Looking for info on user authentication

Hi,

I've just completed my first major application, and I've got some people interested in buying it (or should I say interested in paying for the rights to use it).

What I'm curious about is the best method to set it up so that these paying users will be authenticated. Should I set it up to require a serial number or some similar method? If so, how do I implement it? Are there other options?

Currently, I have a quick fix in place that allows use for a set period of time (ie: 30 days) after which the app just won't run, but this is hardly a permanent solution.

I'd welcome anyone's suggestions on the subject.

Thanks.

should not be a big deal, if you are using db.
look for encryption and similar stuff for securing user authentication.

CLR based applications post some interesting issues in trying to secure them from un-authorized duplication [note: this is NOT User Authentication, but rather Software Licensing].

If you are looking to make it really difficult [nothing can make it impossible], I strongly suggest using a professional package intended for this purpose.

Most "home-grown" solutions can be cracked in less than an hour.