Thanks @jamesj
There are two different things here. The device downloading the snap, and me downloading the snap.
When I try and download the snap I am doing this:
UBUNTU_STORE_ID=foo UBUNTU_STORE_AUTH_DATA_FILENAME=bar snap download --channel current/stable some-snap-name
When I use the same auth file with ubuntu-image, and a signed model assertion, the snaps are downloaded fine.
Secondly, on the device. The device is able to install snaps fine, you can do a snap install some-snap-name, so it must be authenticated ok, but when you try and download a snap with snap download some-snap-name it fails with the above error.
Cheers,
Just