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 Invoices are high-value documents. A single altered field could change a payment amount, tax calculation, client record, or audit trail. Most invoicing systems treat invoices as ordinary database records. That works for normal CRUD workflows, but it does not automatically prove that the invoice data being viewed today is the same data that was created and sent. Reinvoice adds an integrity layer. When an invoice is created, we sign the fields that define the invoice. Later, when someone verifies the invoice, we recompute the signature from the current data and compare it against the original stored signature. If the values do not match, the invoice is flagged. The Implementation The signature is stored in two places: on the invoice record in the database, and behind a public verification endpoint. import { createHmac , timingSafeEqual } from ' node:crypto ' ; const SIGNATURE_FIELDS = [ ' invoiceNumber ' , ' issuerName ' , ' clientName ' , ' totalAmount ' , ' currency ' , ' taxAmount ' , ' issuedAt ' , ' dueDate ' , ' lineItems ' , ' notes ' , ' subtotal ' , ' discountAmount ' , ' shippingAmount ' , ] as const ; export function generateInvoiceHash ( invoice : InvoiceData ): string { const payload = SIGNATURE_FIELDS . map (( field ) => { const value = invoice [ field as keyof InvoiceData ]; return ` ${ field } = ${ JSON . stringify ( value )} ` ; }). join ( ' | ' ); return createHmac ( ' sha256 ' , SIGNING_SECRET )