Ubuntu – How to develop the own display manager

For some reasons we need to create our own display manager.
We need some very special biometric identification of users.
If any one can give me some links to documentation or standarts or any information from which we can start.

We need:

1. Identify user on workstation with our biometric subsystem. Workstation must ask server to find mapping between biometric information and user account. Accounts are stored in LDAP server.
2. Then Work station must say something like "Hello Denis. Enter your password". And after user enter his password workstation have to authenticate user using Kerberos throw PAM subsystem.
3. If user can't pass biometric identification he mast press special button and authenticate using simple user name and password, but this must be logged in security log.

P.S.
Now we are try to understand lightDM source code. But it's very hard because we don't understand fundamental principals of display managers(DM) and principal of interacting DM with over system such as PAM, GNOME, Profiles etc.