RSA, ECDSA, or ED25519 private key (e.g., as generated by the ssh-keygen -t rsa -b 2048 -N "" -m PEM -f my-new-server-key, ssh-keygen -t ecdsa -b 256 -N "" -m PEM -f my-new-server-key or ssh-keygen -t ed25519 -N "" -f my-new-server-key commands).
ssh-keygen -t rsa -b 2048 -N "" -m PEM -f my-new-server-key
ssh-keygen -t ecdsa -b 256 -N "" -m PEM -f my-new-server-key
ssh-keygen -t ed25519 -N "" -f my-new-server-key