With progress in science and technology and the rapid development of the Internet of Things (IoT) which are connected with the cloud-based network, need to communicate between themselves. In addition, a user needs several types of authentication process so that the user can interact with the devices. However, these devices can be under attack, such as a single-point-failure at any time, and some private data can be tampered and stolen by the attackers. In this paper, we propose a blockchain-based approach that assigns a unique device ID and using smart contracts to store the data transaction records in the blockchain for each IoT device as well as cloud and user access. Blockchain-based smart contracts provide distributed digital records that can endure data tampering and data snitching attacks. This method secures the user authenticity and maintains the privacy of IoT data on the cloud using Merkle Tree data structure and Practical Byzantine Fault Tolerance (pBFT). Finally, we implement an open-source blockchain platform that verifies the prototype of the proposed system.