How To Set Up SSH Keys on Linux

Create the RSA Key Pair

The first step is to create the key pair on the client machine.

$ ssh-keygen -t rsa

Store the Keys and Passphrase

Once you have entered command to generate the key, you will get a few more questions:

Enter file in which to save the key (/home/user/.ssh/id_rsa):

You can press enter here, saving the file to the user home.

Enter passphrase (empty for no passphrase):

It’s up to you whether you want to use a passphrase. Entering a passphrase does have its benefits: the security of a key, no matter how encrypted, still depends on the fact that it is not visible to anyone else. Should a passphrase-protected private key fall into an unauthorized users possession, they will be unable to log in to its associated accounts until they figure out the passphrase, buying the hacked user some extra time. The only downside, of course, to having a passphrase, is then having to type it in each time you use the key pair.

The entire key generation process looks like this:

$ ssh-keygen -t rsa
Output
Generating public/private rsa key pair.
Enter file in which to save the key (/home/demo/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/demo/.ssh/id_rsa.
Your public key has been saved in /home/demo/.ssh/id_rsa.pub.
The key fingerprint is:
4a:dd:0a:c6:35:4e:3f:ed:27:38:8c:74:44:4d:93:67 demo@a
The key's randomart image is:
+--[ RSA 2048]----+
|          .oo.   |
|         .  o.E  |
|        + .  o   |
|     . = = .     |
|      = S = .    |
|     o + = +     |
|      . o + o .  |
|           . o   |
|                 |
+-----------------+

The public key is now located in /home/demo/.ssh/id_rsa.pub. The private key is now located in /home/demo/.ssh/id_rsa.

Copy the Public Key

Once the key pair is generated, it’s time to place the public key on the server that we want to use.

You can copy the public key into the new machine’s authorized_keys file with the ssh-copy-id command. Make sure to replace the example username and IP address below.

ssh-copy-id user@198.51.100.0

You will see something like:

The authenticity of host '198.51.100.0 (198.51.100.0)' can't be established.
RSA key fingerprint is b1:2d:33:67:ce:35:4d:5f:f3:a8:cd:c0:c4:48:86:12.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '198.51.100.0' (RSA) to the list of known hosts.
user@198.51.100.0's password: 

This message helps us to make sure that we haven’t added extra keys that you weren’t expecting.

Now you can go ahead and log into your user profile and you will not be prompted for a password. However, if you set a passphrase when creating your SSH key, you will be asked to enter the passphrase at that time (and whenever else you log in in the future).

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