Ok, a couple of days ago I was working with scribus, and as Krita does already manage CMYK I guessed it would be better to use it instead of GIMP. after a few attemps I realized there was a problem with TIFF exporting: it was not saving transparency, no matter the picture settings. Now, as it likely was a problem derived from my lack of expertise I made some tests:

First: Create a blank tabloid size document, in CMYK with US web coated (SWOP) v2 profile, 8 bit, white background.

The new file looked like this:



If I click in the alpha channel symbol this is what it happens:



Now, for a second try this time I made a transparent document and did some doodles on it, the behavior was the same.




Additionaly to that, exporting to TIFF gave these results:



The left one was selecting only "Preserve transparency", and the one on the right with both "Save layers" and "Preserve transparency".
Exporting in other transparency-supporing formats like PNG works ok, with the only detail that I need to convert the image to RGB first.

EDIT: Almost forgot, but the result is the same, no matter if the alpha channel is "active" or not.

Now, I don't know if this is a problem related to my system or anyone else is having it; again, my system settings are pretty much: Krita 2.5 (from backports), KDE 4.9, Kubuntu 12.04 LTS, Nvidia 304 driver, kernel 3.2.0-29 generic.

Sooo... no luck with this? I'll also try with a fresh install in another computer, I guess, this issue's got me puzzled...

Weird, I remember answering this question... I think that tiff doesn't support alpha transparency for cmyk images. That would make the image cmyka, which isn't a regular format. It's why the "store alpha channel (transparency)" check is disabled when your image is in cmyk.

Hi Cezzare,

just to say that the Alpha icon on the layer panel does exactly what it's meant for, but is not related with what you want here.
It's a shortcut to disable the alpha channel of the layer, so that what's painted on it only "appears" if there something painted on layers behind it; it's mostly only useful combined with group layer.
Thing is if you start painting on a white layer, your alpha channel/transparency is already filled with white.
So in your case I would create an empty layer over the white BG layer, paint on it, then hide the white BG before to export.

PS: also I see that you're using the color smudge brush, take care that this engine has an "Overlay mode" which is not compatible with transparency (as it auto merge the stroke with layers behind's colors, and is specificly precised when activating it that it only works correctly with a fully opaque background)
So take care of the presets with "over" in the name, they use this feature.

Thanks everyone! I checked, seemingly in photoshop this can be archieved but in higher bit dephts.