nullcipher

Czech and Slovak birth numbers (rodné číslo)

YYMMDD/SSSC with 50 added to women's month, divisible by 11. Masked after a context word. Below: the exact rule, a live check, why it matters, and what to do if one already leaked.

Live check: Czech or Slovak birth number

What an AI chat would receive after masking

rodné číslo [RODNE_CISLO_1]

1 × Czech or Slovak birth number found and masked.

How the number is built and checked

A rodné číslo is YYMMDD followed by a four-digit ending, usually written with a slash. Women have 50 added to the month, so 855615/1230 is a woman born on 15 June 1985 (56 − 50 = 6). The ending tells apart people born on the same day. [1][2]

The only check: since 1954 the whole ten-digit number must be divisible by 11 without remainder. 8556151230 = 11 × 777 831 930. Numbers given before 1954 have nine digits and do not meet that condition, so there is nothing to test. [1][2]

In the Czech Republic, when the numbers for a birth date run out, the interior ministry issues a supplementary set with 20 added to the month for men and 70 for women. nullcipher accepts both offsets. [1]

One random ten-digit number in eleven is divisible by 11, so nullcipher masks a birth number only when rodné číslo, RČ or birth number stands within 40 characters before it, and only with a real date (years 54–99 read as 1954–1999, 00–53 as 2000–2053).

How nullcipher recognises it

YYMMDD/SSSC or ten digits, with a real date once the month is decoded (women + 50; the Czech supplementary series + 20 for men and + 70 for women) and the whole ten-digit number divisible by 11. Years 54–99 are read as 1954–1999 and 00–53 as 2000–2053; the nine-digit numbers given before 1954 carry no check and are not recognised. Masked with a context word — rodné číslo, RČ, birth number — within 40 characters before it.

This is the rule text of the “Czech or Slovak birth number (rodné číslo)” detector in the masking engine, copied from the engine when this page was built. Detectors run in a fixed order and, where two overlap, the earlier one wins.

Example (fake)

rodné číslo 855615/1230

nullcipher replaces it with [RODNE_CISLO_1]. Constructed for illustration: a woman born on 15 June 1985, with the ending chosen so the number is divisible by 11. Neither law publishes a specimen.

Why it matters in an AI chat

Czech law says only the holder may use their birth number, or someone a law allows to; using another person's birth number without that right is an offence. Where a law does allow it, the number sits in contracts and records next to the name.

It gives away the exact birth date and the sex. The Czech police name the birth number among the details identity thieves are after, because so many institutions use it to look people up.

Masked, the AI sees [RODNE_CISLO_1] and can still help with the rest of the text.

If it already leaked

  1. If documents or sensitive personal details were lost or stolen, go to the Czech police straight away, preferably where it happened, or call 158. [3]
  2. Watch for the signs the police list: payment cards you did not ask for, bank statements that stop arriving, unexplained trouble with offices, and demands to pay for things you never bought. [3]
  3. If it only went into an AI chat and there is no sign of misuse, delete the conversation — how, for each major assistant, is on the page listing every type. The birth number itself stays the same.

Sources

  1. Zákon č. 133/2000 Sb., o evidenci obyvatel a rodných číslech, § 13 — e-Sbírka — checked 2026-10-03.
  2. Zákon č. 301/1995 Z. z. o rodnom čísle — Slov-Lex — checked 2026-10-03.
  3. Ztráta identity — Policie České republiky, KŘP hlavního města Prahy (25 May 2010) — checked 2026-10-03.

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