This paper presents a new method, of recompressing a JPEG crypto-compressed image. In this project, we propose a cryptocompression method which allows recompression without any information about the encryption key. The recompression can be executed on the JPEG bitstream directly by removing the last bit of the code representation of a non null DCT coefficient and adapting its Huffman code part. To be able to apply this approach, the quantization table is slightly modified to make up the modifications. This method is efficient to recompress a JPEG cryptocompressed image in terms of ratio compression. Moreover, since the encryption is fully reversible, the decryption of the recompressed image produces an image that has a similar visual quality compared to the original compressed image.
Vincent Itier, William Puech, "How to recompress a JPEG crypto-compressed image?" in Proc. IS&T Int’l. Symp. on Electronic Imaging: Media Watermarking, Security, and Forensics, 2017, pp 36 - 43, https://doi.org/10.2352/ISSN.2470-1173.2017.7.MWSF-323