ssh-key

1- sudo apt-get install openssh-client
2- ssh-keygen -t rsa
    . Enter file in which to save the key (/home/user/.ssh/id_rsa)
    . Enter passphrase (empty for no passphrase) # can donot use passphrass
    . Enter same passphrase again
3- check :
    . Public Key: /home/user/.ssh/id_rsa.pub
    . Private Key: /home/user/.ssh/id_rsa    

4- ssh-copy-id user@YourServerIPAddress    
or
cat ~/.ssh/id_rsa.pub | ssh User@YourServerIPAddress “mkdir -p ~/.ssh && cat >>  ~/.ssh/authorized_keys”
or
scp /home/user/.ssh/id_rsa.pub  user@YourServerIPAddress:/home/user/.ssh