Image Steganography

Question

I'm working on Steganography application. I need to hide a message inside an image file and secure it with a password, with not much difference in the file size. I am using Least Significant Bit algorithm and could do it successfully with BMP files but it does not work with JPEG, PNG or TIFF files. Does this algorithm work with these files at all? Is there a better way to achieve this? Thanks.

Answer 1

This heavily depends on the way the particular image format works. You'll need to dive into the internals of the format you want to use.

For JPEG, you could fiddle with the last bits of the DCT coefficients for each block.

For palette-based files (GIFs, and some PNGs), you could add extra colours to the palette that look identical to the existing ones, and encode information based on which one you use.

Answer 2

You'll have to distinguish between pixel-based (Bitmap) and palette-based formats (GIF) for which the steganographic technique is quite different. Also be aware that there are image formats like JPG that lose information in the compression process.

I'd also advice some general introduction to steganography including different formats.

Answer 3

Least Significant Bit approach does not work with JPEG and GIF images because you are using the pixel data (raw image) to store hidden information before compression. A pixel p, with data 0x123456 will probably not have this value after compression because its value depends on the compression rate and neighbour pixels. In this case we are talking about algorithms that does not only compact the image (like a ZIP, that keeps the content), but changes the color distribution, texture, and quality in order to decrease the number of bits to represent it.

However, PNG can be used just to compact the image in the same sense of ZIP file, keeping the content. Therefore, you can use the Least Significant Bit for PNG images, so that Wikipedia Steganography page shows example in this format.