Bug 1408323

This does not look like something that can be fixed without introduction of possibly serious security issues.  However, it does look like it can be avoided by detection then selection by the user of a desired action.

At step 5 (above) the dialogue routine can examine the boot configuration for offline update to learn whether LUKS pass phrases will be necessary. If yes, a message then instructs the user either to choose Restart (to reboot and perform the update now) or to confirm Shut Down and do that immediately without updates, which then will be performed at the next boot.

(In reply to Richard Ryniker from comment #1)
> This does not look like something that can be fixed without introduction of
> possibly serious security issues.  However, it does look like it can be
> avoided by detection then selection by the user of a desired action.
> 
> At step 5 (above) the dialogue routine can examine the boot configuration
> for offline update to learn whether LUKS pass phrases will be necessary. If
> yes, a message then instructs the user either to choose Restart (to reboot
> and perform the update now) or to confirm Shut Down and do that immediately
> without updates, which then will be performed at the next boot.

I think my expected result described above would be better, because
1. You've updated
2. You've shut down not restarted
Whereas what you are describing won't update until way later when you are ready to restart. I can already see a concern in that, will the PC remember to update? Will the updates still be the newest? 
Regards.

It looks like the
4.Check the little box to install the updates before switching off
mechanism is intended to give a non-privileged user the ability to update a machine.  In this case, your expected result "it should bring the administrator password popup" is not reasonable - the current user may not be an administrator.

I suggest the use of "offline update" mode is intended to avoid possible conflicts between updated and currently running code that an administrator may handle, but the unprivileged user has neither the knowledge nor the authority to address.

Offline update is already a fragile mechanism in the sense that it is just one of multiple factors that control the boot process - the next boot might start a different operating system.

The question of what updates should actually be installed when offline update runs can have multiple answers, but "the updates staged at the time offline update was requested" makes more sense to me than "whatever updates are available when offline update runs."  If more updates are available when offline update runs, another iteration through the update process can install them if they are desired.

I see what you're saying.
Probably the administrator check is not a must, but it is probably a good idea since the system is still live.
Or maybe it would go into the shutdown blackscreen, and update there before shutting down. 
The idea being it updates first, then shuts down, no restart.

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