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secret msg

Started by plessing May 3, 2004 at 1:47 AM 10 replies 950+ views

Original Post

May 3, 2004 at 1:47 AM

hi I need to make a small image editor for class. In it I''d like to add the ability to hide a secret msg within an image. The user must specify a password when embedding the msg and that password is required to extract it again. The msg is hidden by changing the values of the pixels slighly in a certain way. I was hoping someone could explain to me a couple of used practices for achieving this. Oh, you can assume that all images will be 32bpp. Im not sure if I''m allowed to make use of the 8 alpha bits but at this point any solution will do. Thanks in advance, pl.

TheOnlyToASter

May 3, 2004 at 2:39 AM

You dont have to change the pixels. A friend of mine used plain BMP''s, because per pixel there is an unused byte, but i think only in >16bpp

Sneftel

296

Sneftel

May 3, 2004 at 2:48 AM

What you''re talking about is called "steganography". Google for it; you will find a treasure trove of resources.

"Sneftel is correct, if rather vulgar." --Flarelocke

frostburn

May 3, 2004 at 2:58 AM

here''s a simple idea: use the least significant bit of every pixel/color (R7C1G7C1B7C1) to encode it. Each byte takes 2.66 pixels (or 3 bytes in 8 pixels). The slight variation in color will be impossible to notice. This assumes 24 bit.. If you can use alpha/ 32bit you have much more "space".

Sneftel

296

Sneftel

May 3, 2004 at 3:27 AM

quote:
Original post by Anonymous Poster
Sounds like a homework problem to me. You want people to do your homework for you?

He wasn''t asking for code, only suggestions for high-level techniques. No problem there.

"Sneftel is correct, if rather vulgar." --Flarelocke

plessing

Author

plessing

May 3, 2004 at 5:35 AM

Sneftel : thanks, I did found a whole bunch of resources

now just have to read through them all.

AP : yip, it is a homework assignment and it''d b great if some1 could do it 4 me! Altough, this seems like a fun one to figure out by myself (with the help of google, ofcourse). I have a bunch of other homework assignments that I DONT want to do though...

olrite, thanks guys,
p.

mightyjebus

May 3, 2004 at 8:07 AM

there is a Geek Challenge on Stegonography at http://www.osix.net/ great fun if you have some spare time and think you are a geek

[edited by - mightyjebus on May 3, 2004 9:08:01 AM]

ngill

May 3, 2004 at 9:51 AM

hmm, does lossy compression (jpeg) basically make stuff like this useless?

frostburn

May 3, 2004 at 11:27 AM

I think it''s theoreically possible to make stenography with jpegs as well, at least if you write the jpeg saving and loading code yourself, but if anyone modifies the image you''ll have a problem (also true with all other formats). I think the bits per pixel ratio will be lower as well (you can''t store as much extra information as with say bmp)

technobot

May 3, 2004 at 12:17 PM

I don''t know how jpeg compression works exactly, but with lossy compression that is using some sort of dictionary (e.g. vector quantization based techniques, assuming I got the name right), you can modify the dictionary instead of the pixels, similar to frostburn''s suggestion.

Michael K.,
Co-designer and Graphics Programmer of "The Keepers"

We come in peace... surrender or die!

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