4 GnuPG version compatibility

As of February 2016, there are three active branches of GnuPG: 2.1, 2.0, and 1.4. All those branches should work flawlessly with Emacs with basic use-cases. They have, however, some incompatible characteristics, which might be visible when used from Emacs.

• The key store format used by GnuPG 2.1 is incompatible with 1.4. That means, a key created with GnuPG 2.1 is not visible with 1.4.
• GnuPG 2.1 uses a fixed address for the Unix domain socket used to communicate with gpg-agent. The GPG_AGENT_INFO environment variable, which is used by GnuPG 2.0 and 1.4, is ignored. That means, if your system has both GnuPG 2.1 and 1.4, the gpg command from GnuPG 1.4 is not able to use gpg-agent provided by 2.1 (at least out of box).
• GnuPG 2.1 (2.1.5 or later) has a mechanism to direct the Pinentry password prompt to the Emacs minibuffer¹, which would be useful when you use Emacs remotely or from a text-only terminal. That feature is not available in other versions, and more specifically, with 2.0 (as of 2.0.29), there is no way to avoid the graphical prompt.