D. R. Evans wrote:

https://wiki.trinitydesktop.org/Preliminary_Stable_Builds

Packages are signed using GPG key ID A04BE668. To install the GPG key you
can use any public GPG keyserver, for example:

sudo apt-key adv --keyserver keys.gnupg.net --recv-key A04BE668

regards

> I am trying to install trinity for the first time on my stretch system[*],
> following the instructions at
> https://wiki.trinitydesktop.org/DebianInstall.
> 
> I added the Preliminary Stable Builds sources to sources.list.
> 
> The GPG key was added OK:
> 
> ----
> 
> sudo apt-key adv --keyserver keyserver.quickbuild.io --recv-keys F5CFC95C
> Executing: /tmp/apt-key-gpghome.STqKpDhXXj/gpg.1.sh --keyserver
> keyserver.quickbuild.io --recv-keys F5CFC95C
> gpg: key 96C95152F5CFC95C: public key "QuickBuild Trinity Desktop
> Environment" imported
> gpg: Total number processed: 1
> gpg:               imported: 1
> 
> ----
> 
> But then:
> 
> [HN:apt] sudo apt-get update
> Ign:1 http://ftp.us.debian.org/debian stretch InRelease
> 
> Hit:2 http://ftp.us.debian.org/debian stretch-updates InRelease
> 
> Hit:3 http://ftp.us.debian.org/debian stretch Release
> 
> 
> Get:4 http://security.debian.org stretch/updates InRelease [62.9 kB]
> 
> Hit:6 http://download.virtualbox.org/virtualbox/debian stretch InRelease
> 
> Get:7 ftp://ftp.deb-multimedia.org stretch InRelease [33.4 kB]
> Get:8 http://mirror.xcer.cz/trinity-sb stretch InRelease [46.3 kB]
> Ign:8 http://mirror.xcer.cz/trinity-sb stretch InRelease
> Get:9 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 Sources [8,082 B]
> Get:10 http://mirror.xcer.cz/trinity-sb stretch/main-r14 Sources [58.3 kB]
> Get:11 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 i386 Packages
> [17.0 kB] Get:12 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 amd64
> Packages [17.0 kB] Get:13 http://mirror.xcer.cz/trinity-sb
> stretch/deps-r14 amd64 Contents (deb)
> [9,590 B]
> Get:14 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 i386 Contents
> (deb)
> [9,580 B]
> Get:15 http://mirror.xcer.cz/trinity-sb stretch/main-r14 i386 Packages
> [201 kB] Get:16 http://mirror.xcer.cz/trinity-sb stretch/main-r14 amd64
> Packages [201 kB] Get:17 http://mirror.xcer.cz/trinity-sb stretch/main-r14
> i386 Contents (deb)
> [655 kB]
> Get:18 http://mirror.xcer.cz/trinity-sb stretch/main-r14 amd64 Contents
> (deb)
> [654 kB]
> 
> 
> Fetched 1,972 kB in 8s (238 kB/s)
> 
> 
> 
> Reading package lists... Done
> W: GPG error: http://mirror.xcer.cz/trinity-sb stretch InRelease: The
> following signatures couldn't be verified because the public key is not
> available: NO_PUBKEY 608F5293A04BE668
> W: The repository 'http://mirror.xcer.cz/trinity-sb stretch InRelease' is
> not signed.
> N: Data from such a repository can't be authenticated and is therefore
> potentially dangerous to use.
> N: See apt-secure(8) manpage for repository creation and user
> configuration details.
> 
> ----
> 
> I don't want to install anything that can't be verified. What do I need to
> do proceed?
> 
>   Doc
> 
> [*] Anyone who has tried to use the KDE 5 that is included with stretch
> [will
> know why I've finally given up on the KDE death-spiral and want to get
> back to something that works properly. I would have been here long ago if
> TDE were an official part of debian, but KDE 5 is so painful and
> bug-ridden that I am now forced to overcome my deep-rooted distrust of
> software that is not in the official debian repositories.
>