Comment 18 for bug 1933665

In order for a snap to be asserted, it just has to have been uploaded and accepted into the store. You don't need to download the assertion, as long as the exact same bits were uploaded to the store, the store will create an assertion for it and associate the hash of that file with the assertion.

Snapd when preparing an image will hash the snap file provided with `--snap` and ask the store if there is an assertion for that hash, and if the store says there is such an assertion, then snapd considers the snap file to be asserted.

Typically you would get an asserted snap file by running `snap download ...` and not modifying the resulting .snap file at all. If you unsquash it and modify it at all, the hash no longer matches and thus the .snap file is no longer asserted.