PGPAuth

This app sends GPG-verified requests over the internet to a given server.

Currently there are the actions open and close, but request-type will be
configurable. The request also includes a timestamp to verify the request is
sent by the owner of the gpg-key and is not sent again by someone else.

It is used in the ChaosChemnitz hackerspace in Germany to open and close the
door without the need of a "real" key.

You'll need https://f-droid.org/packages/org.thialfihar.android.apg or
https://f-droid.org/packages/org.sufficientlysecure.keychain to sign requests.

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