Thursday, October 4, 2018

SSH certificate authentication


* You can configure client-side and server-side authentication using SSH certificates with the existing openssh daemon.
* You never need to worry about MITM attacks on the client when connecting to the server the first time
* Significant decrease in management overhead of SSH keys on the server

If you have a remote server to manage and it's running Linux (or even Windows for that matter but that's beside the point) - it's very likely that there is an SSH daemon running on it. You use an SSH client to connect to it and perform administrative tasks on it. While doing so, you can use passwords (by default) or public key authentication which is a bit more secure as it takes out the password-brute-force attacks. It does mean though that there is some management overhead on both the client and the server side.

On the client, you have to add the host that you are connecting to your known_hosts file. So over time, you have a massive list of known_hosts with no clue about the purpose of each host. Similarly on every server, there is a huge authorized_keys file which has the client's public keys added to it. When you want to revoke a client key you have to go in and remove that client's key from this file on every server. When you want to not trust a server any more, you need to remove that entry from your known_hosts manually. This is something that can go wrong easily if you miss one server - so there's probably some automation that is probably required here that can make it more reliable.

Certificate-based-authentication goes one step further, where a client trusts any SSH server signed using an 'SSH-root-CA' and a server can in turn trust a client key only if it is signed by a 'user-CA'. There is a really nice post by Facebook where they automate this process and make it even less error-prone. Those posts do a good job of walking you through step-by-step but I did have trouble replicating it, so I'll do a quick summary of the exact steps here.

Server certificate authentication

1. Configure an SSH daemon on a server (Docker, EC2, VirtualBox doesn't matter - but ideally a separate host as it's the CA). Let's call it ca.
2. Generate an SSH keypair for the server CA.
3. Start a new server up. Let's call it host1. This too should run SSH. This is the server you want to login to and administer.
4. Generate an SSH keypair for host1 in /etc/ssh
5. Copy host1's public key onto the ca server. Sign host1's public key with ca's private key. This will create an SSH certificate.
6. Copy and the certificate you just created from ca to /etc/ssh on host1.
7. Configure /etc/ssh/sshd_config to use the key you created in Step 4 as well as the certificate. This is done using the HostKey and HostCertificate directives.
8. Restart the SSH daemon or reboot your server to reload your SSH config so it uses the certificate
9. Configure the client machine (any machine apart from host1 and ca) to recognize the ca's public key using the @cert-authority directive. This is so you don't get a 'Should I connect? Yes/No' message the first time you connect to host1.

User certificate authentication

1. Generate an SSH keypair for the client. This is the userca.
2. Generate a second SSH keypair for the client. This is the key you use to connect to host1. Call it client.
3. Sign client with userca. This will generate a cert as well on the client.
4. Copy to host1 and configure sshd_config using the TrustedUserCAKeys directive pointing to userca. This is so host1 recognizes that all user certs signed by this cert are to be accepted.

At this point, you should be able to login to host1 from client and never get a popup the first time I connect because I've explicitly trusted the server CA. It's also very cool that there is no need to do any more key management on any server, as long as you trust the user CA used to sign the user keys.


Dockerizing an SSH service
Hardening SSH