[Zope] Zope application offline - how to apply a license ptotection?

[Anton Zubenko]

Greetings to all zope community.

Our company is developing an application, supposed  to be run under dedicated
zope server, placed offline in our customers corporate environment and 
requested via LAN.
The commerical approach is to impose a recurrent fee on our customer for 
using the software. Thus, application needs a renewable license mechanism.
Supposedly, customer gets his updated license in some format upon successfull 
payment and supplies it in order to continue using the application (license 
should contain data like number of users, next expiration date, etc.)
What would be the right way to protect this mechanism from hacking, provied
that zope is an open-source platform, so any code is hypothetically 
accessable by the customer and apllication is run completely offline with no 
intercation  with our information system?

The required level of protection is not really high, but  we need to be safe 
from attempts to hack the application by modest programmers at least.

Thanks for inputs!

-az