Eleven digits. Six of them are your birthday, two are arithmetic, and the three
in the middle are handed out in descending order from the top of a range,
at a rate set by how many people were born in Norway that day. This decodes the
structure, does the mod-11 arithmetic in the open, and counts what is
actually left. For someone born here on a known date it is usually
under a hundred numbers. The point is not that the number is easy to
steal. The point is that it was never a secret to begin with.
🔒Nothing is uploaded. No registry, no lookup, no name attached to anything. It is arithmetic in your browser.
⚠Real format is shown by default so the digits read the way they do on a card. Switch the format to synthetic (Skatteetaten's convention of adding 80 to the month) if you want examples that can never belong to anyone, which is what you want when sharing a screenshot. There is no reason to type a real number in here, least of all someone else's.
1What it contains and whether the check digits agree.
Spaces and dashes ignored. Hide digits with x, ? or * to see what a partial leaves.
Synthetic
2How much does a birthday give away?The part people do not believe until they see the count.
3Try the birth-time hypothesisA transparent model, not a fact about how the register processes births.
Uses the date, sex and format from step 2. The probabilities are normalized weights under one assumption: earlier births tend to be registered earlier. Lovdata establishes descending allocation, but not that timing relationship.