OpenSSH interoperating with ssh.com

[Philip Mak]

Does anyone know how to setup public/private key authentication between
ssh.com's ssh2 client and OpenSSH's ssh server?

If both client and server are from ssh.com, I can do it.

If both client and server are OpenSSH, I can also do it.

But they seem to take different key file formats or something, so they
won't work with each other. Is there a way I can make it work?