Data hiding in homomorphically encrypted medical images for verifying their reliability in both encrypted and spatial domains

In this paper, we propose a new scheme of data hiding of encrypted images for the purpose of verifying the reliability of an image into both encrypted and spatial domains. This scheme couples the Quantization Index Modulation (QIM) and the Paillier cryptosystem. It relies on the insertion into the image, before its encryption, of a predefined watermark, a “pre-watermark”. Message insertion (resp. extraction) is conducted into (resp. from) the encrypted image using a modified version of QIM. It is the impact of this insertion process onto the “pre-watermark” that gives access to the message in the spatial domain, i.e. after the image has been decrypted. With our scheme, encryption/decryption processes are completely independent from message embedding/extraction. One does not need to know the encryption/decryption key for hiding a message into the encrypted image. Experiments conducted on ultrasound medical images show that the image distortion is very low while offering a high capacity that can support different watermarking based security objectives.