signing private openssl key for use with openssh

This is related to Converting keys between openssl and openssh .

I have private keys generated by openssl that I want to use with SSH for connection authentication. I have no issue using ssh-keygen to generate a public key. However, I'm now trying to use CA certificate signing as I'm dealing with over 100 machines that need to connect via SSH.

If I try to do ssh-keygen -s ca.key -I dev1 dev1.key on my openssl generated key I get a message saying 'do_ca_sign: unable to open "dev1.key"'. I noticed that my openssl private keys are all 1675 bytes and the private keys generated by ssh-keygen are all 1679 bytes. Do I need to do some sort of conversion on the openssl keys before being able to sign them with the ca.key? Strangely ssh-keygen has no problem reading the same private key to generate a public key so I'm lost as to what to do.