Cloud computing is a paradigm for offer information technology services on the Internet, such as hardware, software, networking and also the storage can be accessed anywhere at any time on a pay-per-use basis. However, storing private data onto servers of the cloud is a challenging matter. Therefore, cryptography technique and authentication are used in this model to ensure confidentiality and proper access control of sensitive data. Therefore, in this paper I proposed a model to protect data in cloud computing. In this model the algorithm of the Rivest-Shamir- Adleman (RSA) is applied to the private data. Furthermore, the protocol of Challenge-Handshake -Authentication-Protocol (CHAP) is used to improve the security of the authentication as well. The results show this model is secure and practical.