Add Keypair to existing EC2 instance

Nice puzzle, thanks! Here's one answer:

1. Start a new, temporary EBS boot t1.micro instance A, specifying keypair p2. Specify an availability zone where you have another instance B already running and to which you have access. (Start a temporary one if needed).

2. Stop (not terminate) the instance A after it has been in the running state for a few minutes, so it has a chance to save the public key to its authorized_keys file.

3. Detach the root EBS volume from the stopped instance A. Attach and mount it to your running instance B.

4. Copy the public key from the mounted file system.

5. Detach and delete the EBS volume. Terminate the temporary instance A.

By default, when you shutdown/halt an EBS boot instance it is simply moved to the "stopped" state. This leave the instance available for restarting. You are no longer paying for instance hours, but you will continue paying a minimal charge for the EBS volume(s).

You can change this behavior so that a shutdown/halt will terminate the instance by setting the instance-initiated-shutdown-behavior to "terminate".

I've written a bit about this here:

Three Ways to Protect EC2 Instances from Accidental Termination and Loss of Data
http://alestic.com/2010/01/ec2-instance-locking

You'll want to run the instance with the option:

--instance-initiated-shutdown-behavior terminate

With this flag set, either of the following commands will terminate the EC2 instance:

sudo shutdown -h now
# or
sudo halt

You can also "Change Shutdown Behavior" through the AWS console for running instances (right mouse click).