verifactu in go: a sha-256 hash chain is spain's answer to invoice fraud

Wed, 08 Jul 2026 00:00:00 +0000

for years the spanish way to skim a business was the software de doble uso — a till program with a quiet second set of books. you ring up a sale, the customer walks off with their receipt, and then, later, the invoice simply… isn’t there anymore. the record gets deleted, edited, renumbered. the tax office (the aeat) shows up, asks for invoice 47, and invoice 47 was never born. good luck proving a negative.

spain’s answer, live and mandatory, is delightfully low-tech and i mean that as a compliment: make every invoice cryptographically point at the one before it, so deleting number 47 leaves a hole that screams.

spain on krtffl.dev

verifactu in go: a sha-256 hash chain is spain's answer to invoice fraud