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How To Set Up SSH Key

SSH keys provide a more secure way of logging into a server with SSH than using a password alone. While a password can eventually be cracked with a brute force attack, SSH keys are nearly impossible to decipher by brute force alone. Generating a key pair provides you with two long string of characters: a public and a private key.

Step One-Create the RSA Key Pair
The first step is to create the key pair on the client machine:

ssh-keygen -t rsa

Step Two-Store the Keys and Passphrase
Once you have entered the Gen Key command, you will get a few more questions:


Enter file in which to save the key:

You can press enter here, saving the file to the user home (in this case, my example user is called demo).


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.

The entire key generation process looks like this:

ssh-keygen -t rsa
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 (identification) is now located in /home/demo/.ssh/id_rsa. Type in ls into the terminal to help you find it.

Step Three-Copy the Public Key

Once the key pair is generated, it's time to place the public key on the virtual 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@123.45.56.78


 Alternatively, you can paste in the keys using SSH:

cat ~/.ssh/id_rsa.pub | ssh user@123.45.56.78 "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"


 No matter which command you chose, you should see something like:

The authenticity of host '12.34.56.78 (12.34.56.78)' 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 '12.34.56.78' (RSA) to the list of known hosts. user@12.34.56.78's password: 
Now try logging into the machine, with "ssh 'user@12.34.56.78'", and check in: ~/.ssh/authorized_keys to make 
sure we haven't added extra keys that you weren't expecting.

Now you can go ahead and log into user@12.34.56.78 and you will not be prompted for a password. 
However, if you set a passphrase, you will be asked to enter the passphrase at that time (and whenever else you log in in the future).


Step Four Optional-Disable the Password for Root Login

 Once you have copied your SSH keys unto your server and  ensured that you can log in with the SSH keys alone, you can go ahead and restrict the root login to only be permitted via SSH keys.

 

In order to do this, open up the SSH config file:

Choose files or drag and drop files
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