Create a key:
ssh-keygen -t rsa
Copy it to the server:
ssh-copy-id -i ./mainserver.pub archive@5.45.79.2
OR
ssh archive@5.45.79.2 "chmod 700 .ssh; chmod 640 .ssh/authorized_keys"
cat ./mainserver.pub | ssh archive@5.45.79.2 'cat >> .ssh/authorized_keys'
OR
ssh -i /root/.ssh/mainserver.pub archive@5.45.79.2