I'd also wonder where this shared encryption key for message "backups" is stored. If it's available on all of my devices, I suspect it would be available on other devices as well?
At the core of secure backups is a 64-character recovery key that is generated on your device. This key is yours and yours alone; it is never shared with Signal’s servers. Your recovery key is the only way to “unlock” your backup when you need to restore access to your messages. Losing it means losing access to your backup permanently, and Signal cannot help you recover it. You can generate a new key if you choose. We recommend storing this key securely (writing it down in a notebook or a secure password manager, for example).