General :: Make Shared Keys .ssh/authorized_keys And Sudo Work Together?

I've setup the .ssh/authorized_keys and am able to login with the new "user" using the pub/private key ... I have also added "user" to the sudoers list ... the problem I have now is when I try to execute a sudo command, something simple like:

$ sudo cd /root

it will prompt me for my password, which I enter, but it doesn't work (I am using the private key password I set)Also, ive disabled the users password using

$ passwd -l user

I am trying to harden my system ... the ultimate goal is to use pub/private keys to do logins versus simple password authentication. I've figured out how to set all that up via the authorized_keys file.Additionally I will ultimately prevent server logins through the root account. But before I do that I need sudo to work for a second user (the user which I will be login into the system with all the time).

For this second user I want to prevent regular password logins and force only pub/private key logins, if I don't lock the user via" passwd -l user ... then if i dont use a key, i can still get into the server with a regular password.But more importantly I need to get sudo to work with a pub/private key setup with a user whos had his/her password disabled.

1) I've adjusted /etc/ssh/sshd_config and set PasswordAuthentication no This will prevent ssh password logins (be sure to have a working public/private key setup prior to doing this

2) I've adjusted the sudoers list visudo and added

root ALL=(ALL) ALL
dimas ALL=(ALL) NOPASSWD: ALL

3) root is the only user account that will have a password, I am testing with two user accounts "dimas" and "sherry" which do not have a password set (passwords are blank, passwd -d user)

The above essentially prevents everyone from logging into the system with passwords (a public/private key must be setup). Additionally users in the sudoers list have admin abilities. They can also su to different accounts. So basically "dimas" can sudo su sherry, however "dimas can NOT do su sherry. Similarly any user NOT in the sudoers list can NOT do su user or sudo su user.