Unable to decrypt cause: new device, and there were no active devices to receive keys

[richvdh]

When a user logs in on a new device, they should be able to retrieve keys for existing encrypted messages from key backup.

However, this will not work if there were no active devices at the point that the keys for the message were sent. For example:

• The user previously logged out all other devices, or:
• The user suspended down all other devices (eg, closed their laptop/backgrounded the app on their phone), or:
• The keys were sent seconds before the new device logged in, meaning that the keys had not yet been uploaded to backup at the point the new device checked.

All of these problems should be solved by dehydrated devices (MSC3814).

[richvdh]

See also element-hq/element-web#27267 which exacerbates this, though it's far from the main cause.

[richvdh]

A more specific example scenario for this:

• User shuts down their work computer at 17:00 Friday
• At 10:00 Saturday user gets a call about an urgent issue; they log in on their mobile device
• They are unable to decrypt any messages between 17:00 and 10:00, which includes important information about the urgent issue.