The book is about Cloud data security. This book mainly focuses on various homomorphic encryption techniques which are used to provide security of data in cloud. The work is carried in Partial homomorphic encryption techniques using Modified RSA algorithm .In the past decade, the most promising innovation in the computational world is cloud computing. Related with critical data, its usage still faces the challenges of security. To bridge this gap, the most widely used technique is encryption of remotely stored data. The security of data in the cloud is still at big risk due to vast usage of data storage. Computing environments keep on changing. Everyone uses forms of computing devices of different types to access the information that is stored in large servers in cloud. This paradigm shift reflects the fundamental challenge of outsourcing computation, which motivates the power of available asymmetry computing.Homomorphic Encryption is a good idea which enhances the security measures of unreliable systems that stores the information and is venerable to manipulate sensitive data available on cloud.