Run the following command to retrieve the SHA256 fingerprint of your SSH key (-l means “list” instead of create a new key, -f means “filename”):
$ ssh-keygen -lf /path/to/ssh/key
So for example, on my machine the command I ran was (using RSA public key):
$ ssh-keygen -lf ~/.ssh/id_rsa.pub
2048 00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff /Users/username/.ssh/id_rsa.pub (RSA)
To get the GitHub (MD5) fingerprint format with newer versions of ssh-keygen, run:
$ ssh-keygen -E md5 -lf <fileName>
Bonus information:
ssh-keygen -lf also works on known_hosts and authorized_keys files.
To find most public keys on Linux/Unix/OS X systems, run
$ find /etc/ssh /home/*/.ssh /Users/*/.ssh -name '*.pub' -o -name 'authorized_keys' -o -name 'known_hosts'
(If you want to see inside other users’ homedirs, you’ll have to be root or sudo.)
The ssh-add -l is very similar, but lists the fingerprints of keys added to your agent. (OS X users take note that magic passwordless SSH via Keychain is not the same as using ssh-agent.)