ssh keys save failure

I am trying to make a pair of ssh keys so that I can push a local repository to github.
I am using the following command:

*ssh-keygen -m PEM

which results in:

Generating public/private rsa key pair.
Enter file in which to save the key (/SCSI::SSD.$/!BOOT/Resources/!UnixHome/home.ssh.id_rsa): key
Enter passphrase (empty for no passphrase):
Enter same passphrase again: 
Your identification has been saved in key.
Could not save your public key in key/pub

As it says the private key is saved but not the matching public one.

I can’t find anything much in the way of documentation relating to RISC OS and SSH keys.