Half a croissant, on a plate, with a sign in front of it saying '50c'
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Crossword steganography

 Hide information in the grid
 
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Steganography is the art of hiding information where people don't think to look for it.

Data could be hidden in the position of the black squares in a crossword.
If you were the editor of a paper in a hostile regime, you could broadcast small amounts of information surreptitiously. Or if you were a hostile regime, you could tell your spies to do things - all they'd need would to buy a paper, which isn't a suspicious activity.

—Loris, Jul 28 2006

Crossword Steganography http://www.telegrap...5/03/ixnewstop.html
[DrCurry, Jul 28 2006]

[link]






       You'd still be able to code more information in the lonely-hearts adverts of the newspaper, something that has been widely documented in books and films from at least the last 60 years, and is still used today occasionally when communicating with anonymous persons wanted by the police.
—zen_tom, Jul 28 2006
   

       Yes, but then your agents would need a codebook. Doh - they'd need a program to decrypt the crossword too.
Most papers don't run personal ads every day, so it could still have a speed advantage.		   

       Hmmm. Determined counter-intelligence officers could phone every personal ad, and see which numbers didn't get through. Then they'd know to look out for spies. So the other side would have to run ads when they didn't have spies active. Good news for papers, bad news for anyone wanting to buy a cheap second-hand freezer or trying to meet up with a young single GSOH who likes going out, films and playing the Hammond-organ.
—Loris, Jul 28 2006
   

       information has also been hidden into the half tone dots of photographs. Bar codes are also a form of encryption, that do not need a code book.
—xenzag, Jul 28 2006
   

       There was quite a fuss during the Second World War when a number of the code words related to the upcoming D-Day invasion appeared as answers in the Telegraph crossword.   

       But if you are going to use a newspaper for steganography, you have oceans of newsprint to hide your message in - why limit it to the crossword?
—DrCurry, Jul 28 2006
   

       I don't know if this is widely known to exist; I doubt I'm the first to think of it - encode information as subtle differences in colour of individual pixels in apparently identical photos. Changing the colour to the next value is unlikely to be visible to the naked eye, would be lost amongst the adjacent pixels, yet would be brought out immediately when one image was subtracted from the other. Then all you have to do is email the two versions of the otherwise innocuous image to two separate recipients, and have them compare and decode. Naturally you wouldn't be hiding plain text, but digital data, so another level of encoding would be possible.
—david_scothern, Jul 28 2006
   

       [Determined counter-intelligence officers could phone every personal ad, and see which numbers didn't get through. Then they'd know to look out for spies]   

       This sounds like the logic our current regime is applying. [Loris] is correct about running a background noise level in order to make it more difficult to detect signals containing data.
—normzone, Jul 28 2006
   

       ds: widely known. From the early days of JPGs, it's been used to indelibly encode copyright information, and it's been identified as a possible mechanism for malefactors to encode secret messages in publicly distributed files on the Internet.
—DrCurry, Jul 28 2006
   
      
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