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I'd say communicating outside normal means would be "using a covert channel", and reserve "steganography" for such communication as has been whitened to resemble noise[1].

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 ..."

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I'd say communicating outside normal means would be "using a covert channel", and reserve "steganography" for such communication which has been whitened to resemble noise[1].

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.

security per obscurity? (skotadigraphy?)

More "security by hoping they don't look". And in fact, the suggested uses of overlong PNG chunks in the linked article aren't particularly secure ones.

It's hard for me to see how splitting hairs gets you any farther from calling it steganography when you find a place in a binary format where you can write arbitrary bytes. The covert channel is the payload handed over in broad daylight. How the secret is encoded or not is just impl detail.

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.

I agree on the spectrum, the distinction I'm trying to make is one that Bacon wrote about in the seventeenth:

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?

Wouldn't I call the latter "encryption"?

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.

This might be how the term steganography is mostly used these days, but I disagree that it's the definition of 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.

If it includes a marginally hidden container with an obvious structure this sense of "steganography" seems formally correct (secrecy without a secret key) but overly broad in practice.

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).

Applying that scenario to Kabbalah, you may have just come up with an explanation for the canonical status of the Song of Songs.

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