Using SSH keys in Linux

Creating a Key Pair

To create a key pair use the ssh-keygen command. Before running the command check to see if you might already have a key pair. Keys are located in the .ssh/ directory in your home directory. The default key names are id_rsa for the private key and id_rsa.pub for the public key. The command

[name@server]$ ssh-keygen -b 2048 -t rsa

will generate output similar to the following

Generating public/private rsa key pair.
Enter file in which to save the key (/home/ubuntu/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/ubuntu/.ssh/id_rsa.
Your public key has been saved in /home/ubuntu/.ssh/id_rsa.pub.
The key fingerprint is:
ef:87:b5:b1:4d:7e:69:95:3f:62:f5:0d:c0:7b:f1:5e ubuntu@test-key
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|                 |
|           .     |
|            o .  |
|        S    o o.|
|         .  + +oE|
|          .o O.oB|
|         .. +oo+*|
|          ... o..|
+-----------------+

when prompted you should enter a passphrase. If you already have a key pair saved with the default names you may wish to enter a file name for the saved keys so as not to overwrite an existing key pair.

Connecting using a Key Pair

1. Once your key pair has been created, copy the public key (the /home/ubuntu/.ssh/id_rsa.pub key file in the above on your local machine) to the /home/USERNAME/.ssh/authorized_keys file on the server you wish to connect to. If the authorized_keys file already exists add your public key as a new line at the bottom of the file using an editor such as vim or nano.
2. Ensure the file /home/USERNAME/.ssh/authorized_keys and directory /home/USERNAME/.ssh/ have the correct permissions with chmod 600 /home/USERNAME/.ssh/authorized_keys and chmod 700 /home/USERNAME/.ssh respectively.
3. If you were not logged in as USERNAME while you were creating this authrorized_keys file (e.g. you had administrative privileges on the server and used the sudo command before the above commands), ensure the owner of the directory /home/USERNAME/.ssh and the file authrorized_keys are owned by the user USERNAME with the commands sudo chown USERNAME:USERNAME /home/USERNAME/.ssh and sudo chown USERNAME:USERNAME /home/USERNAME/.ssh/authorized_keys.
4. Finally test the new key by sshing to the remote machine from the local machine with

   [name@server]$ ssh -i /home/ubuntu/.ssh/id_rsa USERNAME@ADDRESS
   

   where /home/ubuntu/.ssh/id_rsa specifies your private key file and USERNAME is the user name on the remote machine, and ADDRESS is the address of the remote machine. If you have administrative access on the server and created the account for some one else, rather than giving you their private key they should keep the private key and test the connection out themselves. Private keys should remain private.