Dear Maintainer,

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Have this bug too, started from Jessie, now also on clean-insalled Stretch
(actually, had to clean install after screwing the system trying to resolve
this).

Error: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file
is not readable by user '_apt' executing apt-key.

Allegedly caused by adjusting the repositories vis Synaptic's "Settings ->
Repositories" menu.

More:

https://lists.debian.org/debian-user/2017/07/msg00219.html

https://www.reddit.com/r/debian/comments/6dwicb/apt_aptkey_on_stretch_broken/

https://www.reddit.com/r/debian/comments/5yxyp9/stretch_aptget_update_errors/

I've tried to have a first look at it. So I've installed the nonfree
DVD-Image 9.2 within a VirtualBox container. All went fine, until I've
tried to update the APT database. Both, apt and Synaptics update fails.
In both cases the error reports, that /etc/apt/trusted.gpg is not
accessible by user "_apt".

The recommendation I've found was to delete trusted.gpg.

OK, this was a workaround for using apt. Both update and upgrade works
without errors. Trying Synaptic leads to the previous behavior. The file
trusted.gpg appears again and with this file, the error returns.

It is hard to believe that this bug is known since June and that there
is neither a fix nor a solution. Deleting the file has worked for me,
but I can't say that there are no side effects.

How should a newbie solve this? Without a work-around there are no
updates possible.