Now-a-days Cloud Computing has become more popular in all Information Technology areas. Cloud computing provides a number of services like Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as a Service (SaaS). All these cloud computing models share common resources with different customers to provide services. To provide services to the customers, cloud computing uses a technique called Multitenancy. When cloud uses Multitenancy to provide services, there are so many data and security problems. Cloud security is becoming a major issue in adopting the cloud, where security is considered as one of the major critical concerns for the large customers of cloud. Multitenancy is a technique of sharing resources of cloud commonly across different customers. In SaaS applications, same database is shared by different users. If a common database is shared among different customers, there are chances for so many data related security issues like data loss or leakage, Account or session hijacking, etc., to occur. In this paper, the author has proposed solutions for the problems that occur in cloud computing models with Multitenancy by implementing secure key mechanism using Software Defined Networks (SDN) in the cloud computing service model.