CODEWITHSUNDEEP
matlabimage-processingbit-manipulationsteganography
Chaitali
Chaitali

Reputation: 240

Bit operators issues and Steganography in image processing

Steganography link shows a demonstration of steganography. My question is when the number of bits to be replaced, n =1, then the method is irreversible i.e the Cover is not equal to Stego (in ideal and perfect cases the Cover used should be identical to the Steganography result). It only works perfectly when the number of bits to be replaced is n=4,5,6!! When n=7, the Stego image becomes noisy and different from the Cover used and the result does not become inconspicuous. So, it is evident that there has been an operation of steganography. Can somebody please explain why that is so and what needs to be done so as to make the process reversible and lossless.

Upvotes: 0

Views: 730

Answers (1)

Reti43
Reti43

Reputation: 9796

So let's see what the code does. From the hidden image you extract the n most significant bits (MSB) and hide them in the n least significant bits (LSB) in the cover image. There are two points to notice about this, which answer your questions.

  • The more bits you change in your cover image, the more distorted your stego image will look like.
  • The more information you use from the hidden image, the closer the reconstructed image will look to the original one. The following link (reference) shows you the amount of information of an image from the most to the least significant bit.

If you want to visually check the difference between the cover and stego images, you can use the Peak Signal-to-Noise-Ratio (PSNR) equation. It is said the human eye can't distinguish differences for PSNR > 30 dB. Personally, I wouldn't go for anything less than 40 but it depends on what your aim is. Be aware that this is not an end-all, be-all type of measurement. The quality of your algorithm depends on many factors.

No cover and stego images are supposed to be the same. The idea is to minimise the differences so to resist detection and there are many compromises to achieve that, such as the size of the message you are willing to hide.

Perfect retrieval of a secret image requires hiding all the bits of all the pixels, which means you can only hide a secret 1/8th of the cover image size. Note though that this is worst case scenario, which doesn't consider encryption, compression or other techniques. That's the idea but I won't provide a code snippet based on the above because it is very inflexible.

Now, there are cases where you want the retrieval to be lossless, either because the data are encrypted or of sensitive nature. In other cases an approximate retrieval will do the job. For example, if you were to encode only the 4 MSB of an image, someone extracting the secret would still get a good idea of what it initially looked like. If you still want a lossless method but not the one just suggested, you need to use a different algorithm. The choice of the algorithm depends on various characteristics you want it to have, including but not restricted to:

  • robustness (how resistant the hidden information is to image editing)
  • imperceptibility (how hard it is for a stranger to know the existence of a secret, but not necessarily the secret itself, e.g. chi-square attack)
  • type of cover medium (e.g., specific image file type)
  • type of secret message (e.g., image, text)
  • size of secret

Upvotes: 1

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