The OTA firmware update keys ideally shouldn't be the same as the secure boot keys.

> Forcing the release of signing keys would be a security disaster. The first person to grab the expired domain for the auto update server for a IoT device now gets a free botnet.

Have you seen the state of embedded device security? It is already an unmitigated disaster.

Since you bring up botnets, there are far more exploited security vulnerabilities because a vendor EOLed support (or went bankrupt) and their firmware contained bugs that cannot be fixed because a signed firmware is required, or the source code was not provided than because their signing keys were leaked and someone is distributing malicious updates.

> Forcing vendors to release their security mechanisms to the public and allow anyone to sign firmware as the company is not what you want, though.

Yes, it is what I want. I am perfectly aware of the potential downsides and what I am proposing is worth it. The product is already EOL. In our current era of enshittification, vendor pinky promises to implement a user-bypass in their signed boot chain is not good enough. Look at the Other OS controversy on the PS3 if you want an example of this in practice, or Samsung removing bootloader unlocking in their One UI 8.0 update.

> The only real way to make devices securely re-usable with custom firmware requires some explicit steps and action to signal that the user wants to run 3rd-party firmware: A specific button press sequence is enough. You need to require the user to do something explicit to acknowledge that 3rd-party software is being installed, though.

The vendor has implemented an internal pad on the laser-welded, weather sealed, IP-rated smart watch that must be shorted to disable secure boot. Opening the device to access this will essentially destroy it, but we preserved the vendor's secure boot signing keys so missioned accomplished!