Cloud storage systems have become the primary storage space for cloud users’ data. Despite the huge advantages and flexibility of the cloud storage services, many challenges are hindering the migration of users’ data into the cloud. Among them, the data privacy needs to be considered. In this paper, we design and implement an encryption service namely ESPRESSO (Encryption as a Service for Cloud Storage Systems) to protect the users’ data by using advanced encryption algorithms. The flexible design and the standalone property of ESPRESSO allow cloud storage service providers to easily integrate it without heavy modification and implementation of their infrastructures. ESPRESSO was integrated into two open-source cloud storage platforms: OpenStack/Swift and Nimbus/Cumulus. The real experiments were conducted, and the results assess the performance and effectiveness of ESPRESSO.