**1. Introduction**

The digital image is a technology that contains secret information. In today's highly networked world, web servers, application servers, and database backend all connect through public or shared digital networks. In this environment, image is vulnerable to potentially more destructive attacks such as replay or human-based attacks, bruteforce, statistical attack, etc. Cryptography is the ideal solution to protect numerical images in storing and moving. It provides a deep defense enough against last record. Evidently, security surrounds us daily at a personal level. Cryptography solutions should guarantee the confidentiality, integrity, and authentication of the secret data. The confidentiality is enabled by encryption that transforms secret data from readable forms to unintelligible forms. Encryption provides a good defense against the new generation of attacks. Even if systems are compromised and the information is taken, encryption can keep it unusable. Integrity is that any change in the transmitted data

can be detected at the destination. However, authenticity is that the receiver can verify the identity of the sender. Among encryption schemes, hybrid scheme is considered the ideal idea to protect numerical images that permits both confidentiality and authenticity. Symmetric scheme is effective for large volume data encryption because it is generally hundreds to thousands of execution times faster than asymmetric scheme, but it suffers from secret key distribution. On the other hand, the asymmetric scheme is more secure than the symmetric scheme since it uses different pair keys for encryption than for decryption. However, the hybrid scheme is an innovative idea that combines the symmetric approach for large volume data encryption and the asymmetric approach for secret key exchanging and authentication.

In this paper, we make the following contributions:


The rest of the paper presents the following sections: A survey of existing works is given in Section 2. In Section 3, a preliminary study of Keccak, AES, RSA, and counter encryption mode (CTR) are respectively described. The proposed methodology to create the overall algorithm is detailed in Section 4. Section 5 presents the evaluation and the security analysis of the technique put forward. Finally, the last section gives conclusions and related works.
