git-annex has recently gotten support for running as a
[Tor](https://torproject.org/) hidden service. This is a nice secure
and easy to use way to connect repositories between peers in different
locations, without needing any central server.

## setting up the first peer

First, you need to get Tor installed and running. See
[their website](https://torproject.org/), or try a command like:

	sudo apt-get install tor

To make git-annex use Tor, run these commands in your git-annex repository:

	sudo git annex enable-tor $(id -u)
	git annex remotedaemon
	git annex p2p --gen-addresses

The p2p command will output a long address, such as:

	tor-annex::eeaytkuhaupbarfi.onion:4412:7f53c5b65b8957ef626fd461ceaae8056e3dbc459ae715e4

At this point, git-annex is running as a tor hidden service, but
it will only talk to peers who know that address.

## adding additional peers

To add a peer, get tor installed and running on it.

	sudo apt-get install tor

You need a git-annex repository on the new peer. It's fine to start
with a new empty repository:

	git init annex
	cd annex
	git annex init

And make git-annex use Tor, by running these commands in the git-annex
repository:

	sudo git annex enable-tor $(id -u)
	git annex remotedaemon

Now, tell the new peer about the address of the first peer.
This will make a git remote named "peer1", which connects,
through Tor, to the repository on the other peer.

	git annex p2p --link peer1

That command will prompt for an address; paste in the address that was
generated on the first peer, and then press Enter.

Now you can run any commands you normally would to sync with the
peer1 remote:

	git annex sync --content peer1

You can also generate an address for this new peer, by running `git annex
p2p --gen-addresses`, and link other peers to that address using `git annex
p2p --link`. It's often useful to link peers up in both directions,
so peer1 is a remote of peer2 and peer2 is a remote of peer1.

Any number of peers can be connected this way, within reason.

## starting git-annex remotedaemon

Notice the `git annex remotedaemon` being run in the above examples.
That command runs the Tor hidden service so that other peers
can connect to your repository over Tor.

So, you may want to arrange for the remotedaemon to be started on boot.
You can do that with a simple cron job:

	@reboot cd myannexrepo && git annex remotedaemon

If you use the git-annex assistant, and have it auto-starting on boot, it
will take care of starting the remotedaemon for you.

## speed of large transfers

Tor prioritizes security over speed, and the Tor network only has so much
bandwidth to go around. So, distributing large quantities (gigabytes) 
of data over Tor may be slow, and should probably be avoided.

One way to avoid sending much data over tor is to set up an encrypted
[[special_remote|special_remotes]]. git-annex knows that Tor is rather
expensive to use, so if a file is available on a special remote as well as
over Tor, it will download it from the special remote.

You can contribute to the Tor network by
[running a Tor relay or bridge](https://www.torproject.org/getinvolved/relays.html.en).

## onion addresses and authentication

You don't need to know about this, but it might be helpful to understand
how it works.

git-annex's Tor support uses onion address as the address of a git remote.
You can `git pull`, push, etc with those onion addresses:

	git pull tor-annnex::eeaytkuhaupbarfi.onion:4412
	git remote add peer1 tor-annnex::eeaytkuhaupbarfi.onion:4412

Onion addresses are semi-public. When you add a remote, they appear in your
`.git/config` file. For security, there's a second level of authentication
that git-annex uses to make sure that only people you want to can access
your repository over Tor. That takes the form of a long string of numbers
and letters, like "7f53c5b65b8957ef626fd461ceaae8056e3dbc459ae715e4".

The addresses generated by `git annex peer --gen-addresses` 
combine the onion address with the authentication data.

When you run `git annex peer --link`, it sets up a git remote using
the onion address, and it stashes the authentication data away in a file in
`.git/annex/creds/`

## security

Tor hidden services can be quite secure. But this doesn't mean that using
git-annex over Tor is automatically perfectly secure. Here are some things
to consider:

* Anyone who learns the address of a peer can connect to that peer,
  download the whole history of the git repository, and any available
  annexed files. They can also upload new files to the peer, and even
  remove annexed files from the peer. So consider ways that the address
  of a peer might be exposed.

* While Tor can be used to anonymize who you are, git defaults to including
  your name and email address in git commit messages. So if you want an
  anonymous git-annex repository, you'll need to configure git not to do
  that.

* Using Tor prevents listeners from decrypting your traffic. But, they'll
  probably still know you're using Tor. Also, by traffic analysis,
  they may be able to guess if you're using git-annex over tor, and even
  make guesses about the sizes and types of files that you're exchanging
  with peers.

* There have been past attacks on the Tor network that have exposed
  who was running Tor hidden services.
  <https://blog.torproject.org/blog/tor-security-advisory-relay-early-traffic-confirmation-attack>

* An attacker who can connect to the git-annex Tor hidden service, even
  without authenticating, can try to perform denial of service attacks.