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I always been fascinated by steganography. I understand that its purpose alone is to provide security by obscurity. However, combining steganography with encryption can actually make it desirable overall.

In my favorite TV show, Mr. Robot, the main character uses a program called "Deepsound". I see that it has an option to encrypt the secret files which sounds promising. Can someone explain to me how this works? For example the steps it takes to provide it's purpose and the pros and cons of it.

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    Mr Robot is not a great place to get your security information. With that said, all the encryption does is, well, encrypt the data. In part that's to make it appear as random noise which allows it to hide better and blend in with, say, white noise from an audio recording. – forest May 23 '18 at 1:17
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Steganography is any method of hiding a message inside something that looks innocent. It has different goals to encryption, in that encryption stops a party reading the message even if they know it's there, whereas steganography hides the message so if you don't know it's there, you won't discover it. That's why the two can be used together usefully: to encrypt the message and then hide it, so that somebody not knowing the existence of the message won't discover it, but if they do, then they won't be able to read it.

Steganography should be able to hide the existence of the message with no tell-tale signs that a message is hidden in it. In an image or audio file its effect on the image or the audio must be imperceptible to human ears, so it's not good enough if it creates a "weird noise in the background" or something to that effect. Ideally it should even not indicate its presence under regular scrutiny such as zooming in, or analysis with a scope.

Typically it superimposes a noise pattern over the signal in such a way that it's not going to be seen or heard by humans but it can be detected by a digital examination of the file itself, by ensuring that its noise pattern is outside the range that humans will pick up on or is sufficiently masked by actual signal.

Note: steganography doesn't have to be this sophisticated. Some files allow arbitrary data to be hidden in the file format somewhere that it won't be read by software that reads that file format, by placing it into redundant space that is not referenced by the header of the document. This is notably possible with ZIP files where you can have innocent looking compressed files in the archive itself but have other data in the file that will never be accessed by a ZIP reader. The side effect of this type of steganography is that the increase in file size may arouse suspicion, whereas this may not be the case in an uncompressed audio signal where adding extra information will not increase file size.

  • To add a stone to the wall, you can hide your data bytes in the least significant bit of earch pixel of an image. This will not change the size and won't add noize. – Guillaume Beauvois May 23 '18 at 12:09
  • This is the case if it's an uncompressed, full RGB (eg 24 bit color, or 8 bit grayscale) image. For (lossless) compressed images it will increase size, and for palette based images it will significantly harm the image. For lossy compressed images the added data won't be preserved. – thomasrutter May 24 '18 at 0:52
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    Regarding your last paragraph, one technique me and a group of friends used to use was appending audio data to the end of images and upload them to imageboards so we could listen to music while browsing. The nice thing about using images is that they're widely supported, and appending data to the end of them does not mangle the image data at all. – forest May 24 '18 at 1:07
  • Yes it is very common in file formats that you can append arbitrary data at the end. ZIP (and zip based formats like docx, jar etc) are a notable exception as they are read starting from the end, but then you can craft the ZIP to leave unallocated parts earlier on into which you can put arbitrary data, such as at the start of the file - meaning something can be a valid ZIP file and another file type, say .GIF, at the same time. – thomasrutter May 24 '18 at 3:19

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