This is a pam module that will authenticate a user based on them having an ssh
certificate in their ssh-agent signed by a specified ssh CA.

This is primarily intended as an authentication module for sudo. Using it for
something else may be unsafe (we haven't tested it anyway). We'd be happy to
learn of other potential uses though.

An example usage would be you ssh to a remote machine and sshd authenticates
you (probably using your ssh cert, because if you're using it for this, you're
probably using it for sshd as well). At that point when you want to run a
command that requires authentication (eg. sudo), you can use pam-ussh for
authentication.