AutoFirma is a Java desktop application built by the Spanish government to let citizens sign documents and identify themselves on government websites.

Users load their digital certificates either directly into AutoFirma or into their OS/browser keystore, so that they can use them to sign or authenticate. When a website requires AutoFirma, it usually launches it through a deep link and then communicates with it through WSS (WebSocket plus TLS).

For the WebSocket to work, the browser needs to trust its TLS certificate. But AutoFirma communicates over localhost, which means it cannot ship a trusted leaf certificate. Instead, it messes with the local keychain (at least on macOS) to add a local certificate authority and a leaf certificate.

I don’t particularly like any of this. I don’t want it to add its own CA to my system trust store. Plus, macOS support is wonky.

Nowadays most government websites rely on TLS authentication, with the browser supplying the certificate. Others, however, require users to use AutoFirma to sign in or authorize operations. For those websites, there seems to be no way around AutoFirma.

To contain all this, I asked an AI to bundle a browser and AutoFirma into a Nix VM guest. The VM launches Firefox in a GUI and asks me to import a certificate into a tmpfs store. When websites require it, AutoFirma automatically opens and handles the signing.

The VM runs only essential services to keep the attack surface small. It builds in CI, so I can just download it and run it with QEMU or Baguette. The result is in this GitHub repository. It has made my life a bit easier.