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Just a quick question about DCT Steganography. Do you embed the data before or after you send the 8*8 block through the quantization table?

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The quantization table is part of the compression or decompression stage in JPEG, but for using steganography you should not compress/decompress the image, because these operations modify the DCT coefficients.

The usual process to hide a message in JPEG files is:

  1. Read DCT coefficients from the JPEG file.
  2. Modify the DCT coefficients to hide the message.
  3. Write the DCT coefficients into the JPEG file.

So, the compression needs to be done before you hide the message. And if you decompress and compress the image again you could destroy the message.

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