Comment by bmacho
8 hours ago
> you never have to make thumbnails ever again, you just truncate the output stream at the right proportion of pixel data (!)
It's an attack vector for images to have different thumbnail/first bytes than the final image so software still have to decode the whole image.
How would that work in an attack?
For serious, general purpose programs when thumbnails are shown people expect them to correspond to the full images, and not something else. Windows explorer, android gallery, gnome file picker etc can't show made-up thumbnails, or people would send or delete the wrong images, or would have no idea what images do they actually have.
png/webp (just as jxl) can store thumbnail in their meta, but no operating system, gallery, browser etc uses that because it has no usage that is safe.
I am not sure how easy it is to create a jxl image that starts differently from how it ends. If it is super easy then I expect programs not relying on it for thumbnailing (especially that they already have a way to generate and store thumbnails). Maybe browsers will support progressive loading, but it has no use for local images.
I believe the image would need to be present in the full size image, so I think that would limit to some extent the attack vector, but I don't know how far you could push it.