I'm trying to find a secret message, a string, in a 256x256 png image. It's supposed to have "used an old school trick to hide the data", and apparently that method is mentioned in the steganography Wikipedia article.
I tried what appeared to me as most oldschool an straightforward first: LSB steganography. But no luck. I know the first and last characters of the string ("F" and "}"), and I thought they may have mixed the common lsb method up a bit, so I inspected the very first pixels and the very last pixels of the picture myself. However, no apparent combination (like only red values of each pixel) would allow for the correct character. Hence I'm pretty positive it's not using lsb.
In a second, rather desperate try I saw that Wikipedia talks about stripping the most significant six bits, leaving only the least significant two, and then normalizing the picture. I wrote a little script to do this, but no luck here either.
I also looked at the metadata with identify -verbose image.png. Nothing. The file ends as it should after the IEND chunk, so nothing hidden beyond that either.
I'm running out of ideas, so here my question:
Any hints what might classify as old school trick, that I haven't already tried? I'm sure I missed something obvious. This exercise came with a few others, and they all looked harder at first glance than they really were.
Thanks a lot. :)
It turned out that there was a chunk in the middle of the picture with a long text, which contained the wanted string, hidden in the least bits of the blue values only, in least bit first order. Somehow I missed that combination in my preliminary tests. So there you go. :)
To anybody having a similar problem: I find it's best to write a script to test all more commonsense variations (like only single colors, vertical, least-bit or greatest-bit first, etc.) in one large run. It's too easy to miss a simple one otherwise and get hopelessly stuck in crazy complicated theories.