I'm building a centralized desktop application using Python/wxPython. One of the requirements is User authentication, which I'm trying to implement using LDAP (although this is not mandatory).
Users of the system will be mechanical and electrical engineers making budgets, and the biggest problem would be industrial espionage. Its a common problem that leaks occur commonly from the bottom on informal ways, and this could pose problems. The system is set up in such a way that every user has access to all and only the information it needs, so that no one person but the people on top has monetary information on the whole project.
The problem is that, for every way I can think to implement the authentication system, Python's openness makes me think of at least one way of bypassing/getting sensible information from the system, because "compiling" with py2exe is the closest I can get to obfuscation of the code on Windows. 
I'm not really trying to hide the code, but rather make the authentication routine secure by itself, make it in such a way that access to the code doesn't mean capability to access the application. One thing I wanted to add, was some sort of code signing to the access routine, so the user can be sure that he is not running a modified client app.
One of the ways I've thought to avoid this is making a C module for the authentication, but I would rather not have to do that.
Of course this question is changing now and is not just "Could anyone point me in the right direction as to how to build a secure authentication system running on Python? Does something like this already exist?", but "How do you harden an scripting (Python) against wrongful modification?"
Possibly:

The user enters their credentials into the desktop client.
The client says to the server: "Hi, my name username and my password is password".
The server checks these.
The server says to the client: "Hi, username.  Here is your secret token: ..."
Subsequently the client uses the secret token together with the username to "sign" communications with the server.
How to deal with user authentication and wrongful modification in scripting languages?