[[!comment format=mdwn
 username="joey"
 subject="""comment 3"""
 date="2020-01-29T15:04:06Z"
 content="""
I'm not sure how to implement this in git-annex's Remote API.
retrieveKeyFile/retrieveExport would need to check it and download
the url, so that would need modifications of those methods of every
remote that implements this. And it would need to be possible to
enable the remote in readonly mode.

It might be possible to use a mixin to modify a Remote to support this?
"""]]
