Sign/encrypt files with Nitrokey

What it says on the tin!
This issue is thematically well suited to the following discussions:
elementary/pantheon-agent-polkit#33
elementary/pantheon-agent-polkit#40
elementary/greeter#230
elementary/mail#345
https://github.com/elementary/switchboard-plug-onlineaccounts/issues/89
https://github.com/elementary/installer/issues/368
https://github.com/elementary/appcenter/issues/936

Unlike a Yubikey, it is Free Hardware and Free Software, which is mostly manufactured locally (in Berlin, Germany).

Here you can find more general information: https://www.nitrokey.com/ & https://github.com/nitrokey

This integration can also be used to decrypt your hard disks - see LUKS/LUKS2 - or as a solution for Two-factor authentication in the Installer and Switchboard Online Accounts Plug. Integration with Mail (to sign, encrypt and decrypt emails) and for installing Software with AppCenter it's also very useful.

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