Encrypting JPEG-compressed Images by Substituting Huffman Code Words

This paper presents a method for encrypting JPEG-coded images that preserves both compression ratio and format of a bit stream. Such solutions allow for selectively hiding information: image contents can be encrypted, while in-file meta-data remain readable. Our algorithm is a symmetric, polygram substitution cipher, as it replaces Huffman code words and rearranges value bits that describe the main results of the Discrete Cosine Transform (DCT) of a pixel block: the DC coefficient and the first non-zero AC coefficient. Both length and format of a file are preserved, because bits are modified under constraints on their numbers. Such encryption is a kind of post-processing of a compressed bit stream, and thus it can be built on the top of an existing JPEG codec, without accessing its internals. Compared to previous similar solutions, our approach better hides image contours, exchanging AC for DC energy. Our work also reveals some properties of Huffman code tables and bit streams related to the JPEG standard.