What Is the e.firma?
The e.firma (formerly called FIEL) is the advanced electronic signature that Mexico's tax authority, the SAT, issues to individuals and companies to identify them legally in digital transactions. Think of it as the electronic equivalent of your handwritten signature: when you use it to sign a document or authenticate on a government portal, it carries the same legal weight as if you had signed in front of a notary. It's made up of three files that always travel together: a certificate (.cer), a private key (.key), and a password you set yourself when you request it.
Any individual or company that files taxes, issues invoices, handles a more complex tax procedure, or needs to interact formally with the SAT will eventually need a valid e.firma, though the most common use among businesses operating in Mexico is as the required first step to generate the certificate used to stamp electronic invoices.
e.firma vs. SAT Password: What's the Difference?
It's easy to confuse the e.firma with the SAT portal password (also called CIEC), but they serve different purposes:
- The password (CIEC) only gives you access to basic services inside the SAT portal, such as checking your tax mailbox or filing certain simple returns. It doesn't carry the legal weight of a signature.
- The e.firma does have the legal standing of a handwritten signature. It's required for higher-stakes procedures: filing complex annual tax returns, filings with other government agencies that accept the e.firma, requesting your tax status certificate with extra validations, or generating the certificate used to stamp your invoices.
In short: the password opens the door to the portal, but the e.firma is what legally identifies you when the procedure calls for it.
e.firma vs. CSD: Which One Do You Need to Invoice?
Another common point of confusion is the e.firma versus the Certificado de Sello Digital (CSD, or Digital Seal Certificate). They're separate documents with separate purposes, though one depends on the other:
- The e.firma is your electronic identity with the SAT for any procedure.
- The CSD is a specific certificate used only to stamp your electronic invoices, or CFDI. It has no use outside of invoicing.
The relationship between the two: you need a valid e.firma to generate the CSD through the SAT portal in the first place. If your e.firma has expired, you won't be able to generate or renew the CSD you use to stamp invoices, and your invoicing stops until you sort it out. You can read the full breakdown in our guide to what a CSD is.
Requirements to Apply for the e.firma for the First Time
If you've never had an e.firma before, the initial application requires an in-person appointment at a SAT office. There's still no fully online path for a first-time application, because the SAT needs to verify your identity in person before issuing the certificate. You'll need to bring:
- A valid official ID: INE (Mexican voter ID), passport, or professional license card.
- Your CURP (unique population registry code).
- A recent proof of address, no older than three months.
- A USB drive, ideally new or freshly formatted, where the system stores your .cer and .key files.
- A personal email address you can access, to receive confirmations about the process.
Before you go, book your appointment through the SAT portal, selecting the "e.firma for individuals" or "e.firma for legal entities" service, depending on your case. At the appointment, a SAT agent takes your biometric data (fingerprints and photo) and issues your certificate on the spot. If your RFC is already tied to an invoicing setup, this is also a good time to review the requirements to invoice in Mexico you'll need afterward.
How to Renew the e.firma Online
Unlike the first-time application, renewal can be done entirely online, as long as your e.firma is still valid or expired less than a year ago. If more than a year has passed since it expired, the SAT requires you to redo the in-person process from scratch, as if it were your first time.
Within that window, you have two free options:
- The Certifica program, available through the SAT portal: you download the application, generate a renewal file using your current e.firma (or CSD), and upload it to the system to receive your new certificate.
- The SAT ID app: you enter your RFC, email, and phone number, upload an official ID, and record a short video to verify your identity. The SAT reviews the request and issues the new certificate without you having to visit an office.
Either way, the new certificate keeps a four-year validity period from the renewal date. It's worth setting a reminder several months ahead of time, because an expired e.firma halts tax filings and CSD generation right when you least expect it.
What the e.firma Is Used for in a Business
Beyond the application itself, the e.firma has concrete, everyday uses for a company operating in Mexico:
- Generating and renewing the CSD used to stamp your CFDI electronic invoices.
- Filing tax returns that require an electronic signature, not just the portal password.
- Signing contracts and legal documents on platforms that accept an advanced electronic signature with legal validity.
- Handling procedures with other government agencies that also accept the e.firma, such as IMSS (social security) or certain filings with the Public Registry of Commerce.
- Authenticating on services that require your RFC validated directly against the SAT's own record, instead of relying on a password alone.
For a business that issues invoices regularly, keeping the e.firma current isn't a one-off task: it's the foundation that the rest of your electronic invoicing depends on, from the first CFDI to the last complement of the fiscal year.
Frequently Asked Questions
What Is the e.firma, and Is It the Same as the FIEL?
The e.firma is the advanced electronic signature the SAT issues to identify you legally in digital transactions. FIEL was the original name for this certificate (Firma Electrónica Avanzada), and the SAT renamed it e.firma several years ago, but it's the same document with the same function.
Can I invoice using only my e.firma?
Not directly. To stamp CFDI invoices you need the Digital Seal Certificate (CSD), which you generate using your valid e.firma as a prerequisite. The e.firma on its own doesn't stamp invoices.
What happens if my e.firma expires for more than a year?
You lose the option to renew it online and have to redo the entire in-person process at a SAT office, with an appointment and all the original documents, as if you never had one.
Does the e.firma cost anything?
No. The SAT does not charge for the initial application or the renewal, whether you renew in person, through the Certifica program, or through the SAT ID app.
Is the e.firma useful for anything besides tax matters?
Yes, although its most common use for businesses is tax-related. It's also accepted for procedures with other government agencies and on platforms that recognize an advanced electronic signature with legal validity.
If your business already has a valid e.firma but needs the rest of its invoicing process cleaned up — from generating the CSD to stamping and sending CFDI without manual errors — at AISDC we build CFDI electronic invoicing systems integrated into your daily operations, so the e.firma is the only step you ever have to remember on your own.