$ ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (/home/pi/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/pi/.ssh/id_rsa.
Your public key has been saved in /home/pi/.ssh/id_rsa.pub.

Copy your keys to the target server:

$ ssh-copy-id pi@raspixxx
pi@raspixxx's password:
 
Now try logging into the machine, with ssh 'pi@raspixxx', and check in:

.ssh/authorized_keys
to make sure we haven't added extra keys that you weren't expecting.

Finally check logging in

$ ssh pi@raspixxx

pi@raspixxx:~$