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.