How to use publickey from x509 certificate?

[Damien Miller]

Sorry, I had you backwards - I thought that you wanted a PEM public key
for your smartcard, not an OpenSSH public key from your smartcard.

An OpenSSH private key is just a PEM RSA key, so you can extract a
OpenSSH-formatted public key using:

ssh-keygen -yf privatekey.pem

I don't know of any easy way to convert a PEM public key into an OpenSSH
public key. It probably wouldn't be very difficult to modify ssh-keygen
to do this.

-d