SSH
from https://help.ubuntu.com/8.04/serverguide/C/openssh-server.html
SSH Keys
SSH keys allow authentication between two hosts without the need of a password. SSH key authentication uses two keys a private key and a public key.
To generate the keys, from a terminal prompt enter:
ssh-keygen -t dsa
This will generate the keys using a DSA authentication identity of the user. During the process you will be prompted for a password. Simply hit Enter when prompted to create the key.
By default the public key is saved in the file ~/.ssh/id_dsa.pub, while ~/.ssh/id_dsa is the private key. Now copy the id_dsa.pub file to the remote host and appended it to ~/.ssh/authorized_keys2:
cat id_dsa.pub >> .ssh/authorized_keys2
Finally, double check the permissions on the authorized_keys2 file, only the authenticated user should have read and write permissions. If the permissions are not correct change them by:
chmod 644 .ssh/authorized_keys2
You should now be able to SSH to the host without being prompted for a password.
Top 20 OpenSSH Server Best Security Practices
http://www.cyberciti.biz/tips/linux-unix-bsd-openssh-server-best-practices.html
Beginning SSH on Ubuntu
http://principialabs.com/beginning-ssh-on-ubuntu/