The default file association for `.7z` files in KDE Linux is Flatpak Ark. However, Flatpak Ark on KDE Linux can't properly handle encrypted 7z files - probably because the 7z plugin is unavailable (it shows as greyed out in the Plugins screen). It looks like the libarchive plugin is used instead, but this fails when used on an encrypted 7z file. This doesn't seem to be a generalised problem with Flatpak Ark - for example, on Fedora, it does have the 7z plugin available and is able to open encrypted 7z files. KDE Linux does in fact include the non-Flatpak Ark, and this works fine for encrypted 7z files. However, the non-Flatpak Ark isn't visible to a GUI user (the desktop file at `/usr/share/applications/org.kde.ark.desktop` is masked by the Flatpak one). So this isn't a great UX because: (1) there's no indication in the app launcher that the non-Flatpak Ark exists (2) in File Associations in the System Settings, the non-Flatpak Ark doesn't show as a choice in the tree view. (You can create the association by typing `/usr/bin/ark` in there.) (3) even if both apps were visible and the default file associations were directed appropriately, it still wouldn't be very obvious how you should open your encrypted 7z file.