Companies are increasingly using cloud storage, where an estimated 60% of enterprise data is stored. This makes cloud data security crucial for preventing data privacy breaches as they can have a serious impact on individuals and businesses. Commonly used techniques for ensuring data security during storage and transmission are encryption, hashing, and tokenization. Methods currently being used for securing cloud data, which involve use of single or multiple encryption keys, have limitations in terms of security and efficiency. In shared-tenancy cloud environments, secure data sharing becomes challenging due to its reliance on server-enforced access control. This research aimed at developing a Hierarchical Key-Aggregate Cryptosystem (HKAC) with homomorphic encryption for scalable and secure cloud data sharing. The proposed system combines hierarchical key structures with key aggregation allowing users to share multiple files with a single key, reducing key management overhead. The system allows users to encrypt multiple files with unique keys and generate a single aggregate key for decrypting a selected subset of encrypted data. The Hierarchical Key-Aggregate Cryptosystem (HKAC) further enhanced this system by structuring data hierarchically and allowing decryption of all data within a specific subtree using a single aggregate key. Additionally, homomorphic encryption employed in the system enables computations on encrypted data without revealing the original data, enhancing privacy. Homomorphic encryption was also integrated, enabling computations on encrypted data without decryption, increasing privacy. RSA and Paillier encryption were used to support multiplicative and additive homomorphism, respectively. The web application was built using Flask, and frontend web technologies like HTML, CSS and JavaScript to allow users to handle encryption keys, aggregate keys, and perform homomorphic operations. The system’s performance was evaluated by assessing encryption/decryption time, key aggregation time, and computational cost. Multiplicative and additive homomorphism implemented using RSA and Paillier encryption, allowed secure operations on encrypted numbers, lists of numbers, and strings. Decryption of ciphertext consistently recovered accurate plaintext showcasing the reliability of RSA and Paillier encryption schemes. The use of aggregate keys in a tree-based hierarchical structure effectively reduced key management overhead by enabling the decryption of entire data sub-branches with a single aggregate key. The Flask-based web application provided an intuitive interface for encrypting, decrypting, and performing computations on encrypted data, demonstrating the system’s practicality for real-world applications. The approach proved to be scalable and efficient for secure cloud-based data management. Potential future improvements include optimizing the system for homomorphic operations on multimedia data, and exploring distributed computing for improving efficiency.