Cloud computing is a wide architecture based on diverse models for providing different services of software and hardware. Cloud computing paradigm attracts different users because of its several benefits such as high resource elasticity, expense reduction, scalability and simplicity which provide significant preserving in terms of investment and work force. However, the new approaches introduced by the cloud, related to computation outsourcing, distributed resources, multi-tenancy concept, high dynamism of the model, data warehousing and the nontransparent style of cloud increase the security and privacy concerns and makes building and handling trust among cloud service providers and consumers a critical security challenge. This paper proposes a new approach to improve security of data in cloud computing. It suggests a classification model to categorize data before being introduced into a suitable encryption system according to the category. Since data in cloud has not the same sensitivity level, encrypting it with the same algorithms can lead to a lack of security or of resources. By this method we try to optimize the resources consumption and the computation cost while ensuring data confidentiality.