Your OpenSSH Private Key looks like this
-----BEGIN OPENSSH PRIVATE KEY-----
b34ahC
-----END OPENSSH PRIVATE KEY-----
First, backup your OpenSSH Private Key
$ cp server_ssh.key server_ssh.key.backup
Convert to RSA Private Key
$ ssh-keygen -p -N "" -m pem -f server_ssh.key
Now your server_ssh.key looks like this
-----BEGIN RSA PRIVATE KEY-----
b34ahC
-----END RSA PRIVATE KEY-----
permissions 0644 for are too open
The keys need to be read-writable only by you
$ chmod 600 server_ssh.key
Failed to load key idrsa_data: invalid format
Add new line to the server_ssh.key
Tk3AQIDBAUGBw==
-----END OPENSSH PRIVATE KEY----- (press Enter button here, to add new line)
Replace CRLF to LF
$ sed 's/\r$//' server_ssh.key > server_ssh_backup.key