I have separated internal and external users with cohorts and uses categories to make sure only users with the right cohorts has access to certain courses. The idea is that external users - customers - can earn point to use in a point shop (on another site - where they have a user id and a password), by taking a course, that ends with a quiz - somewhat simular to a competition.
The users should get access to the courses /categories by a link (key) to the course from a news-letter. They should ONLY be able to participate once in one course and not more (to avoid them from earning more points in the same course).
So i wonder - how to make sure they only participate once and secure it against fraud, like encryption?
Can this be set up without much coding?