Abstract: This paper presents various disputes associated with public Auditing and security for storing userís data on untrusted cloud. There is a huge amount of research being made to discover the disputes with these cloud service providers and cloud security. In this paper system proposes a HMAC algorithm to enhanced public integrity auditing Shared Dynamic Cloud Data. Typically message authentication code is used in between sender and receiver that share secrete key in order to authenticate information transmitted between this parties. This standard defines a MAC that uses a cryptographic hash function in conjunction with a secret key in order to authenticate information transmitted between these parties. The main goal of this paper is to proposal a secure and an effective cloud data storage system to decrease the bandwidth and to increase the data integrity. The key is part of HMAC, since it is shared secrete known between two parties only and only they can create .The main intention of these is to overcome the length extension attack and to protect the original performance of the hash function without incurring a significant degradation.

Keywords: Cloud Computing; Hash Message Authentication Code (HMAC); Message Authentication Code (MAC); integrity.