ReadyNAS 300/400 series SSH keys

SSH Keys

Hi

I wish to change the SSH private and public keys on the NAS server.

At the moment I have a client server backing up to another server offsite.

Backup Server <--- Client Server

I am using a backup job on the NAS box to use remote SSH.

I have port forwarding enabled for port 22 on the clients side so the server can connect to it.

A recent penetration test has found that the SSH is not secure enough at 1024 bits.

I wish to change the public key to 4096 bits.

The following has not worked, with each stage restarting the Backup server.

I have gone through the process with PuTTY and replaced the public key in the /root/.ssh/ directory

Same with both public and private.

And lastly with just the private key.

When the new public key is given to the client server (under a new user I have given - SSH) it does not connect and asks for a login.

The backup server needs to pull the information from the client server and I can not allow open ports on the Backup servers internet, so pushing the backup from the clients server does not work.

Is there a way to increase the bits, or make it more secure, so the clients network is not at risk?

Am I changing the keys wrong?