We provide an encryption protocol for storing highly confidential data of a user on a public cloud storage. We show that the protocol provides unconditional security. More specifically, we prove that the protocol is semantically secure against an all-powerful adversary with unbounded computational power and storage capacity who has complete access to the communication line. The provided protocol is very simple, it is implemented by exploiting finite group modular addition and XOR operations. The protocol provides a high security gain defined as the ratio of the amount of data (in bits) that can be securely stored on the public cloud and the number of secret bits required to be stored at the user end. As an illustrative example, according to the protocol, by using 40.2 Terabytes (TBs) of secret bits, a user is able to store 5.9 ×10⁸TBs of data on a public storage with guaranteed everlasting security.