Simple Access Code generation for enabling offline App features

Hi all

I need to implement an access code that can be used to enable some features in my app.

This solution has to work offline.

I would like to use the iOS device serial number combined with a unique GUID for later online data access.

Something like [DEVICE SERIAL]+[GUID]

But how do I make this secure in a simple way, so the generated access code to enter is not to long for the user ?

I have been thinking about just using base64 on the [DEVICE SERIAL]+[GUID], but the resulting string is very long.

The solution must be secure in a way so a third party can not just enter their own device serial and a generated GUID and get access to the offline features. I need to be able to block a third party already from the offline features and not at the point where a user is able to access online stuff using the GUID code.

Any suggestions as how to solve this is greatly appreciated ?