It is possible to use certificate authentication of clients: If
a user provides at TLS negotiation a certificate which DN matches a known
user then nuauth assumes the users is authenticated and doesn't ask for a
password.
To activate this functionnality, you need to set the option
nuauth_tls_auth_by_cert to 1.
Note that, nuauth_tls_request_cert has to be set
to 1 or 2 in the mean time.