SSH authentication with pre-shared key

Create new key on client
#ssh-keygen -t rsa
#(confirm with 3x with enter to leave passphrase empty)
ssh-keygen -q -f ~/.ssh/id_rsa -N ''

Copy public key to server
ssh-copy-id ${USER}@

Test login
ssh -v ${USER}@

Login with private key
ssh -i ./backup_ssh_key/id_rsa USER@YOUR_SERVER

Import own ssh key by using previous / master ssh key
cat ~/.ssh/ | ssh -i ./backup_ssh_key/id_rsa USER@YOUR_SERVER 'cat >> .ssh/authorized_keys'

OPTIONAL: Disable password login on server