How to Set Up SSH Keys on Linux

What is SSH?

SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. When working with an Ubuntu server, chances are you will spend most of your time in a terminal session connected to your server through SSH.

In this guide, we’ll focus on setting up SSH keys for an Ubuntu installation. SSH keys provide a secure way of logging into your server and are recommended for all users

Creating the RSA Key Pair

The first step is to create a key pair on the client machine (usually your local computer)

ssh-keygen

By default ssh-keygen will create a 2048-bit RSA key pair, which is secure enough for most use cases (you may optionally pass in the -b 4096 flag to create a larger 4096-bit key).

After entering the command, you should receive the following output:

OutputGenerating public/private rsa key pair.
Enter file in which to save the key (/your_home/.ssh/id_rsa):

Press ENTER to save the key pair into the .ssh/ subdirectory in your home directory, or specify an alternate path.

If you’ve previously generated an SSH key pair, you may receive the following prompt:

Output/home/your_home/.ssh/id_rsa already exists.
Overwrite (y/n)?

If you choose to overwrite the key on disk, you will not be able to authenticate using the previous key anymore. Be very careful when selecting yes, as this is a destructive process that cannot be reversed.

The next prompt will ask you to enter a secure passphrase:

OutputEnter passphrase (empty for no passphrase):

Here you have the option to enter a secure passphrase, which is highly recommended. A passphrase adds a layer of security to prevent unauthorized users from logging in

Copying the Public Key to Linux host

The quickest way to copy your public key to the Ubuntu host is to use a utility called ssh-copy-id. Due to its simplicity, this method is highly recommended if available. If you do not have ssh-copy-id available to you on your client machine, you may use one of the two alternate methods provided in this section (copying via password-based SSH, or manually copying the key).

If you do not have password-based SSH access to your server available, you will have to complete the process manually.

This section outlines how to manually append the content of your id_rsa.pub file to the ~/.ssh/authorized_keys file on your remote machine.

To display the contents of your id_rsa.pub run the following command on your local computer:

cat ~/.ssh/id_rsa.pub

This will return the key’s content in the command’s output:

Outputssh-rsa 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 test@demohost

Access your remote host using whichever method you have available.

Once you have access to your account on the remote server, you should make sure the ~/.ssh directory exists. This command will create the directory if necessary, or do nothing if it already exists:

mkdir -p ~/.ssh

Now you can create or modify the authorized_keys file within this directory. You can add the contents of your id_rsa.pub file to the end of the authorized_keys file, creating it if necessary. For this command, substitute the public_key_string with the output from the cat ~/.ssh/id_rsa.pub command that you executed on your local system. It should start with ssh-rsa AAAA...:

echo public_key_string >> ~/.ssh/authorized_keys

Finally, ensure that the ~/.ssh directory and authorized_keys file have the appropriate permissions set:

chmod -R go= ~/.ssh

This recursively removes all “group” and “other” permissions for the ~/.ssh/ directory.

If you’re using the root account to set up keys for a user account, it’s also important that the ~/.ssh directory belongs to the user and not to root. In this tutorial our user is named sammy but you should substitute the appropriate username into the following command:

chown -R johnwick:johnwick ~/.ssh

Now you can attempt passwordless authentication with your linux server.

Authenticating to Ubuntu Server Using SSH Keys

The process is the same:

ssh username@remote_host

If this is your first time connecting to this host (if you used the manual method), you may receive something like this:

OutputThe authenticity of host '192.168.25.6 (192.168.25.6)' can't be established.
ECDSA key fingerprint is fd:fd:d4:f9:77:f7:73:84:e1:55:00:ad:d6:6d:22:fe.
Are you sure you want to continue connecting (yes/no)? yes

This means that your local computer does not recognize the remote host. Write “yes” and then press ENTER to continue.

If you did not supply a passphrase for your private key, you will be logged in immediately. If you supplied a passphrase for the private key when you created the key, you will be prompted to enter it (note that your keystrokes will not display in the terminal session for security). After authenticating, a new shell session should open for you with the configured account on the Ubuntu server.

If key-based authentication was successful, continue on to learn how to further secure your system by disabling password authentication