Public Key from Private Key

I fall over this every so often. I have the private key file but would either have to trawl servers for authorized_keys files to get the public password or remember how to obtain the public key from the private key.

Time to document it here so I don’t have to hunt for it with Google again.

For an RSA PEM format public key

$ openssl rsa -in private.key -pubout

-----BEGIN PUBLIC KEY-----
MIIBIDA ...
 -----END PUBLIC KEY-----

For an SSH putty friendly version

$ ssh-keygen -y -f private.key

 ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAQE ...