createLastDeviceRecoveryKey

Creates a new offline last-device recovery key for this user (docs/last-device-recovery.md). Returns it to the application and stores nothing: the application backs it up offline (for example LastDeviceRecoveryKey.encode printed or in a password manager) and registers it with registerLastDeviceRecoveryKey. Whoever holds it can replace the server authentication key of every registered device of the user. No network I/O.