quote:
Originally posted by Vilkku
You can add information, like text, to images so that you can't see it viewing the image. I have no idea how you could protect it. 
It isn't about adding comments or stuff like using the author field to add messages, nor is it about pasword protecting files like Lee Jeffery suggested, but it is about actually using unused bits and bytes or using unsignificant bits and bytes to encode messages (like KeyStorm said). For example: bits and bytes wich normaly would be used to contain graphical data.
a very crude example of one possebility:
when you save the pixels of an image the actualy pixel values are saved. If you construct your image (or rather the colors) so that you use only high values, the lower values wouldn't make much of a difference (especially when you use many colors like in 32bit graphics). In other words, you can use the lower bit values to encapsulate text messages.
pixel values (bitwise) for four original pixels: 01100000 11110000 11100000 10010000
Now for the human eye a pixel value of 01100000 will be roughly the same as 01101010. So you can use the lower 4 bits to actually hold text information. and when you combine such two lower fields of 4 bits, you have 1 full byte, in other words 1 character.
Thus when we save our image we can instead of saving
01100000 11110000 11100000 10010000
save
01101001 11110000 11101101 10011001
and thus the lower bit fields will be
1001 0000 and 1101 1001
and they are the ASCII codes for:
10010000 = H
11011001 = i
When viewing this image, you will notice nothing out of the ordinary. But put through the correct decoder, it can extract the plain text message "Hi".
This is one of billions of possible methods you can use. Another group of encoding is by manipulating graphic data so it contains other graphical data (this can of course be text again, but it must be viewed with a graphic editor in that case, so it isn't quite the same as extracting plain text from something, but the technics are the same though).
(and of course you could protect it by any method you choose like coding the text further with a password or something only you and the reciever would know... now isn't this fun for all you paranoid people
I'd suggest you all to investigate all those graphics found on the net for hidden messages
lol)
quote:
Originally posted by KeyStorm
I think this is called steganography.
yup, that's the one
quote:
Originally posted by WDZ
http://en.wikipedia.org/wiki/Steganography
Quite interesting. I've heard about it but never bothered to try it.
That one shows indeed a very nice exmaple of another method of encapsuling graphic data within another graphic.
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