Personal identification number (Denmark)
The Danish Personal Identification number (Danish: CPR-nummer or personnummer) is a national identification number, which is part of the personal information stored in the Civil Registration System (Danish: Det Centrale Personregister).
The register was established in 1968 by combining information from all municipal civil registers of Denmark into one.
It is a ten-digit number with the format DDMMYY-SSSS, where DDMMYY is the date of birth and SSSS is a sequence number. The first digit of the sequence number encodes the century of birth (so that centenarians are distinguished from infants), and the last digit of the sequence number is odd for males and even for females.
Any person registered as of 2 April 1968 (1 May 1972 in Greenland) or later in a Danish civil register, receives a personal identification number. Any person who is a member of ATP or is required to pay tax in Denmark according to the Tax-control Law of Denmark, but is not registered in a civil register, also receives a personal identification number.
The civil register lists only persons who:
Danish citizens, including newborn babies, who are entitled to Danish citizenship, but are living abroad, do not receive a personal ID number, unless they move to Denmark.
The sequence numbers used to be chosen (and still are, preferentially) so that the last digit of the sequence number functions as a check digit for the entire personal identification number. In this case, the number satisfies the equation 4x1 + 3x2 + 2x3 + 7x4 + 6x5 + 5x6 + 4x7 + 3x8 + 2x9 + x10 ≡ 0 (mod 11) where the xi are the ten digits of the complete ID number, and the coefficients (4, 3, 2, 7, …) are all nonzero in the finite field of order 11.
However, in 2007 the available sequence numbers under this system for males born on 1 January 1965 ran out, and since October 2007 personal identification numbers do not always validate using the check digit. This had been predicted and announced several years in advance. Thus, most IT systems are presumed updated to accept numbers that fail the check-digit validation.
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