Nov 302011
 
Mastering SSH Keys

Welcome to the wonderful world of SSH keys! If you don’t yet share my enthusiasm you soon will! SSH keys are a perfect way for you to control access to your machine, whether that be a very secure way for only you to have access, locking down other authorized users and preventing their passwords from getting distributed or stolen, or even allowing access to scripts for very specific purposes. SSH keys accomplish all of it and more. First, let me just lay out how simple SSH keys are in case you aren’t clear and maybe get discouraged by the length of this post. The basics of it are, you have a host server create a private and public key pair. You can optionally provide a password for the keys. You copy the public key to servers you wish to log on to, while keeping private key secured and on your system. Then when you login to the system the private key is used to decrypt the public (also with optional password) and you get in. It is that simple at its core, there are just a lot of various configuration options you can use to accomplish certain features or functionality that may be more desirable for you. The keys are very secure as using this method alone prevents against man-in-middle attacks and eliminates a few other possible break-ins from password only authentication.
The most common way I see SSH keys being used is as a password-less easy login for an admin to use for his remote linux systems. Although this is better than using a password only system you still are vulnerable to someone obtaining your private key and using it for instant access to any system with the corresponding public key. I prefer to use them as a type of two factor authentication for the ssh logins where I need to provide the password to the key plus (of course) have the actual key. Regardless, let’s get a basic configuration going and start using ssh-keys. First on the server side you can follow a config I’ve used in a previous post but general settings to ensure:

  • Port 2992 – (I always recommend changing the default port)
  • Make sure both “RSAAuthentication” and “PubkeyAuthentication” are both set to yes
  • PermitRootLogin no
  • AuthorizedKeysFile %h/.ssh/authorized_keys
  • I’d also only allow users in a certain group to use ssh for further protection so “AllowGroups sshlogin” for example and add users who will ssh into this system into the sshlogin group.

Now on your machine, the computer you will be ssh’ing from, you need to create the key pair and copy the public key to the host(s). Generate the key pair as follows:
Create the .ssh directory in your linux home directory if it doesn’t exist already. Ensure correct permissions with chmod 0700 .ssh. Then create the actual keys and when prompted specify a pass-phrase to use (This pass-phrase is the optional part if you just want to authenticate with the keys alone but as I mentioned above I prefer supplying one so you have a two factor authentication going on). Keep in mind this can be a passphrase not just a password.
ssh-keygen -b 2048 -t rsa -f ~/.ssh/MainKey
The -b specifies the length of the key and the -t the type (which is rsa or dsa).
I’d use RSA 2048 or if you are the more paranoid type RSA 4096. I won’t delve into the RSA/DSA debate except to say that I’ve done a LOT of reading on the topic and I’m choosing RSA keys and with sensitive servers I’d go with RSA at 4096.
[Update, Dec 2015] :: DSA is being deprecated in OpenSSH so I now strongly recommend RSA over it. If you are running version 6.5 or newer of openssh I’d actually recommend using ed25519 keys over RSA and you can read this blog post for more details. Typically you can check your version with ssh -V command. Your command to generate this key would simply be
ssh-keygen -t ed25519 -f ~/.ssh/MainKey
Note for Mac OSX users, the version of ssh is too old for ed25519 unless you are on OSX 10.11, El Capitan. If you are on an older Mac OSX version simply google for homebrew install of openssh to upgrade your version and use that.
[End Update]
Once the keys are created scp the public key to the server but copy the public key into the .ssh directory named as “authorized_keys” (or wherever you specified in sshd_config the AuthorizedKeysFile). You can append multiple public keys to the authorized keys file just cat mykey.pub >> authorized_keys for each public key.

Now you may have noticed above when generating the key pair we passed the -f option. This way we can name our private key something other than the default. If you do not specify the -f the default will be id_rsa with the corresponding id_rsa.pub. So now to ssh into our system we will want to specify the identity file to use as:
ssh -i /home/user/.ssh/MainKey user@myserver.com -p2992
The reason I want to show the more complicated use case of specifying a particular identity file is that you may want to have different key pairs for servers that are yours versus your employers’ or any other separation that would be important to you. You can simplify the management of which keys are used for which hosts by specifying in the ~/.ssh/config file.
vi ~/.ssh/config

Add both host names and their identity file as follows:
Host server1.myserver.com
IdentityFile ~/home/user/.ssh/MainKey
Host server1.workserver.com
IdentityFile ~/home/employee/.ssh/id_rsa

That’s all there is to your basic key based login! Now lets briefly go over many of the popular use cases for these ssh-keys so you can see how powerful and helpful they can be for you.

First, as I mentioned earlier, I frequently see administrators using these without a password so they can quickly login to the servers they manage. The problem is you are now solely relying on the security of that private key for complete access to those systems. I will strongly recommend (yet again) putting a pass-phrase on your keys and then if you want a quick password-less login then simply configure ssh-agent to cache your pass-phrase on your system for that session. For details just search for “ssh-agent cache password” and you’ll find a few examples. In this way if your key is somehow stolen that person still has no access to the servers as they don’t know the password, yet you have the efficiency of not typing a pass-phrase once you set up and configure ssh-agent to cache it on your system. Win-win here.

Next, we can limit by IP or IP subnet where users with ssh-keys are able to login FROM. So say you are the server admin and you have other users you manage on the server. Maybe you (especially after reading this) have a requirement that they only login with ssh-keys. Being security conscious you might have a few concerns, not the least are:

  • Did they create a passphrase or leave blank?
  • Will they give their key and pass to someone else to use?
  • What if their key is stolen or compromised?

One thing you can do if you just control the server with the public key, is limit by IP or subnet where they can be coming from. In this way if the users key is stolen it can’t be used anyway if that person isn’t trying to login from the same IP (or subnet) as where the original user is from. You can either wait till they login the first time and see what IP they are coming from or ask them to send you the subnet or possible IP’s they might come from. Once you have this IP or list of IP’s just open the authorized_keys file on your server and add a “from=” line to the beginning of their public key as shown below:

[root@mainserver .ssh]# cat authorized_keys
from="10.20.30.*,172.16.31.*,192.168.1.*" ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEArAkcHTOXZiDxcJEHNmrJRoM2HJE9Rq1uoiVHuTSjgl0THp0UFDNepnmCvk5bX22KzjAUa8PWnq1ZDw5Uf9/i6N/SiXittnxT0dFnZGj6RRep5Ae3AmOJbg0i69XL9o2zBJnYo2JVPXJkCDhSvVWokZUn5QjaJiGNigP9plA1He94Slkhn2jTxx1iehx9Vy/ojnxsJDqpTa8hX1GQK/b1jzvWdN3Qg+EhlxBDfKSA8u4uGsPP+6hXGgFLRluG/6yizj8LDF1LRWIKYaBvaLNJ+720sAI9O4miHyxY4n3ghBDbULPoLGz5a6bYRJ9pY9i0ySQEmXNSD+0u31+fAaGGpQ== user@sauronseye

So if you have this public key and it is supposed to be used by a user at Company A and that user gives it to someone else at Company B or it is compromised by evil haxor, then that key whether it had a password or not will not allow them access. Then with your normal audits you do (right?) you see failed logins from a foreign IP and realize the key was compromised. Server is still secure your world is safe from harm :).

The last and yet another very common way of using ssh-keys is for specialized scripts use. You have a script on one server and at a certain point in the script you’d like it to kick off a command or script on another server. How do you do this? Well you can create an ssh-key specific for this purpose. Create your keys just as above but name its identity for the job at hand so you don’t get confused.
ssh-keygen -t rsa -f ~/.ssh/login_audit
For an example lets say you have primary internal server and you want it to go out and gather a report of all the last logins from your remote servers. You could put a script like this on the remote server:

# Script: last_logins.sh
last -a > /home/user/last_logins.barney
scp /home/user/last_logins.barney fred@mainserver:.
# End Script

Then in the authorized_keys file on the remote server edit the public key just as we added the “from=” line but instead use “command=”:

command="/home/user/scripts/last_logins.sh",no-port-forwarding,no-X11-forwarding,no-agent-forwarding [rest of pub key...]

Now whenever you ssh in using that identity file the only thing it will do is run that command and exit. There are countless uses for this alone and being aware that you can use ssh-keys like this will help you realize WHEN you should use them like this.
There is a bunch more you can do with ssh and ssh-keys so use this as a starting point and take advantage of the tools at your disposal!

~~~~~BONUS TWO-FACTOR AUTHENTICATION SECTION~~~~~
We discussed above using ssh-keys as two-factor authentication to login to a server via ssh. Some of you may be aware that Google offers a two factor authentication to login to your Google services (gmail, apps, etc.). They accomplish this with an application you can install called Google Authenticator.
If you don’t use google authenticator and 2-factor authenication on your google account you should strongly consider it. Gmail accounts are prime targets for being compromised and a lot of people use that account for a wide array of things that would make the compromise that much more damaging. Read some of the links below for starters:
http://www.multitasked.net/2011/jun/27/hacked-gmail-google-account/
http://threatpost.com/en_us/blogs/new-password-not-enough-secure-hacked-e-mail-account-100410

Once you have google authenticator installed cause now you fear for the safety of your gmail… what else can you do with this thing? Well we can use it for two-factor authentication to login to our machines, much in the same way we used the ssh-keys.
I’ve found a great post that details how to do this and don’t forget to read the comments there as they include a lot of valuable information as well:
http://www.mnxsolutions.com/security/two-factor-ssh-with-google-authenticator.html

Linked also in that post is how to use a YubiKey to login with two factor authentication. These are all great and really secure methods to login and authenticate to your systems.

Hopefully you can realize the importance of two factor authentication to secure the things that really matter to you. There are countless hacks and exploits and more found every day. Trust me it is much easier to take the extra 10 minutes to set this up than the hours or days of recovery from a security breach. One needs to be proactive about security because if you are reactive then you are too late.