Month: July 2007

Image Encryption

Problem The trouble with encrypting documents is that its quite obvious that they have been encrypted which makes people want to decrypt them. So what about some kind of sexy algorithm that encodes a message within a picture? If the picture only gets slightly changed, then no one will ever realise it contains a secret message, since it looks to all intents and purposes like its a genuine picture. Analysis So, an evenings hacking later and I've come up with an algorithm that does it. The program takes an image file (gif, jpeg, etc) and outputs a bitmap (it cannot be output to a format that compresses with loss like JPEG, otherwise its not possible to decrypt the message since what gets saved is saved with bits lost). Here is an example of a red image that has been modified to contain a message: OK, if you look close (see below which is zoomed in) you can see the modified dots. But its just an example. So here is a photo that has been modified to contain a different secret message: Again, without zooming in, its hard to see the changes. Below is a zoomed in version. You can see the changes as speckled dots. The algorithm has been tuned to try and match the changes to the pixels around the changed pixel. If the surrounding area is blue, the change will be blue, whereas if the area around the change is red, the result will be a red pixel.…

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