Let me suggest that there are should be few rules for password validation. Special characters don't seem to be valid rules for passwords: you want to make them hard to guess, so inserting a character that ends up invisible (if you could) would be a clever move. Therefore this can only be a type of question from a textbook or course material that treats the topic in an academic context.
While I agree with Lynne, one reason you "design" the test at a functional level is that you need to be aware of what your GUI accepts. For example if I want to insert a TAB character that puts me in the next input field on a browser. That might help you to establish the rules for passwords. In terms of unit testing, you want to make sure that these data come in verbatim. If my password might contain a "," then I do not want it parsed with only the portion prior to the "," validated. That is just a starter for ideas, take it from there.