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diff --git a/website/signing-host-keys.mdwn b/website/signing-host-keys.mdwn new file mode 100644 index 0000000..1eb61a0 --- /dev/null +++ b/website/signing-host-keys.mdwn @@ -0,0 +1,127 @@ +# 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: + +1. verify the identity of the person. +2. 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: + +1. verify the identity of the server. +2. 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. |