To create key only authentication the users public key needs to be put on the server in their home directory under .ssh/authorizated_keys.
Create the users private key on their workstation:
# ssh-keygen -t rsa
Send the private key to the server:
# cat .ssh/id_rsa.pub | ssh newmachine “cat >> .ssh/authorized_keys”
Make sure the authorized_keys file it set to the correct permissions.
Login to the server and change the permissions.
chmod 644 .ssh/authorized_keys
Configured sshd to use the key autentication and disable password authentication
# vi /etc/ssh/sshd_config
Make sure the following lines are in the file:
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
PasswordAuthentication no
Save the file.
Restart sshd
/etc/init.d/sshd restart
Test by connecting from the users workstation. They should be logged in right away without the need for a password. Try to login from the server, there should be an error message.
See http://open.bsdcow.net/tutorials/ssh_pubkey_auth for more information.