Encryption Method for JPEG Bitstreams for Partially Disclosing Visual Information

Abstract

In this paper, we propose a novel encryption method for JPEG bitstreams in which encrypted data can preserve the JPEG file format with the same size as that without encryption. Accordingly, data encrypted with the method can be decoded without any modification of header information by using a standard JPEG decoder. In addition, the method makes two contributions that conventional methods allowing bitstream-level encryption do not: spatially partial encryption and block-permutation-based encryption. To achieve this, we propose using a code called restart marker for the first time, which can be inserted at regular intervals between minimum coded units (MCUs) for encryption. This allows us to define extended blocks separated by restart markers, so the two contributions are possible with restart markers. In experiments, the effectiveness of the method is verified in terms of file size preservation and the visibility of encrypted images.