It is only a security problem is somebody keeps track. You can be issued a secure key ('a passport') to make new ids ('HN registration') but nobody needs to kn

It is only a security problem is somebody keeps track. You can be issued a secure key ('a passport') to make new ids ('HN registration') but nobody needs to know who ('the passport') generated the website specific ids once your key has been used. The passport only gives access to generate further keys.