GnuPG
=====

Introduction
------------

PGP and GPG
~~~~~~~~~~~

`Gnu Privacy Guard`_ (GnuPG) is an Open Source implementation of the
`Pretty Good Privacy`_ (OpenPGP) specification.
Its main purposes are digital authentication, signature and encryption.

It is often used by the `FLOSS`_ community to verify:

* Linux package signatures: Debian `SecureApt`_, ArchLinux `Master Keys`_
* `SCM`_ releases & maintainer identity

.. _FLOSS: https://en.wikipedia.org/wiki/Free_and_open-source_software
.. _Gnu Privacy Guard: https://gnupg.org/
.. _Master Keys: https://www.archlinux.org/master-keys/
.. _Pretty Good Privacy: https://en.wikipedia.org/wiki/Pretty_Good_Privacy#OpenPGP
.. _SCM: https://en.wikipedia.org/wiki/Revision_control
.. _SecureApt: https://wiki.debian.org/SecureApt

Trust
~~~~~

To quote Phil Pennock, the author of the `SKS`_ key `server <http://sks.spodhuis.org/>`_:

.. pull-quote::

   You MUST understand that presence of data in the keyserver (pools) in no way connotes trust.
   Anyone can generate a key, with any name or email address, and upload it.
   All security and trust comes from evaluating security at the “object level”,
   via PGP Web-Of-Trust signatures.
   This keyserver makes it possible to retrieve keys, looking them up via various indices,
   but the collection of keys in this public pool is KNOWN to contain malicious
   and fraudulent keys.
   It is the common expectation of server operators that users understand this
   and use software which, like all known common OpenPGP implementations,
   evaluates trust accordingly.
   This expectation is so common that it is not normally explicitly stated.

Trust can be gained by having your key signed by other people
(and signing their keys back, too :-) ), for instance during `key signing parties`_:

* `The Keysigning Party HOWTO
  <http://www.cryptnet.net/fdp/crypto/keysigning_party/en/keysigning_party.html>`_
* `Web of Trust
  <https://en.wikipedia.org/wiki/Web_of_trust>`_

.. _key signing parties: https://en.wikipedia.org/wiki/Key_signing_party
.. _SKS: https://bitbucket.org/skskeyserver/sks-keyserver/wiki/Home

Generate a GPG key
------------------

* `Generating a GPG key for Git tagging <http://stackoverflow.com/a/16725717>`_ (StackOverflow)
* `Generating a GPG key <https://help.github.com/articles/generating-a-gpg-key/>`_ (GitHub)

``gpg`` - provide identity information
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: bash

   $ gpg --gen-key

   gpg (GnuPG) 2.1.6; Copyright (C) 2015 Free Software Foundation, Inc.
   This is free software: you are free to change and redistribute it.
   There is NO WARRANTY, to the extent permitted by law.

   Note: Use "gpg2 --full-gen-key" for a full featured key generation dialog.

   GnuPG needs to construct a user ID to identify your key.

   Real name: Marvin the Paranoid Android
   Email address: marvin@h2g2.net
   You selected this USER-ID:
       "Marvin the Paranoid Android <marvin@h2g2.net>"

   Change (N)ame, (E)mail, or (O)kay/(Q)uit? o
   We need to generate a lot of random bytes. It is a good idea to perform
   some other action (type on the keyboard, move the mouse, utilize the
   disks) during the prime generation; this gives the random number
   generator a better chance to gain enough entropy.

``gpg`` - entropy interlude
~~~~~~~~~~~~~~~~~~~~~~~~~~~

At this point, you will:

* be prompted for a secure password to protect your key
  (the input method will depend on your Desktop Environment and configuration)
* be asked to use your machine's input devices (mouse, keyboard, etc.)
  to generate random entropy; this step *may take some time*

``gpg`` - key creation confirmation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: bash

   gpg: key A9D53A3E marked as ultimately trusted
   public and secret key created and signed.

   gpg: checking the trustdb
   gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
   gpg: depth: 0  valid:   2  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 2u
   pub   rsa2048/A9D53A3E 2015-07-31
   Key fingerprint = AF2A 5381 E54B 2FD2 14C4  A9A3 0E35 ACA4 A9D5 3A3E
   uid       [ultimate] Marvin the Paranoid Android <marvin@h2g2.net>
   sub   rsa2048/8C0EACF1 2015-07-31

``gpg`` - submit your public key to a PGP server
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code-block:: bash

   $ gpg --keyserver pgp.mit.edu --send-keys A9D53A3E
   gpg: sending key A9D53A3E to hkp server pgp.mit.edu