With an unprecedented pace of developments in Cloud computing technology, there has been an exponential increase of users of these services and an equal rise of cloud services providers. Clouding Computing is a virtual pool of resources provided to users as service through a web interface. These resources may include Software, Infrastructure, Storage, Network, Platform etc. With more and more organizations migrating their data over cloud, it is imperative to ensure security and integrity of their data. In this paper we 1) discuss the security challenges posed to data on the cloud computing. 2) Survey cryptographic algorithms that can be used to overcome these challenges. 3) Survey Security designs of 5 leading cloud service providers. 4) Perform a comparative study of security and features of these providers.