D-numbers (D-nummer), introduced in 1978, allow people which are not resident to be issued an ID number, including foreign-resident workers and foreign land-owners.
The ID numbers were initially issued by Statistics Norway, but along with the operation of the Population Register this was turned over to the Norwegian Tax Administration in 1992.
I1 contains information about the century of birth, while the gender is coded in I3 (females are even, males are odd numbers).
As both the Tax Administration and the National Insurance Service were increasingly using computers for their data processing, they introduced various identification and serial numbers of people and employees.
[1] The population register in Oslo, introduced in 1959, was at the time using an eight-digit ID-number, consisting of six digits representing the birth date and two individual numbers.
[2] The Ministry of Finance took initiative to establish a common government identity number, and the process was led by Statistics Norway.
Two main systems were considered, one which encoded the person's birth date and gender, and an eight-digit number that did not carry any information.
At the time a major advantage of this approach was that because the birth date and gender were encoded, it was possible to record this information without additional fields during data processing.
[1] The decision to introduce the numbering scheme was taken by Statistics Norway in 1964, with the backing of the Ministry of Finance, but without any legislation.
This changed in 1991, after the registers had been fully digitized and the responsibility for the content transferred to the Tax Administration.
Initially this was mostly sailors working on Norwegian ships, and who were therefore liable to pay taxes to Norway.
[1] An ID number is issued to a person when they are registered in the National Population Registry, which is operated by the Norwegian Tax Administration.
Similarly, a person who undergoes a judicial sex change is issued a new ID number.
There were two errors that could happen at this stage—the resident could remember their number wrong, alternatively the clerk would misunderstand the person.
A major restriction of the algorithm was that it needed to be supported by the IBM 402 tabulating machines used by Statistics Norway at the time.
Increased use also means that people are more likely to remember their number, significantly reducing the need for control digits.
[3] A D-number is issued to people who have to interact with Norwegian authorities, but are not regarded as resident in the country.
[4] Non-EEA citizens, including landed refugees, receive a DUF-number upon registering their resident applications.
[6] The confusion caused a vicious circle, where people became increasingly reluctant to share or use their ID number, perpetuating the problem.
Identity theft due to accepting ID numbers as authentication has been common in Norway.