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Image steganography techniques have indeed reached a level of sophistication that requires significant expertise to implement correctly. A good parallel is comp

Image steganography techniques have indeed reached a level of sophistication that requires significant expertise to implement correctly. A good parallel is compression--most CS undergrads can figure out how to implement Huffman or even LZW compression, but state-of-the-art ANS (asymmetric numeral systems) is pretty dense. I believe a special type of wet paper code called a syndrome-trellis code is at the forefront of image steganography: http://dde.binghamton.edu/filler/pdf/Fill10tifs-stc.pdf http://dde.binghamton.edu/filler/pdf/Fill10tifs-stc.pdf. Good luck parsing that without a solid foundation in information theory. Steganography is the current focus of my PhD research, and from my perspective the greatest security challenges are in systems engineering. Embedding secret messages into your vacation photos is one thing, but who are you sending these messages to? And why? How will they know when and where to go looking for the secret message? Encryption entered its golden age with the advent of public-key cryptography and Diffie-Hellman key exchange, allowing two parties to share the secrets required for secure communication knowing only public information about one another. However, steganographic systems are still dependent upon lots of out-of-band secret sharing, since attempting to share the necessary secrets over public channels tends to reveal the existence of the system and opens up DoS attacks (e.g. Tor and the identity of relays). IMHO, figuring out better solutions to the system security of particular steganographic applications is a much more pressing research problem than improving statistical embedding profiles for image steganography.

nullc 6y ago
You can use public key encryption which is indistinguishable from uniform. E.g. using ECDH w/ ellegator and ed25519 for the key agreement. Then the recipient of the stego need only publish the public key. If the encoding software were part of standard linux distributions (even if just specialized security conscious distros), then the sender could obtain it without identifying themselves. I don't think you can tackle the problems in isolation: There isn't much point in making uniform encryption if the embedding gives it away to anyone who's looking. :) > but who are you sending these messages to? One scheme I've been fond of is where the recipient announces their public key and that they'll accept messages in images posted to some moderately high traffic public picture board, like r/naturepics. Then then simply download and process all images submitted there. The sender has an easy and obvious cover story for their posting and can produce as many images they need just by visiting a park with a camera.
- betterunix2 6y ago
  "the recipient announces their public key" A typical threat model is that there is a "warden" who can selectively block messages, so this would not really work since the warden would block the announcement (in theory the warden is not going to block all messages, although a formal security definition should only focus on whether or not the warden is more likely to block stegomessages than innocent messages, which is trivial if the warden blocks everything). Thus out of band communication is needed to distribute secrets; likewise, the stegosoftware itself has to be distributed out of band. "indistinguishable from uniform" Actually you need to be indistinguishable from the "background noise" (the thermal noise from the camera sensor) of the image, which is not uniform. In fact, the noise is correlated with the camera itself and can be used to identify which camera took a particular photo.
  - nullc 6y ago
    > Thus out of band communication is needed to distribute secrets; likewise, the stegosoftware itself has to be distributed out of band. This is why public key cryptography is so important. Because you can use a broadcast channel -- like a Linux distro to communicate the tools and required knowledge in a way that doesn't give a strong signal that you're using it. > "indistinguishable from uniform" Actually you need to be indistinguishable from the "background noise" (the thermal noise from the camera sensor) of the image This isn't the case for code based encoding. The encoded values are a pseudorandom linear combination of a huge number of bits from the image and end up being extremely uniform. Consider my silly xor example: the xor of all the bits in the image is going to be close to uniform, even if the bits in the image are fairly biased. But also more generally: If you have a way to map encrypted messages onto the space of uniform strings, then you can also remap from them onto any other required distribution in a fairly straight forward way. Having a good error metric is important though, and it's one of the areas that seems a bit under researched.
    - jkhdigital 6y ago
      > But also more generally: If you have a way to map encrypted messages onto the space of uniform strings, then you can also remap from them onto any other required distribution in a fairly straight forward way. This is true, but it begs the question: where does the “required distribution” come from? Most (all?) steganographic techniques make heuristic arguments about some plausible cover distribution but there is no provable security as with cryptography. There are some theoretical results on how to make steganography provably secure, but they make use of objects (“channel oracles”) which cannot feasibly be instantiated in the real world except in a handful of unique cases.
jarekd 6y ago
Implementing rANS is simpler than arithmetic coding or Huffman. This syndrome-trellis code sounds like picture-resembling QR codes from https://ieeexplore.ieee.org/document/7348695 https://ieeexplore.ieee.org/document/7348695 - here is implementation: https://github.com/JarekDuda/PRcodes/blob/master/PRcodes.cpp https://github.com/JarekDuda/PRcodes/blob/master/PRcodes.cpp

nullc 6y ago

You can use public key encryption which is indistinguishable from uniform. E.g. using ECDH w/ ellegator and ed25519 for the key agreement. Then the recipient of the stego need only publish the public key. If the encoding software were part of standard linux distributions (even if just specialized security conscious distros), then the sender could obtain it without identifying themselves. I don't think you can tackle the problems in isolation: There isn't much point in making uniform encryption if the embedding gives it away to anyone who's looking. :) > but who are you sending these messages to? One scheme I've been fond of is where the recipient announces their public key and that they'll accept messages in images posted to some moderately high traffic public picture board, like r/naturepics. Then then simply download and process all images submitted there. The sender has an easy and obvious cover story for their posting and can produce as many images they need just by visiting a park with a camera.

betterunix2 6y ago
"the recipient announces their public key" A typical threat model is that there is a "warden" who can selectively block messages, so this would not really work since the warden would block the announcement (in theory the warden is not going to block all messages, although a formal security definition should only focus on whether or not the warden is more likely to block stegomessages than innocent messages, which is trivial if the warden blocks everything). Thus out of band communication is needed to distribute secrets; likewise, the stegosoftware itself has to be distributed out of band. "indistinguishable from uniform" Actually you need to be indistinguishable from the "background noise" (the thermal noise from the camera sensor) of the image, which is not uniform. In fact, the noise is correlated with the camera itself and can be used to identify which camera took a particular photo.
- nullc 6y ago
  > Thus out of band communication is needed to distribute secrets; likewise, the stegosoftware itself has to be distributed out of band. This is why public key cryptography is so important. Because you can use a broadcast channel -- like a Linux distro to communicate the tools and required knowledge in a way that doesn't give a strong signal that you're using it. > "indistinguishable from uniform" Actually you need to be indistinguishable from the "background noise" (the thermal noise from the camera sensor) of the image This isn't the case for code based encoding. The encoded values are a pseudorandom linear combination of a huge number of bits from the image and end up being extremely uniform. Consider my silly xor example: the xor of all the bits in the image is going to be close to uniform, even if the bits in the image are fairly biased. But also more generally: If you have a way to map encrypted messages onto the space of uniform strings, then you can also remap from them onto any other required distribution in a fairly straight forward way. Having a good error metric is important though, and it's one of the areas that seems a bit under researched.
  - jkhdigital 6y ago
    > But also more generally: If you have a way to map encrypted messages onto the space of uniform strings, then you can also remap from them onto any other required distribution in a fairly straight forward way. This is true, but it begs the question: where does the “required distribution” come from? Most (all?) steganographic techniques make heuristic arguments about some plausible cover distribution but there is no provable security as with cryptography. There are some theoretical results on how to make steganography provably secure, but they make use of objects (“channel oracles”) which cannot feasibly be instantiated in the real world except in a handful of unique cases.

jarekd 6y ago

Implementing rANS is simpler than arithmetic coding or Huffman. This syndrome-trellis code sounds like picture-resembling QR codes from https://ieeexplore.ieee.org/document/7348695 https://ieeexplore.ieee.org/document/7348695 - here is implementation: https://github.com/JarekDuda/PRcodes/blob/master/PRcodes.cpp https://github.com/JarekDuda/PRcodes/blob/master/PRcodes.cpp