What restrictions can I place on a device whose software is licensed using GPL v3?

Question

Let's say I'm selling a device with my software pre installed whose license is GPL v3. I want to prevent an unauthorized OS from being installed. According to GPL v3, I will have to share the private key with the user that can sign the image that will be accepted by the device.

My question is, when am I required to share the key? Can I share the key only when the user requests it? Can I stop sending updates and stop providing phone support if the user asks for the signing key? (Assuming it's already part of terms and conditions)

Answer 1

GPLv3 §6 calls the

methods, procedures, authorization keys, or other information required to install….

the Installation Information. The next paragraph requires that when you (paraphrasing) sell a device with the software installed, you must accompany it with the Installation Information.

So, the answer to when is: it must accompany the device.

The next paragraph explicitly says you do not need to

provide support service, warranty, or updates for a work that has been modified or installed by the recipient, or for the User Product in which it has been modified or installed.

So you do not need to provide updates or support.

For example, you could probably have some "unlock" procedure the user must go through, that would indicate the device is no longer supported. But you must not "break" the device just because it's unlocked, GPLv3 prohibits that. Talk to a lawyer to review your specific plans.