Test Signal in Tails

There is currently no official Debian package for Signal.

Our options seem to be:

use the Flatpak as documented on https://bisco.org/notes/installing-and-running-signal-on-tails/ → Tails is not 100% ready for this yet
use the upstream Debian repository → that’s putting a lot of trust in Signal upstream and infra; most of the reasons why we’ve rejected the Electrum AppImage idea probably apply here

Parent Task: #14567

Original created by @u on 15200 (Redmine)

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