This is very nitpicky, but does adding extra data at the bottom of the image count as steganography? I've always thought of steganography as something that is very hard to detect, like changing the LSB of each pixel to a part of your message.
"steganography" is simply the act of hiding messages, so I don't see why this wouldn't count.
What term for hiding information outside of the bounds of the image would you propose instead?
Obviously tastes may differ, so someone else might call the former "steganography" and distinguish the latter as "deniable steganography", or at least "near-noise steganography".
(with a little information theory, one can even communicate reliably via sub-noise steganography)
cryptography vs. steganography?
[1] compare Bacon (1605): https://archive.org/details/advancementofl00baco/page/256/mo...
"... a method of expressing and signifying one's mind to any distance by objects that are either visible or audible — provided only the objects are but capable of two differences ..."
Ah. I think I see where the confusion is coming from. You're saying this isn't steganography because the information isn't hidden in the image data. It's there in the 'plaintext'. I thought this is steganography because the image is hidden in the file data by exploiting the fact that PNG renderers don't display it.
I'm going to change my mind and say that your definition is actually better than mine. The information itself isn't hidden. You don't need a trick, or a key, or any special knowledge to view it. The fact that common PNG libraries don't display it isn't really enough to say that it's enciphered at all - you can't really say a message is definitely hidden if you assume something about the PNG library the attacker is using to view it. I don't know what the right term is, but it probably isn't steganography.
An early example of steganography is writing on the scalp of a person and concealing it with the regrowth of hair: https://en.wikipedia.org/wiki/Steganography#History . The message is clear-text, you just have to know where to look for it. Which is the same as in the blog post.
Some other non-computer examples: https://www.commonlounge.com/discussion/4bc16dbc2c7145ff87ad...
At the risk of "appeal to authority", my field of study is cryptography, and we used steganography in this broad sense.
tl;dr: I agree with the blog post in that it is steganography.
Consider, for example, that you can apply "normal" steganography to the image data in the overlong PNG content (claiming, for example, that you just "hid" a porn picture at the bottom of a cute kitten while there's a secret message in the whole composite picture).
There's obviously a spectrum of how hidden the message is. e.g. tacking bytes onto the end of the image blob vs encoding your message by inc/decrementing rgba values away from the original.
The former is trivial to remove in this case (can just diff images with different messages or run the image through an optimizer), the latter is impossible to remove though you can ruin the encoded message, but surely they both check the high level definitions of steganography.
https://archive.org/details/advancementofl00baco/page/254/mo...
> "For if a letter should come into the hands of such as have a power over the writer or receiver, though the cipher itself be trusty and impossible to decipher, it is still subject to examination and question, unless there be no room to suspect or examine it."
I doubt many slaves' heads were shorn looking for messages (and if they had been, the eavesdropping would be, as in QE, obvious), but overlong images are begging to be flagged by a linter https://news.ycombinator.com/item?id=24341108 , and anyone who naively stuffs plaintext there should hope no one ever runs strings(1).
Anyway, as mentioned above, I don't care if the general case is called steganography, I'm just curious what then the specific case (of writing that has a "cover story") would be called?
Nothing stopping you encrypting a steganographic message. Denying message existence isn't going to be possible in a system that also compresses data and leaks file size - the latter rules out having null states (all RF communication?) or large blocks of zeros (most, if not all, storage?).
You could do something to avoid that, by doing a LOT of padding with random bits - but again, that's back to encryption, isn't it? Except encryption is something you can apply in the real world, where constant broadcast is usually impractical.
Actual image-based steganography schemes, as used e.g. for watermarks, usually alter pixels in a way that is approximately (and hopefully) undistinguishable from plausible noise.
cf. https://en.wikipedia.org/wiki/Portable_Network_Graphics#"Chu...
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?
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.
> so why bother
The answer is - because it's more deniable. You bother because more deniable is better.