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The PNG format supports arbitrary data being included in private (nonstandardized) chunks, which will be ignored by decoders that don’t recognize them. It’s ea

by kd5bjo 6y ago

The PNG format supports arbitrary data being included in private (nonstandardized) chunks, which will be ignored by decoders that don’t recognize them. It’s easier to extract and more reliably hidden, so why bother inserting the extra data as pixels? cf. https://en.wikipedia.org/wiki/Portable_Network_Graphics#"Chunks"_within_the_file https://en.wikipedia.org/wiki/Portable_Network_Graphics#"Chu...

chrisseaton 6y ago
> [you can use] private (nonstandardized) chunks ... so why bother inserting the extra data as pixels? Because these extra chunk are super-obvious? As soon as someone looks in your file they're going to wonder what you've added these for. They will likely also not survive re-encoding. The point of modern stenography is that you can't tell it's there, and it's just a part of the core image data, and that it's completely deniable. How do you deny your nonstandard chunks?
- Beldin 6y ago
  The "completely deniable" part is not necessary for steganography. Otherwise, you dismiss just about any examples of steganography prior to steganography on computers. Which is fine, but as far as i know, not the commonly accepted definition.
  - chrisseaton 6y ago
    The question was > so why bother The answer is - because it's more deniable. You bother because more deniable is better.
crpatino 6y ago
The threat model is that of an active attacker that is able to tamper the message,- either inserting his own malicious data in the extra chunk, or zeroing it out if you are using strong authentication,- but that does care of the quality of the cover message (the image itself). By encoding the secret message in the pixels themselves, you ensure that the attacker cannot significantly alter the secret message without degrading the quality of the image itself. One application is watermarks containing copyright notices. The idea is that the infringing party cannot render the watermark undetectable without also degrading the quality (and by proxy, the economic value) of the image they are trying to plagiarize. Of course, that goal cannot be achieved by simply storing the secret message in the least significant bit of the image. You need to have a model of what type of tampering the attacker is going to perform, and then design an encoding that will be resistant to such attack.

by kd5bjo 6y ago

chrisseaton 6y ago

> [you can use] private (nonstandardized) chunks ... so why bother inserting the extra data as pixels? Because these extra chunk are super-obvious? As soon as someone looks in your file they're going to wonder what you've added these for. They will likely also not survive re-encoding. The point of modern stenography is that you can't tell it's there, and it's just a part of the core image data, and that it's completely deniable. How do you deny your nonstandard chunks?

Beldin 6y ago
The "completely deniable" part is not necessary for steganography. Otherwise, you dismiss just about any examples of steganography prior to steganography on computers. Which is fine, but as far as i know, not the commonly accepted definition.
- chrisseaton 6y ago
  The question was > so why bother The answer is - because it's more deniable. You bother because more deniable is better.

crpatino 6y ago

The threat model is that of an active attacker that is able to tamper the message,- either inserting his own malicious data in the extra chunk, or zeroing it out if you are using strong authentication,- but that does care of the quality of the cover message (the image itself). By encoding the secret message in the pixels themselves, you ensure that the attacker cannot significantly alter the secret message without degrading the quality of the image itself. One application is watermarks containing copyright notices. The idea is that the infringing party cannot render the watermark undetectable without also degrading the quality (and by proxy, the economic value) of the image they are trying to plagiarize. Of course, that goal cannot be achieved by simply storing the secret message in the least significant bit of the image. You need to have a model of what type of tampering the attacker is going to perform, and then design an encoding that will be resistant to such attack.