Sign your agreements with the SAT e.firma (FIEL)

How it works

1. Propose the agreement and check "Both sign with e.firma," or reinforce one already signed.
2. Sign with your code, then choose your .cer, your .key and enter your password.
3. Your browser signs; only the signature and public certificate are sent.
4. We check the certificate's status with the SAT and keep its answer.
5. Download the evidence package as a ZIP.

On Multilistado you can sign your commission agreements with your SAT e.firma (FIEL), on top of the code sent to your phone. Before we accept the signature, we ask the SAT whether your certificate is in force and keep its signed answer. You then download an evidence package that an expert witness or mediator can check without relying on Multilistado. It's free and optional.

What it is and what it's for

The e.firma is the certificate Mexico's tax authority (SAT) issues to each taxpayer: a .cer file (public), a .key file (private) and a password. It's an advanced electronic signature. A code by text or email shows someone had access to that phone or inbox; the e.firma ties the signature to a certificate the SAT issued under a person's name and RFC.

On Multilistado it does two things:

• Reinforce your signature on any agreement already signed with a code (optional).
• Require it from both parties when you propose the agreement, by checking "Both sign with SAT e.firma (FIEL)." The agreement is then signed only once both agents have signed with their code and their e.firma.

It works on agreements between Multilistado agents and on agreements with outside agents.

How it works

1. Sign the agreement with your code, as usual.
2. In the e.firma box, choose your .cer file and your .key file and enter your password.
3. Signing happens in your browser. Your .key and password never leave your device; we only receive the signature and your public certificate.
4. We check that the certificate is an e.firma issued by a SAT certificate authority, that it's within its validity dates, that it's not a Digital Seal Certificate (CSD, which is only for invoicing) and that the signature matches the certificate.
5. We ask the SAT for the certificate's status (OCSP). If the SAT reports it revoked, the signature is rejected. If the SAT doesn't answer right then, the signature is saved as "check pending" and checked again later.
6. If the name on the certificate doesn't match your account, the agreement flags it.

What you sign with the e.firma includes the agreement text's SHA-256 fingerprint and your code signature, so it can't be moved to another document. When it's required from both parties, the agreement includes an advanced electronic signature clause and, once all signatures are in, the sealed record (text, signatures and SAT answers) gets an RFC 3161 time stamp.

The evidence package

From the agreement you download a ZIP with:

• The exact agreement text and a summary of signers, certificates, fingerprints, dates and SAT status.
• For each signer: their certificate, the exact text they signed, the signature, the SAT's answer on the certificate status (signed by the SAT) and the certificate of the SAT authority that issued it.
• The RFC 3161 time stamp.
• A README with the OpenSSL commands to verify everything.

Legal standing

Mexico's Commercial Code (articles 89 to 114) governs electronic signatures in commercial transactions and gives an advanced or reliable electronic signature the same effect as a handwritten one when it meets the code's requirements. Signing with the e.firma, backed by the SAT's answer and a time stamp, is stronger proof of who signed and when than a text-message code.

There are limits. The e.firma doesn't replace the notarized deed for the sale itself, and how an agreement is weighed in court depends on its content and the judge. This isn't legal advice: for high-value deals, have your lawyer review the agreement.

Requirements

• A verified Multilistado account (sign up or claim your profile).
• A current e.firma: .cer, .key and password. If it's expired or revoked, renew it with the SAT.
• An up-to-date browser on a computer or phone with access to your files.

Tips

• Use the e.firma on large deals or when you don't know the other agent.
• Give the other agent a heads-up before you require it, so they have their files handy.
• Keep the evidence package with the deed and your payment receipts.
• See also how to document a shared commission.

Frequently asked questions

Are my .key file or password uploaded to Multilistado?

No. Signing happens in your browser; we only send the signature and your public certificate.

What if my e.firma is expired or revoked?

The signature is rejected and you'll need to renew it with the SAT. If the SAT doesn't answer right then, it's saved as a pending check and retried.

Can I use my digital seal certificate (CSD)?

No. The CSD is only for invoicing; you need the e.firma (FIEL).

Is it legally valid?

Mexico's Commercial Code (arts. 89–114) recognizes advanced electronic signatures in commercial transactions, but how it's weighed depends on the case and the judge. This isn't legal advice; consult your lawyer on large deals.

How much does it cost?

Signing with the e.firma and downloading the evidence package is free.