Propellor's security model is that the hosts it's used to deploy are
untrusted, and that the central git repository server, if any,
is untrusted too.

The only trusted machine is the laptop where you run `propellor --spin`
to connect to a remote host. And that one only because you have a ssh key
or login password to the host.

Since the hosts propellor deploys do not trust the central git repository,
and it doesn't trust them, it's normal to use git:// or http:// to pull
from the central git repository, rather than ssh://.

Since propellor doesn't trust the central git repository, it checks
that any commit it fetches from it is gpg signed by a trusted gpg key,
and refuses to deploy it otherwise.

That is only done when privdata/keyring.gpg exists. To set it up:

	gpg --gen-key             # only if you don't already have a gpg key
	propellor --add-key $MYKEYID

In order to be secure from the beginning, when `propellor --spin` is used
to bootstrap propellor on a new host, it transfers the local git repository
to the remote host over ssh. After that, the host knows the gpg key, and
will use it to verify git fetches.

Since the propellor git repository is public, you can't store
in cleartext private data such as passwords, ssh private keys, etc.

Instead, `propellor --spin $host` looks for a
`~/.propellor/privdata/privdata.gpg` file and if found decrypts it,
extracts the private data that the $host needs, and sends it to to the
$host using ssh. This lets a host know its own private data, without
seeing all the rest.

To securely store private data, use: `propellor --set $field $context`  
Propellor will tell you the details when you use a Property that needs
PrivData.
