idea: use openpgp (pgp / gnupg) for authentication

[ThomasWaldmann]

Original report by Thomas Waldmann (Bitbucket: thomaswaldmann, GitHub: thomaswaldmann).

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let the user sign or encrypt some challenge, check with his public key wether the response is valid.

[ThomasWaldmann]

Original comment by Thomas Waldmann (Bitbucket: thomaswaldmann, GitHub: thomaswaldmann).

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see also issue #380 - maybe easier.