Proactive visual cryptographic schemes for general access structures

In the modern digital era, long-term protection of digital data, stored in a server, is essential, especially when it is in the format of medical images like electronic health records, MRI scans, etc. Visual cryptography is an efficient method to protect secret image data by encoding it into shares and keeping in different servers. However, if the shares, stored in several servers, remain unaltered for a long time, an adversary may capture the shares one by one and in the end, break the confidentiality of the secret data. In the current paper, we propose, to the best of our knowledge, the first proactive visual cryptographic scheme for general access structures. In our scheme, the shares are embedded within meaningful cover images and are stored in different servers. The main advantage of our scheme is that the meaningful share images are updated periodically without changing the original secret image. The updating/renewal procedure makes the previous shares statistically independent from the current shares. The secret image can be reconstructed only by the lastly updated share images. This technique prevents the aforementioned attack. The renewal procedure can be performed an unlimited number of times still keeping the quality of the reconstructed image unchanged. The mathematical analyses along with the experimental results exhibit the practicality of our proposed scheme.