SAT e.firma (Electronic Signature): What It Is, How to Get It, and What It's Used For

The e.firma (formerly known as FIEL — Advanced Electronic Signature) is the most important digital identity instrument for any taxpayer in Mexico. Issued by the SAT, it allows signing electronic documents with legal validity, performing advanced tax procedures, and securely authenticating on government portals.

What Is the e.firma?

The e.firma is a set of digital files (a .cer certificate and a .key private key) that, together with a password, form the taxpayer's advanced electronic signature. It functions as a digital equivalent of a handwritten signature and has full legal validity under Mexico's Federal Tax Code.

Unlike the SAT Password (formerly CIEC), which only allows basic procedures on the SAT portal, the e.firma enables operations of greater complexity and security.

Difference Between e.firma and SAT Password

| Feature | SAT Password | e.firma |

|---|---|---|

| Security level | Basic | Advanced |

| Validity | Indefinite | 4 years |

| Available procedures | Limited | Complete |

| Document signing | No | Yes, legally valid |

| CFDI issuance | Not directly | Yes |

| Obtainment | Online | In-person at SAT offices |

What Is It Used For?

The e.firma is required for multiple operations:

Electronic invoicing: It is necessary to obtain the Digital Seal Certificate (CSD) used to issue CFDI.

Tax filings: It allows submitting annual and supplementary returns.

Activating the [Tax Mailbox](/blog/buzon-tributario-sat): Mandatory for receiving official SAT notifications.

Obtaining the [Tax Compliance Opinion](/blog/opinion-cumplimiento-sat): Needed to query and authorize access to this document.

Online procedures: Changes to tax regime, RFC data updates, refund requests.

Signing electronic contracts: With legal validity equivalent to a handwritten signature.

Access to government portals: IMSS, INFONAVIT, and other agencies accept e.firma as an authentication method.

How to Obtain the e.firma

Requirements:

Valid official ID (INE, passport)

Recent proof of address

CURP

USB device to store the files

Personal email address

Process:

Schedule an appointment on the SAT portal (sat.gob.mx > Appointments)

Visit the SAT office on the scheduled date

Bring the required documentation

The SAT will capture your biometric data (fingerprints, photo, signature)

Your files (.cer and .key) will be generated along with the private key password

Save the files on your USB — they are your digital identity

e.firma Renewal

The e.firma has a validity of 4 years. You can renew it:

Online if your e.firma is still valid or expired less than a year ago

In person at SAT offices if it expired more than a year ago

For online renewal you need your valid certificate (.cer), private key (.key), and password.

Security Best Practices

Never share your private key (.key) or password with third parties.

Back up your files on a secure medium. If you lose them, you'll need to visit SAT for replacement.

Revoke your e.firma immediately if you suspect it has been compromised.

Check the validity periodically to avoid interruptions in your procedures.

Importance in the Financial Ecosystem

For financial institutions, the e.firma represents an additional layer of identity verification. A taxpayer with a valid e.firma:

Has a biometrically verified identity with the SAT

Can perform complex tax procedures, indicating a level of business formality

Is enabled to issue electronic invoices and comply with tax obligations

At CRiskCo, we integrate RFC validation and Tax Status Certificate checks as part of our automated analysis, verifying that evaluated companies maintain an active and updated fiscal identity.

---

Want to automate fiscal identity verification for your clients? [Explore our credit risk solutions](/solutions/credit-risk).