How We Built Cryptographic Invoice Signatures for a SaaS Invoicing Platform

How Reinvoice Uses HMAC Signatures to Detect Invoice Tampering

Every invoice sent through Reinvoice includes a cryptographic integrity signature.

It is not a PDF stamp, a visual badge, or a checkbox. It is an HMAC-SHA256 hash generated from the invoice payload and a server-side signing secret. If signed invoice data changes after creation, Reinvoice can recompute the hash, compare it to the stored signature, and flag the invoice as potentially tampered with.

Here is why we built it, how it works, and what we learned.

Why Integrity Checks Matter for Invoicing