8.1.5 Additional repositories
You may add more repositories, broadening your package choices beyond what is available in your target distribution. These may be, for example, for backports, experimental or custom packages. To configure additional repositories, create config/archives/your-repository.list.chroot, and/or config/archives/your-repository.list.binary files. As with the --mirror-* options, these govern the repositories used in the chroot stage when building the image, and in the binary stage, i.e. for use when running the live system.
For example, config/archives/live.list.chroot allows you to install packages from the debian-live snapshot repository at live system build time.
deb http://debian-live.alioth.debian.org/ sid-snapshots main contrib non-free
If you add the same line to config/archives/live.list.binary, the repository will be added to your live system's /etc/apt/sources.list.d/ directory.
If such files exist, they will be picked up automatically.
You should also put the ASCII-armored GPG key used to sign the repository into config/archives/your-repository.key.{binary,chroot} files. (highlighting by me)
Should you need custom APT pinning, such APT preferences snippets can be placed in config/archives/your-repository.pref.{binary,chroot} files and will be automatically added to your live system's /etc/apt/preferences.d/ directory.
Similarly, if you need custom APT_AUTH.CONF(5) authentication configuration, this can be placed in config/archives/your-repository.auth.{binary,chroot} files and will be automatically added to your live system's /etc/apt/auth.conf.d/ directory
I went here:
https://wiki.trinitydesktop.org/Debian_Trinity_Repository_Installation_Instructions#TDE_R14.x.y_series
I installed the file: trinity-keyring.deb
But the problem remains-------------------I used to build TDE in Live Build but haven't done so for a couple of years. Anyway you need to download the appropriate keyring for TDE from TDE itself, read the live build manual (it tells you exactly what to do) and place the relevant file(s) in the relevant place in the build environment. Live build will see the keyring, install it, read the repositories, and install the packages you tell it to install plus their dependencies and recommends if you allow installation of recommends.