Private repos are implemented by extending the identity document with a
`visibility` attribute, that can either be `"public"` (default) or
`"private"`.
In case of `private` visibility, only the delegates are allowed to view
the repo, as well as any DIDs added to the allow list.
To implement repo visibility, we simply block fetches from and
announcements to peers for whom the repo should remain invisible.
Private repos are also not announced in the `inventory` message, since
the full list of peers that *may* have the repo is retrievable from the
repo identity. This could cause errors if eg. a peer who is allowed to
view the repo doesn't actually have it. However this is an ok trade-off
for now to keep the complexity low. For repos to truly be private, it's
important that the RIDs don't leak either.
Finally, we modify `radicle-httpd` for now to only list public repos.
Eventually, we would want to change this depending on whether an allowed
peer is authenticated with the service or not.
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It's also worth mentioning why this approach was taken, vs. end-to-end
encryption. The reasons are as follows:
1. Nodes that do not have access to a private repo will generally not want to
replicate encrypted data that they cannot examine or use.
2. The chosen solution is trivial, while encrypting git objects isn't.
3. Performance of the chosen solution is much better, there is no
overhead.
4. Privacy of the chosen solution is better: RIDs are never leaked, and
neither is the existence of a private repo, nor who has access to it.
There is one downside: Paying for storage of private repos is no better
in terms of privacy than what GitHub offers. Hosting providers will have
access to your private repos, if this solution is used.
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