Last modified: March 11 2019.
Contact: hpchelp [at] iitd.ac.in

How To Set Up SSH Keys

Step One

Create the RSA Key Pair : The first step is to create the key pair on the client machine. In the case of HPC cluster, it will be any one of the login nodes. 
ssh-keygen -t rsa

Step Two

Store the Keys and Passphrase Once you have entered the keygen command, you will get a few more questions:
Enter file in which to save the key (/home/demo/.ssh/id_rsa): 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):
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

Step Three

Copy the Public Key to the server. 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.
OR
Alternatively, you can paste in the keys using SSH: 
cat ~/.ssh/id_rsa.pub | ssh user@hpc.iitd.ac.in "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@hpc.iitd.ac.in'", 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@hpc.iitd.ac.in 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). In the case of HPC cluster, you will be on a login node and to check you will try ssh user@hpc.iitd.ac.in.