> a national ID number isnt usually publicly available info
A couple of counterexamples:
Italy's "fiscal code" is generated with a simple algorithm which can be executed manually:
https://en.wikipedia.org/wiki/Italian_fiscal_code
Sweden's "personal identity number" is public information and can be looked up by anyone:
https://en.wikipedia.org/wiki/Personal_identity_number_(Swed...
Belgian National Number is your birth date, then an increment from 1 to 999 for every baby born that day, then a base-97 checksum.
Not sure if still the case: odd numbers for males and even numbers for females.
This also means a person’s Belgian RRN can change.
Yep, but they do not change on sex change, IIRC.
I wrote a system for a medical lab decades ago and IIRC I did actually have to change some RRN's manually. It's been a while so I'm not 100% sure, but I do think the changes were due to switching gender.