Recompression of JPEG crypto-compressed images without a key

International audience; The rising popularity of social networks and cloud computing has greatly increased number of JPEG compressed image exchanges. In this context, the security of the transmission channel and/or the cloud storage can be susceptible to privacy leaks. Selective encryption is an efficient tool to mask image content and to protect confidentiality while remaining format-compliant. However, image processing in the encrypted domain is not a trivial task. In this work, we present a JPEG crypto-compression method which allows us to recompress a JPEG crypto-compressed image several times, without any information about the secret key or the original image content. Indeed, using the proposed method in this paper, each recompression can be done directly on the JPEG bitstream by removing the last bit of the code representation of each non-zero coefficient, adapting the entropic code part, and slightly modifying the quantization table. This method is efficient to recompress JPEG crypto-compressed images in terms of ratio compression. Moreover, the decryption of the recompressed image produces an image with a very similar visual quality when compared to the original image, according to the obtained results.