Q said: "Archive everything offline"

I'm trying to devise a scheme that would cryptographically verify that the archive is valid (HTTPS signs content). I know there was something like this developed before, but I can't find it. I could try to do something with a centralized signature for verification, but it kind of defeats the purpose.

I have a few more ideas, for example the archive being a .png file concatenated with a .zip, so it could be shared on imageboards as an image and unzipped using any unzip software.