linux: added gpg and links

wiki/linux/gpg.md

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+# GPG
+
+[GNU Privacy Guard](https://gnupg.org/) - short GnuPG or just GPG - is a free and open-source
+implementation of OpenPGP from [RFC 4880](https://datatracker.ietf.org/doc/html/rfc4880).
+It is used to savely encrypt and decrypt messages or files using assymetric encryption.
+
+## Setup
+
+On most linux distributions GPG can be installed with the `gnupg` package.
+
+## Usage
+
+### Generating a Key Pair
+
+A fast way to generate a key pair is by running the following command.
+
+```sh
+gpg --generate-key
+```
+
+### Backing Up and Exporting Keys
+
+A total backup or the export of all private, public and subkeys can be achieved by running the
+following commands.
+Replace `<id>` with the identifier of the key that should be backed up.
+
+```
+gpg --export --armor <id> > <id>.pub
+gpg --export-secret-keys --armor <id> > <id>.prv
+gpg --export-secret-subkeys --armor <id> > <id>_priv.asc
+gpg --export-ownertrust > ownertrust.txt
+```
+
+Especially the first command can be run in individually to export public keys without owning the
+private key.
+
+### Import Keys
+
+Keys that have been imported in the way described in
+[the previous section](#backing-up-and-exporting-keys) can be imported by the following commands.
+Replace `<id>` with the identifier of the key that should be imported.
+
+```sh
+gpg --import <id>.pub
+gpg --import <id>.priv
+gpg --import <id>_priv.asc
+gpg --import-ownertrust ownertrust.txt
+```
+
+Again the first command can be run individually to import single public keys that are not owned.
+
+Afterwards the following commands can be run to set the ultimate trust level to the key pair.
+Caution is advised.
+
+```sh
+gpg --edit-key [email protected]
+gpg> trust
+Your decision? 5
+```
+
+### Encrypting Files and Other Messages
+
+A file - in this case named `message.txt` - can be encrypted using the a specific key with the
+following command.
+Replace `<id>` with the identifier of the key that should encrypt the message.
+
+```sh
+gpg -e -r <id> message.txt
+```
+
+This command will create a file with the same name as the input file but with an added `.gpg` - in
+this case its called `message.txt.gpg`.
+
+### Decrypt Files and Other Messages
+
+A file - in this case named `message.txt.gpg` - can be decrypted with the following command.
+
+```sh
+gpg --decrypt message.txt.gpg
+```
+
+This command will output the content of the file.
+
+### Restart the GPG Server
+
+The GPG server can be restarted by simply running the following command.
+
+```sh
+gpgconf --kill all
+```
+
+### Receive a Key and Trust It
+
+If importing a key does not properly work it can be useful to manually receive and trust a key.
+This is done by running the following lines:
+Caution is advised.
+Replace `<id>` with the identifier of the key that should be received and trusted.
+
+```sh
+gpg --recv-key <id>
+gpg --lsign <id>
+```