FA-72744 / Check-digit algorithms / Member archive

CPF accepts repeated-digit numbers · case 04

Placeholder numbers such as 111.111.111-11 pass both check digits.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Brazilian CPF taxpayer number: dots and hyphens removed, then 11 ASCII digits that are not all the same digit (else "malformed"). k1 = (sum of digits 1-9 weighted 10..2) * 10 mod 11 mod 10; k2 = (digits 1-9 and the computed k1 weighted 11..2) * 10 mod 11 mod 10. Return [k1, k2, whether digits 10-11 equal them].

Why this case matters

Onboarding and invoicing flows validate CPF numbers before tax reporting.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression ["111.111.111-11"]	`[1, 1, true]`	`"malformed"`	Failed

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