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I recently came across an odd JPEG file: Resolution 400x600 and a filesize of 2.9 MB. I got suspicious and suspected that there is some additional information hidden. I tried some straight forward things: open the file with some archive tools; tried to read its content with an editor, but I couldn't locate anything interresting.

Now my questions: What else can I do? Are there any tools available that analyze images for hidden data? Perhaps a tool that scans for known file headers?

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I agree that is strange, but keep in mind it might have been encoded with very lax JPEG settings. – Konrad Feb 14 '11 at 15:59
Good question - thanks. Hope you don't mind that I tried to give it a more descriptive title. – nealmcb Feb 14 '11 at 17:34
Not an answer to the asked question, but to the actual situation you had: It might not be hidden data, but hidden binary code, e.g. GIFAR. See also http://security.stackexchange.com/q/600/33. – AviD♦ Feb 14 '11 at 19:15
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@Konrad, I doubt it. Even at three bits per pixel (24-bit color), a basic bitmap would be only approx 720,000 bytes. (400*600*3). I'd bet a trip to the Chinese buffet that there's something there not related to the obvious image. @Chris: Please post your findings, or even the file if you'll part with it. – pboin Feb 15 '11 at 20:57

5 Answers

up vote 17 down vote accepted

To detect Steganography it really comes down to statistical analysis (not a subject I know very well).
But here are a few pages that may help you out.

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A small matter of semantics, here: Steganography on its own isn't encryption, it's obfuscation. While the hidden data may in fact be encrypted, it is not a necessary part of steganography for it to be so. – Iszi Feb 14 '11 at 16:12
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You are right Iszi I'll correct that. – Mark Davidson Feb 14 '11 at 16:13
Another vote for Stegdetect here. It works really well. – Rory Alsop♦ Feb 14 '11 at 17:54
up vote 0 down vote

You must try to read its binary using software like Winhex , as they might provide you some clue about some other file extension appended to your file.

Also open it with Notepad in window and vi editor in linux as they also provide certain information about file extensions. If nothing is detected go for some most used stego detecting tools available on internet.

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This doesn't really add anything to the existing answers... – Terry Chia Mar 8 at 7:35
up vote 5 down vote

It's probably just a file appended to the end of the JPEG. Look for the EOF or the start of a known header such as PK RAR PE etc

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up vote 6 down vote

There's some great general references in the other answers here, so I'll just give some input specific to your situation:

When hiding data in pictures without changing the file size, you put it in the low-order bits; this can be detected by opening in an editor with a histogram and looking for jagged edges. But this sounds like a concantenation of a file to the image; *chan denizens often use this technique for distributing illicit files. Looking for file signatures after the first one--say, using 'grep -a' with a list of known filetype magic numbers--should reveal this technique. The combination of encryption and steganography is beyond the scope of this comment :D

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up vote 8 down vote

I'll second the reccomendation for Stegdetect: here's another good source for information http://www.outguess.org/detection.php as well as downloads for stegbreak and XSteg

You can go right to the source for the research on this if you're interested; Neil Provos's page is here http://www.citi.umich.edu/u/provos/stego/

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