Cloud computing (CC) environments offer cost-efficient and flexible resources, appealing to users despite concerns about the reliability of cloud service providers (CSPs) and data privacy. To address these concerns, encrypting data before outsourcing to the CC environment is essential. However, encryption introduces challenges such as data leakage through side-channels in virtual machines (VMs). To address these issues, a Feistel cipher symmetric encryption with multiclass vector (FCSE-MV) side-channel attack detection method was developed, leveraging blockchain technology in CC. Initially, an aggregated Byzantine fault tolerance-based block generation model was employed for efficient block production. Subsequently, the presence or absence of side-channel attacks was determined using an FCSE-MV-based block validation model. Experiments conducted with the SCAAML database in JAVA demonstrated that FCSE-MV improved accuracy and throughput by 17.5% and 19%, respectively, and reduced communication complexity and attack detection time by 24% and 21%, compared to traditional attack detection methods. The proposed FCSE-MV method offers a secure and efficient solution suitable for CC environments.