Overriding authenticate method - Django admin

I'm trying to figure out how to enhance the authenticate method with additional functionality.

e.g.

• Expiring passwords
• special password formats
• length requirements
• etc...

It is pretty straight forward for the site's frontend, but what about the admin panel?

I reckon that I should override the User's Manager object, as authenticate probably resides there. This is quite a tough one to figure out I think.

Thanks in advance! :)