Notes for GnuPG users on several platforms
Always use a current version of GnuPG Version >= 2. (There are only a few rare exceptions to this rule.) GnuPG 2.1.x is "modern", not all applications may already support the new features fully. So when in doubt, use GnuPG 2.0.x.
gnome-keyring or seahorse may pose as gpg-agent, but known versions today (2015-03) are not fully compatible with the gpg-agent protocol. So applications may show errors or cannot work when they believe they are talking to gpg-agent and try to use its features. Examples: ERR 280 not implemented or a protocol error in the Kleopatra self-tests.
You can detect the situation with GnuPG 2.0.x when checking the environment variable on a shell like echo $GPG_AGENT_INFO. You are talking to a gpg-agent for good, if it has something with gpg-agent in it and a corresponding gpg-agent process is running. Otherwise the connection has probably been taken over.
Note that the gnupg2 Package may search for files in paths that differ from the paths of a vanilla build and what its written in the documentation.
E.g: The gpgsm[2.0.19-2+deb7u1] in Wheezy looks in /etc/gnupg2/trustlist.txt but the docs differ (Debian defect #725804).
Mac OS X
- https://guardianproject.info/ , they have experimental builds of a GnuPG port, see https://guardianproject.info/2013/05/09/gnupg-for-android-progress-we-have-an-app/
- See APG and OpenPGP-Keychain via OtherFreeSoftwareOpenPGP