When I try to sign, it says that the .cer or .key file I selected do not correspond to an e.firma issued by the SAT

The .cer file you selected does not belong to an e.firma issued by the SAT

To sign documents with e.firma, Mifiel only supports public certificates (.cer files) of advanced electronic signature issued by the SAT. If you try to use a different certificate such as HTTPS/SSL certificates used on websites, you will not be able to sign your document in Mifiel.

You selected a file with the extension .key that does not belong to an e.firma issued by the SAT

For the private key, we only support .key files for advanced electronic signatures issued by the SAT. For example, if you try to use a .key file from a Keynote presentation, you won’t be able to sign.

What to do if your files are corrupted

Both files can become corrupted if they’re handled incorrectly, for example, if you try to open them with the wrong program.

Make sure you’re using .cer and .key files that belong to an e.firma and aren’t corrupted. If your .cer file is corrupted, you can download it again from the SAT website. However, if your .key file is corrupted and you don’t have an intact backup, you’ll need to apply for a new e.firma at a SAT office.