Unfortunately alpha-channel steganography is one of the simplest and most trivially detected of image steganography algorithms, making it not very useful. For an example of what goes into a serious and secure steganographic algorithm see perturbed quantization:

http://dde.binghamton.edu/download/pq/Fri05pq.pdf

http://dde.binghamton.edu/download/pq/

"Hiding" your data in the alpha channel of most images doesn't really seem at all secure to me. That's like hiding your valuables by placing them in your basement.

Steganography that I'm used to seeing intertwined ones data with the actual image data. Which is much more difficult to detect (though certainly not undetectable).

There is a joke here somewhere about pictures and specific number of words. I just can't work it out.

A picture is worth a thousand DWORDS.

I had issues with precomputed alpha when I played with storing data in the alpha channel of images in the browser. In the end, since I didn't really need to 'hide' the data, I just wrote a few pixels at the top of the image, but even then, I found some services (i.e. gravatar) was modifying the pixel values slightly. I had to just use a bunch of the most significant bits from each pixel.

If you're interested why I was playing with all this, it was for storing public keys in gravatar images: http://kybernetikos.github.io/VisualSecrecy/

I didn't see a useful answer on "Why use steganography.js?" block, it just explains "How?".

I wonder answers about "When and why?" questions. Can you guys answer it?

This doesn't really attempt to hide the fact of communication from computers - only from humans casually viewing the image.

steghide has a far superior approach:

http://steghide.sourceforge.net/

But that can be broken too:

http://dl.acm.org/citation.cfm?id=1578445

@eigenschinkpete come by the next viennajs meetup (www.viennajs.org) next wednesday there will be a talk about a similar topic