). In paper [5] they discuss main challenges for achieving cloud computing services, this problem focuses on accountability in cloud computing. Accountability means verification of access control policies. 3. PROPOSED WORK 3.1 System Modules: 1. Client: Client is that entity who is using of cloud services and who has to store data on cloud. Multiple clients can use cloud storage services. 2. TPA: TPA is an optional entity. It has expertise and capability to expose dummy client. E.g. authentication of client. 3. CSP: CSP is an entity which provides cloud services. E.g. client want to upload file then CSP give call to CS. 4. CS: CS is an entity which allow client to perform operation on data stored on itself. 5. Main Backup server: It is an entity which stores complete file. 3.2 System Architecture Figure 1. Proposed cloud storage service architecture (using 4 machines) Here client, CSP, TPA and main backup server working on single machine. For storing client’s data i.e. input file is