Use SSH Keys for Login Instead of Passwords

Introduction

In this tutorial, we'll have a look at how to use the more secure SSH keys for logging in to your server instead of the weaker password method.

Requirements

• Server (root or VPS)
• PC/laptop running Linux

Step 1 - Generate SSH keys

For this tutorial, we assume that you have a Linux OS running (Windows users can use WSL).

First, we need to generate our SSH key pair, which we can use later to log in to our remote machine. To do this, we start by running the following command:

ssh-keygen

This command will by default generate a key with 2048-Bit-RSA encryption. If you need more security, you can also add the -b 4096 flag to the command to generate keys with 4096-Bit-RSA encryption. Now you have to define where to store the key. The default setting is ~/.ssh/id_rsa (where ~ points to the home folder of your user on Linux). Then the script prompts you to enter a password that will prevent the key from being used without a password. If you don't want to protect this key with a password, just hit Enter (not recommended).

If the key was successfully generated, the output should look like this:

Your identification has been saved in ~/.ssh/id_rsa
Your public key has been saved in ~/.ssh/id_rsa.pub
The key fingerprint is:
SHA256:G+UCCe2r45jt7az1cTVEZj4R1/B8U7eAMNmw0DsAOA username@host
The key's randomart image is:
+---[RSA 3072]----+
|   .oo. oo+oB=oo |
|  .  ..o . @=o+oo|
|   E .o o + =o o+|
|      .. + . .  .|
|       .S . o    |
|      .  + . .   |
|     .. o .      |
|   +o+ . o       |
|  oo=++ .        |
+----[SHA256]-----+

Step 2 - Copy the public key to the remote server

Step 2.1 - Using ssh-copy-id

We now have the private and the public keys on our local system, but we need to transfer the public key to our server. For that we can use the built in ssh-copy-id of our Linux OS.

Use this command:

ssh-copy-id username@host

Example:

ssh-copy-id maxmusterman@37.34.15.3

where 37.34.15.3 is the IP of the server.

When connecting to the server for the first time, the system will ask whether it should trust the key fingerprint of the server:

The authenticity of host '37.34.15.3 (37.34.15.3)' can't be established.
ECDSA key fingerprint is fd:fd:d5:f9:88:fe:73:84:e1:55:00:ab:d6:6f:22:fe.
Are you sure you want to continue connecting (yes/no)?

When you type yes, the script connects to the server:

/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
maxmusterman@37.34.15.3's password:

Next enter the password of your remote server. If the connection was successful, the script will copy the public key into the ~/.ssh folder of this user account:

Number of key(s) added: 1

Now try logging into the machine, with: "ssh 'maxmusterman@37.34.15.3'"
and check to make sure that only the key(s) you wanted were added.

Step 2.2 - Manually copying the key

If ssh-copy-id is not available on your system, you can also connect to your server via SSH and then copy the key manually. Run the following command on your local machine to get the content of the key:

cat ~/.ssh/id_rsa.pub

The output should look like this:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABgQDAuEmfbUGlpPtjpxZU7jcLmmgQ5OAsfPumzvadL4qjLhv3FFbpanQpsSlCfE8kDvWqCkHRhXm29TpjtGLU0mJisg3dz9zHkoG1LoRTnPACmmzjW3Ca+MX56cgWKRFB/d2QN9riPCkRfvFCBgeHlZIa5hGlmbcu+uJNfTu/1vDjlhnyWMB9gxuesOPEB08YT64Ro84ACPAIdfFC4RJICilO+L9FgeYjKwi9jUvYOL3Ywhgs/DkC+EvTkZGyUL8cX8ITkFjLpVh7c/8KkFZCB6DQe1CJQKo8Y5TUM2wit0UlrsrO2AhnlfXXTtIOELQ3ygug6YI1CPaqGpLPbzlM1hfnros5S2UJB6OmiDM17YuyMdbQj2O5HyDUTEtGzgfKt/IG3Na6wFSGEOZhY4TIV9Z6FwXvJ+F4GK4srMgq/HJAIthbRP3x30K29Sfb8edgzDtaJbR38vrIjGUAVbr+mSqVZ1yeithXjuQ9hVOc1GaCxaMuVAw/CHtwdnq100= username@host

Now copy this key (all of it including the ssh-rsa part and also the username@host in the end) and connect to your server via SSH. If you are connected to your remote server, create a new file in the ~/.ssh folder named authorized_keys:

nano ~/.ssh/authorized_keys

Paste the previously copied public key content into the file, save it with Ctrl+O and close the file with Ctrl+x.

Now the key should be ready for use.

Step 3 - Test the key

The last step is to test the key. Therefore, log out of your server if you are still logged in and log in again using the usual SSH command:

ssh username@host

Instead of username/password, the SSH key is now used for login. If you have previously set a password for the key file, you must use it for the key to work.

Step 4 - Disable password login (optional)

To make the whole login process more secure, you can disable password login in general and use only SSH key. For this we need to edit our sshd_config file:

sudo nano /etc/ssh/sshd_config

Next we have to change this option:

- #PasswordAuthentication yes
+ PasswordAuthentication no

and restart the SSH service:

sudo systemctl restart ssh

From now on, only SSH key login will work.

Conclusion

There should be an SSH key on your server that you can use to log in to it. This method is considerably more secure than the usual password method and helps to increase server security.

License

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