Mexico's e.firma (SAT) vs. electronic signature: which one you need

In Mexico, "electronic signature" usually brings to mind the SAT's e.firma (formerly FIEL). That's why many people think they need one to sign a contract remotely. They don't: they're two different tools with different uses.

It's an advanced electronic signature issued by the SAT, Mexico's tax authority: a digital certificate (.cer file), a private key (.key file) and a password. You apply in person at a SAT office, by appointment, with official ID and biometric data, and it's valid for four years.

It's mainly for tax and government procedures: tax returns, filings with the SAT, some IMSS and other agency procedures, and setting up electronic invoicing.

It's what you use to sign private agreements: service contracts, leases, NDAs, quotes, employment contracts. Mexico's Commercial Code has recognized it since the 2003 reform, and its evidentiary weight comes from the record of who signed, when and how their identity was verified.

It needs no prior application: the signer gets a link, signs on their phone and confirms with a code. Platforms like Firmiu also add a certificate with the document's SHA-256 fingerprint and public verification by reference number.

When the procedure is with the SAT or another agency that explicitly requires it, or when a law or institution demands an advanced electronic signature. In those cases there's no substitute.

For the vast majority of contracts between individuals and businesses. Asking your client for their e.firma to sign a quote or a service contract is impractical — not everyone has a valid one, and using it on another platform means handling their private key — and in most cases it isn't necessary.

A good rule: e.firma for the government, an electronic signature with an audit trail for your private agreements. And if the contract is high-value or there's doubt, check with your lawyer or accountant.