Signing a host's SSH key using OpenPGP
This page is meant to address the issue of signing OpenPGP-based SSH
host keys. Machines are not people, so the circumstances under which
one should sign a host key are different from those under which one
should sign another person's key.
Why are signatures on an SSH host key important?
In order for users to validate a host (an SSH server) in a
monkeysphere-enabled network, the host key must have full calculated
validity from the perspective of the connecting user. If the user has
not themselves signed the server's key, then the server's key can only
be valid if other people that the user trusts have signed the key.
If only one person has signed the server's key, then the user must
fully trust the single person who has signed the host key. Full trust
should be granted sparingly and with consideration, though, so unless
the user knows the server admin very well, they will in general not
have full trust of this person.
However, full trust of the host key can also be achieved if the
server key has been signed by three or more people that the user has
marginal trust of. In other words, three or more marginally
trusted signatures equals one fully trusted signature. It is much
more common for users to have marginal trust of other users in the Web
of Trust. For this reason, it is advisable to have as many people
sign the server key as possible.
What information should you have before signing a host key?
Before signing the key of a person, you want to do two things:
- verify the identity of the person.
- verify that the person is actually in control of the key that you
are signing.
For a server, you want to do basically the same thing:
- verify the identity of the server.
- verify that the server is actually in control of the key that you
are signing.
However, verifying these things for a server is less intuitive than it
is for a human.
Verifying that the host is in control of the key is, in principle,
straightforward. If you are logged on to the machine in question,
then you can check directly that the key exists on the system.
What is not so straightforward is what exactly it means to "verify the
identity" of a remote server on the internet? The identity in this
case is the fully qualified domain name (FQDN) of the host. Verifying
this identity amounts to being sure that the host in question really
is located at that FQDN.
Signing the host key
If you are the person (or persons) that actually setup the server and
configured Monkeysphere and ssh on the server, then you should sign
the host key as part of that process. When the server is first set
up, the administrators who set it up are the only ones who can
actually vouch for the server key, so their signatures are necessary
to get things going. Their signatures are also necessary so that they
can validate the host key themselves and log into the server via
monkeysphere-enabled ssh in the future.
If you did not set up the server initially, you do not have an
accumulated full trust of the person(s) who did, and you do not
necessarily have console access to the server directly, it's hard to
confidently verify the server identity and key ownership. You would
like to be able to walk up to the server, log in at the console, and
get the fingerprint of the ssh host key directly. But this is usually
impossible.
However, it is still possible to verify the server identity and
server ownership of the key, even in this case.
Remotely verifying host identity and key possession
It is in fact possible to verify the identity and key ownership of a
server in one fell swoop with monkeysphere-enabled ssh. Here is the
procedure:
Attempt to make a monkeysphere-enabled ssh connection to the host in
question. Monkeysphere will check that the ssh host key offered by the
host matches the OpenPGP key with the correct host FQDN user ID. If
the ssh host key and the OpenPGP key with the correct user ID match,
then you will have effectively:
1. verified the host identity, because you actually connected to the
host in question, which you know because you:
2. verified the host is in control of the key, because the ssh host
key offered by the host matches the OpenPGP key with correct host FQDN
user ID.
Here is an example:
servo:~ 0$ ssh zimmermann.mayfirst.org
-------------------- Monkeysphere warning -------------------
Monkeysphere found OpenPGP keys for this hostname, but none had full validity.
An OpenPGP key matching the ssh key offered by the host was found:
pub 2048R/860E8F9C 2008-10-29 [expires: 2009-02-26]
uid [marginal] ssh://zimmermann.mayfirst.org
sig! 76CC057D 2008-11-15 Jamie McClelland <jamie@mayfirst.org>
sig!3 860E8F9C 2008-10-29 ssh://zimmermann.mayfirst.org
sig! D21739E9 2008-10-29 Daniel Kahn Gillmor <dkg@fifthhorseman.net>
sig! 1CF2D62A 2008-11-16 Micah Anderson <micah@riseup.net>
RSA key fingerprint is 81:96:13:3e:24:c9:3c:5b:3c:6d:55:ba:58:85:e9:9e.
-------------------- ssh continues below --------------------
The authenticity of host 'zimmermann.mayfirst.org (<no hostip for proxy command>)' can't be established.
RSA key fingerprint is 81:96:13:3e:24:c9:3c:5b:3c:6d:55:ba:58:85:e9:9e.
No matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)? no
Host key verification failed.
servo:~ 255$
I have attempted to connect to the host zimmermann.mayfirst.org.
zimmermann's host key has only marginal validity for the FQDN user
ID in question, so I am not able to connect. However, the
Monkeysphere has checked that the ssh host key actually does match the
OpenPGP key with the correct user ID ssh://zimmermann.mayfirst.org.
I have therefore verified the identity of zimmermann, and verified
that zimmermann is in possession of the key in question.
