How to generate SSH keys in Ubuntu

In Ubuntu simply use the terminal to create a SSH key pair. You then enter your server with the public key, giving yourself the chance to set up a secure connection. How does this work exactly?

What is an SSH key?

Those who would like a secure connection via SSH can authenticate using an SSH key instead of entering a password. To do this, you need an SSH key pair, made up of a public, and a private key. The public key is stored on the system to which the SSH connection is to be created, for example, on a server. The private SSH key should be stored securely and created on the client side. It is then used for encrypting the public SSH key when the connection is made.

Create and generate SSH keys in Ubuntu step by step

Securely log in on servers or other services without entering a password. Using an SSH key pair for authentication is, above all, practical. In the following guide we will show you the example of Ubuntu 22.04 and how you can generate SSH keys in Ubuntu and create them for authentication purposes on a server, you can read through our article on using SSH keys for your network for more information.

Step 1: Create an SSH key pair

Quickly generate SSH keys in Ubuntu. You can create a key pair including a public and private key at any time using the terminal. Open the command center in Ubuntu and enter the following:

ssh-keygen

This automatically generates an RSA key pair with 3,072 encryptions, more than enough for most uses. You can also create a larger key with 4,096 bits by entering the command with ‘-b 4096’ at the end.

ssh-keygen -b 4096

As soon as you enter the command Ubuntu generates a key pair. You will be asked if you want to save the SSH keys in the Ubuntu user directory (subfolder / .ssh), confirm this by pressing the enter key. Alternatively, you can enter another pathway.

Once you have chosen where to save your key pair, Ubuntu will give you the option of setting a secure password. Entering a password to the encrypted authentication method is another level of security preventing those who are not authorised. Enter your chosen phrase or leave it blank if you do not want an extra level of security.

In both instances, use the enter key to confirm your choice. You will receive confirmation of the key being successfully created in the way of a ‘randomart image’ of the SSH key.

Step 2: Adding the public key to the server

The next step is to add the public key to the target server, where you want to later create an SSH connection. To do this you need the username used to log in to the system, as well as the IP address or domain of the system. If you haven’t got a username, you can use the user root instead.

To add the public SSH key, Ubuntu has the tool ssh-copy-id available. Enter the command as follows:

ssh-copy-id username@ip-adresse/domain

If you’re creating a connection to the target host for the first time, the terminal will display a warning that the ‘authenticity of the host…can’t be established’. Enter ‘yes’ and confirm the entry to carry on with the process.

Following on from this, the service will look for the key that’s already been created. When found, you can continue with the process by entering the password to log in to the target system. The setup will end, and you will get the following output if the public key can be successfully added.

Step 3: SSH authentication on the target host

You can now log in to the device or server which has the key added to it via SSH. As with the transfer of the key, you need the username as well as the IP address or domain of the target host. Enter these as follows in the command field:

ssh username@ip-adresse/domain

If you set up a password for the key pair, you will be asked to enter it in the next step. Enter the password requested and then press ‘Unlock’ to connect to the destination system.

Step 4: Deactivate password

If the SSH key pair was successfully set up, then you will no longer need to use standard password logins. In principle, this authentication procedure is still active, even for unauthorised persons who want to access the system, for example in the form of brute force attacks.

In order to improve security, you can deactivate password login on the server or end device. For this log in via SSH and open the configuration file of the SSH server.

sudo nano /etc/ssh/sshd_config

Look for the entry ‘PasswordAuthentication’ in the configuration file and change this entry to ‘no’.

Save the changes in the configuration file and then restart the SSH service using the following command:

sudo systemctl restart ssh

How to manually add your SSH key to Ubuntu

The second step, ssh-copy-id is not available on all systems. In this case, you can manually add the public key to the target system.

To do this, display your public key by running the following command:

cat ~/.ssh/id_rsa.pub

Now copy the key being displayed in the authorized_keys file in the ssh directory of the target host. If the directory is not available, use the following command:

mkdir -p ~/.ssh

To add the code, you then use the following command:

echo key >> ~/.ssh/authorized_keys

Instead of entering ‘keys’, enter the output obtained previously, beginning with ‘ssh-rsa’ in the command.

In the last step, ensure that all groups and permissions are removed for the ‘ssh’ directory and the ‘authorized_keys’ file.

chmod -R go= ~/.ssh