Question

Can somebody help me, to find a library, or a detailed description of algorithm, that could embed a Digital watermark(invisible watermark, just a kind of steganography) to a jpeg/png file. But the quality of algorithm, should be great. It should be possible to extract this mark after rotation and expansion(if possible) of image.

Mark is just a key 32bytes.

I found a good site, but the algorithm are made for the NetPBM format, that is dead…

I know that there is a LSB method, but it is not stable to the expansion. Are there something better?

Changing metadata, is not suitable, because it is visible changes.

Answer 1

This maybe won’t really be an answer, as I don’t think it would be easy to give a magical, precise answer on this question.Watermarking is complex, and the best way to do it is by yourself : this will make things more hard for an attacker trying to reverse engineer your code. One could even read your question here, guess what library you used, and attack your system more easily.

Making Steganography resist to expansion in JPEG images is also very hard, because the JPEG compression is reapplied after the expansion. There are in fact a bunch of JPEG steganography algorithms. Which one you should use, depends on what exactly do you require :

• Data confidentiality ?
• Message presence confidentiality ?
• Message coherence after JPEG changes ?
• Resistance to “Known Cover” attacks (when attackers try to find the message, based on the steganographic system) ?
• Resistance to “Known Message” attacks (when attackers try to find the steganographic system used, based on the message) ?

From what I know, usually, algorithm that resist to JPEG changes (picture recompression) are often really easier to attack, whereas algorithms that run the “encode” stage during the JPEG compression (after the DCT (lossy) transform, and before the Huffmann (non-lossy) transform) are more prone to resist.

Also, one key factor about steganography is scale : if you have only 32bytes of data to encode in a, say, 256*256px image, don’t use an algo that can encode 512bytes of data in the same size. Either use a scalable algorithm, either use an algorithm at its efficient scale.

Also, the best way to do good steganography is to know its limitations,and to know how steganalyzers work. Try these tools, so you can understand what attackers will do to your picture.^

Now, I cannot tell you what steganographic system will be the best for you, but I can give you some indications :

• jSteg – Quite old, I don’t think it will resist to JPEG changes
• OutGuess – Quite old too, but one of the best algorithms
• F5 (and F3/F4) – More recent, good algorithm, scientifical research behind.
• stegHide

I think all of these are LSB based : the encoding is done during the JPEG compression, after the DCT and Quantization. The only non LSB-based steganography system I heard of was mentionned in this research paper, however, I did not read it to the end yet, so I cannot tell if this will meet your needs.

However, I’m not sure there exists a real steganography algorithm resisting to JPEG compression, to JPEG resize and rotation, resisting to visual and statisticals attacks. Or I’m not aware of it.

Sorry for the lack of precise answer, I tried to give you what I know on the subject, as it’s always better to be more informed. Sorry also for the lack of proper English, I’m French, nobody’s perfect 🙂