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Recently I learned of the steghide application (a command-line tool allowing the use of steganography, the art of hiding information inside other information, used in practice with JGP and WAV files primarily with gzip compression and AES encryption).

Excitedly, I hid sensitive data in my desktop background, deleting the original desktop background in it's place.

After more research, I realized that the default algorithm used (128-bit AES) is considered weak (for my standards, anyway. It's nice to be ahead of the game, using 4096-bit RSA and 256-bit AES for things).

Is there a way to cleanse this JPG file of the irrelevant bits that the embedded file is stored in?

I'm not entirely sure how the use of the passphrase (aside from encryption) is implemented to hide the file in a recoverable place using steghide, but I would assume that if you know the passphrase, you can find the exact bits that the file is located in. Is there a way to locate these bits and overwrite them in the same manner you would use to shred a file, but keeping the original image the same (to the human eye)? Or am I boned?

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  • $\begingroup$ And yes, I'm well aware of the overkill that is 256-bit symmetric encryption. I just don't like that the cryptography community has to wait until something is deemed insecure before we switch to more advanced techniques (like switching from 1024-bit RSA to 2048-bit RSA, meanwhile we could simply be using 4096-bit RSA and be much safer for more time to come with very little performance tradeoff) $\endgroup$ – TheFuzzyFish Oct 2 '16 at 1:23
  • $\begingroup$ I don't know but I conjecture that the stego embedding process involved is not reversible, i.e. one can't exactly recover the original file. If you don't think so, a practical way for you would be to carefully study the algorithm used, if that's publically available in details, in order to find means of reversal. Or else simply question the implementor of the scheme. $\endgroup$ – Mok-Kong Shen Oct 2 '16 at 11:20
  • $\begingroup$ Steghiding sensitive information in the desktop background image is probably not the best idea because your data might disseminate all over your system (screenshots, swap files/partitions) and beyond that through electromagnetic emissions that can easily be detected by a technique called Van Eck phreaking. However, this question seems to be more about information security than cryptography. $\endgroup$ – aventurin Oct 3 '16 at 20:13
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As you stated steganography hide data in less important piece of information of a source (because the goal is hide data within other data affecting them less as possible).

In no way pure steganography protect the message hidden, it's only a tool for hiding.

Image you would hide message "AB" in a grey scaled bitmap 4x4 (0-255 value per pixel).

Eg :

Convert message in bit :
"AB" > hex -> 65 66 -> 01000001 01000010

Do the same thingh with your image (0 is black 255 is white, grey between) :
 ____
|####|    0   0   0   0       00000000 00000000 00000000 00000000
| #  |--> 255 0   255 255 --> 11111111 00000000 11111111 11111111
| #  |    255 0   255 255     11111111 00000000 11111111 11111111
|__#_|    255 255 0   255     11111111 11111111 00000000 11111111

Foreach bit of your messaage, sobstitute the less significant bit of the
corresponding byte of the picture so :

Message  Original Picture              Resulting Picture
0        00000000                      00000000
1        00000000                      00000001
0        00000000                      00000000
0        00000000                      00000000
0        11111111                      11111110
0        00000000                      00000000
0        11111111                      11111110
1        11111111                      11111111
0        11111111                      11111110
1        00000000                      00000001
0        11111111                      11111110
0        11111111                      11111110
0        11111111                      11111110
0        11111111                      11111110
1        00000000                      00000001
0        11111111                      11111110

Original Picture                    | Picture with Message
00000000 00000000 00000000 00000000 | 00000000 00000001 00000000 00000000
11111111 00000000 11111111 11111111 | 11111110 00000000 11111110 11111111
11111111 00000000 11111111 11111111 | 11111110 00000001 11111110 11111110
11111111 11111111 00000000 11111111 | 11111110 11111110 00000001 11111110

As you can see message stored in that way (in the bitmap) does not have a great impact on the way on which human percept the image (pixels are pretty equals before and after). The algorithm i have described is very naive but the principle is the same in every steganography algorithm.

In your case there is not bitmap but JPEG (a complicated way to represent image, complicated because there are compression algorithm), and there is a password and encription. Encription is there for two reason, secure the message (steganograpy alone does not guarantee any confidentiality) and create a noise like distribution on less significative bits (normally less significant bits has such distribution but an hidden message may deviate that distribution in the Resulting Picture, so an attacker can identify which pictures hide a message).

If you wuold be sure that an image does not hide secret data you don't have to try to find them. Mainly beacuse every steganograpy algorithm is particular and store information in a different way (the only limit is the immagination), secondly because if steganograpy is done properly (with robust encription) you can not distinguish between noise and hidden data.

So the only approach that seems to me pratical is try to manipulate the candidate picture (a picture that could contain hidden data but also could be a regular picuture) in a way that a side channel will be broken.

In my naive implementation a manipulation for cleanup could be a compression with JPEG, JPEG is pretty close to bitmap (to human eye), but the less significant bit will be whipped out (due to compression) and hidden information with them.

In your specific case i suppose that taking the picture and compress it again with JPEG (using a greater factor of compression) should also work. Remember that every steganography algorithm is particular so one may store hidden data in a redundant way, tryin to be more resilent to your cleanup endeavour (see watermark tecnique : Digital Watermark).

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It would seem to me, conceptually, that if you could find the original message from the steganographied (if that's a word) message then the steganography in use is quite poor (i.e. the hidden text is not very hidden).

If an algorithm exists for this, a person could simply subtract the newly found original from the steganographied and discover the actual message.

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There are countless ways to do steganography, "hide in the least significant bits" is just one way. Unless you have more information what that program actually does, no one can tell you how to get rid of the hidden information with certainty - although JPEG compression will work most likely.

If you want to actually find where the data was hidden, try to find some part, e.g. the few least significant bits, which look like white noise (uniform random bits). Today's encryption schemes require ciphertext to be indistinguishable from randomness, as long as you don't know the key. And then if you overwrite this with $0$s, $1s$ or other randomness, you will destroy the original information. Even if you just overwrite a small part of each block of ciphertext, the original message is impossible to reconstruct.


But what I actually find more worrying:

  • Your point of view on cryptography is somewhat pessimistic: AES 128 and RSA 2048 surely aren't "weak" in the sense that they can be broken if you use enough computation power. And there isn't really a "game", like the arms race in other areas of IT security.
  • But despite that, you trust some implementation without any documentation of which algorithm they actually use to hide your information, if they have done any kind of testing or quality assurance, and if there was any security analysis done.

Before you worry about using AES 256 instead of 128, it would be way more important to look into the kind of software you are using. Security breaks at its weakest link, and that's definately not AES 128.

And yes, I'm well aware of the overkill that is 256-bit symmetric encryption. I just don't like that the cryptography community has to wait until something is deemed insecure before we switch to more advanced techniques (like switching from 1024-bit RSA to 2048-bit RSA, meanwhile we could simply be using 4096-bit RSA and be much safer for more time to come with very little performance tradeoff

Actually, key length suggestions tend to be absolute worst case assumptions, and statements like "AES 128 is weak" are on a scale "using all computational power of the earth for millions of years". Going from RSA 2048 to RSA 4096 can surely be done, but that's an overkill on an even larger scale. And considering that "little performance tradeoff", this is true if you do just one RSA opration. But already the key generation takes quite a lot longer, and this statement can't be generalized to all kinds of algorithms and protocols.

I do think using AES 256 and RSA 4096 is really good. But not because of some unfounded worries about smaller keys. The reason could be: It's the upper limit of what is currently considered state-of-the-art (and commonly used).

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