Wikipedia has lots of interesting info about South Korea's RRN number:<p><a href="http://en.wikipedia.org/wiki/Resident_registration_number" rel="nofollow">http://en.wikipedia.org/wiki/Resident_registration_number</a><p>Basically it's a 13-digit number with format yymmdd-sbbbbnc consisting of year, month, and day of birth, sex, birth location, a check digit, and single extra digit to differentiate persons who happen to have the same date and place of birth.<p>The US Social Security Number (SSN) encodes some personal info too, but not to this degree.<p>The nature of identity numbers--being permanent or very difficult to change--means that you can expect much confidentiality. But the South Korean design has eliminated all possibility of keeping it private. If you know a person fairly well, or they're a public figure, you can deduce the RRN. Conversely, if you get the RRN, you automatically get a lot of personal info.