SSH – Start to Finish Architecture – Securing The Private Key

Our previous post showed how to generate the bare bones public/private key pair without using a passphrase. This is sometimes the desired configuration, but it is better to lock down the private key using a passphrase. When you generate the key pair, you can add a passphrase at the prompt that we just hit “enter” on last week, but you might want to change an existing passphrase or add a passphrase to a key that doesn’t already have one. The means for doing this is shown below:
ssh-keygen -f ~/.ssh/id_rsa -p

If the existing phrase is empty (like the one we generated last week,) this will prompt you for your new passphrase right away. If there is an existing passphrase, it will first prompt for that before prompting for the new one. Setting a passphrase on a private key is an important step to securing that key. If someone unauthorized to use that key managed to get a copy of it somehow, they won’t be able to use the key until they figure out the passphrase for it. While it is possible to brute force crack a key, if you use a decently long phrase that isn’t something commonly spoken or written, the chances of cracking it go down. Also note that SSH key passphrases allow for spaces, so you can literally write nonsense sentences, spaces and all. There is more that can be done to reduce the risk of someone using a stolen private key to do harm, but it’s on the client side, and there are caveats. We’ll cover that next week.

Now that we have a passphrase protecting our private key, what has changed in how ssh works? For starters, if you don’t use an Agent to load your keys to, every time you go to log into a server using this key, you will be prompted for the passphrase like you used to be prompted for a password. This makes convenience worse, not better. To use the agent, run ssh-add. If you’re using a standard key name such as id_rsa, id_dsa, or id_ecdsa, it will automatically find and load that key for you. For each key with a standard name it finds, it will prompt for the passphrase. You give it the phrase and it handles the rest. It acts on your behalf from then until it is told to unload a key or is stopped. When you go to login, the SSH client will see that the agent is running, and when prompted for the key by the server, it will pass that request through to the agent, which will provide proof that it knows the key, and thus you won’t be prompted. It’s like promptless SSH, but requires the extra step of loading the agent first.

If you get an error message when you run ssh-add, there is a chance that ssh-agent isn’t already running. If that is the case, you can start ssh-agent first, take the output it gives, and export those variables. For example:

SSH_AUTH_SOCK=/tmp/ssh-w8iG9Aq6KWLR/agent.1070; export SSH_AUTH_SOCK;

If you have a key with a custom name such as id_rsa_2016, you can load these by passing their name, like so:
ssh-add /home/User_A/.ssh/id_rsa_2016

Using the agent is dangerous in a shared environment where other people have elevated privileges. Anyone with root can potentially pull the private key from memory while the agent is running on your behalf. You can unload the keys before locking your workstation if you’re paranoid enough, by using -D or -d as below:
ssh-add -D #Delete all identities
ssh-add -d /home/User_A/.ssh/id_rsa_2016 #Deletes just the id_rsa key from the agent list

You can also lock and unlock your agent using the “-x” and “-X” flags respectively, if you don’t want to completely unload for security’s sake. These will prompt you for a password to use for locking and unlocking the agent, if you choose to use them.

If you want to see which keys are loaded, you can list them with “ssh-add -l.” And if you need to be sure which public key matches the loaded private key, you can use “ssh-add -L.”

Finally, if you want to set a time limit on a key being loaded, you can use the -t flag to make it temporary. It requires a number (in seconds) for how long the key should remain loaded by the agent.

The rest of the flags are for more advanced stuff I will be covering separately, so that’s all we’ll cover for today. If you’ve kept up thus far, you’re pretty much at the level of the average SSH user at this point. (And there’s so much more to be covered.) Next week, we’ll go over some client configuration options to make session management easier.

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