using rsync through secure ssh in a cron job

Solution 1:

rsync since 2.6.x defaults to using ssh for transfers. So it's already taken care of as long as your remote NAS rsync is supported.

As to password less unattended transfers, setup ssh key based authentication for the user you are running the cron as and that should be done. Here is how you set it up -- http://www.cyberciti.biz/tips/ssh-public-key-based-authentication-how-to.html

Solution 2:

You can also pass this flag to rsync to use a file for the password:

--password-file

Can easily chmod the file to 400 so no one but your user can read that file (and root of course)