This authentication module is built on top of ctypes_pam.py which is included with Gate One.
It was originally written by Alan Schmitz (but has changed quite a bit).
The only non-obvious aspect of this module is that the pam_realm setting is only used when the user is asked to authenticate and when the user's information is stored in the 'users' directory. It isn't actually used in any part of the authentication (PAM doesn't take a "realm" setting).
This is used by PAMAuthHandler in Authentication to authenticate users via PAM.
Processes the client's Authorization header and call self.auth_basic()