Email encryption using GPG/PGP on Linux and Android

It is the identifier of the created key it will be different for each person (i used random letters as an example but they are that length for me at least).
It is so applications know your preferred key in the event you have multiple secret keys created and stored in your secure keyring.

I too use KDE, and found on both Fedora 19 and Slackware two files 'missing' when attempting to seamlessly use gpg. KDE seems to have full support for nicely integrated use of gpg, but on both distributions, there seems to be a missing link when attempting to actually do that.

Do feel free to correct my misunderstandings...

The files included below are shell scripts. The indentation was lost in the cut & paste action, to this forum, so please forgive the obfuscated code, it is not intentional.

What these files do is that they start/stop the gpg-agent program for each user that logs in/out, and make sure that the communication socket of the agent is known to all programs for that user that want to access it. The scripts do this by setting, at an early stage in the start of KDE, an environment variable that points to the socket of gpg-agent for that user.

Without this -to name a thing- kleopatra will report errors when started, and is unable to manage/generate keys at all. 
Without this, kmail will not be able to encrypt/decrypt mails.

Adding these two files and logging out/in will remedy both these problems. With both files installed, you will still have to do 'the usual' such as actually generate keys, configure kmail to actually use them, but that all is well 'covered' in the manuals of the respective programs. The little trick mentioned here is not.

This to the best of my knowledge, feel free to enlighten me if i'm mistaken.

On Fedora, these files reside in /etc/kde/env/start-gpg-agent.sh and /etc/kde/shutdown/stop-gpg-agent.sh respectively, and are thus available/use to/by all users.
On Slackware i've put them in ~/.kde/env/start-gpg-agent.sh and ~/.kde/shutdown/stop-gpg-agent.sh respectively, an they are present for the current user only.

Both installation locations seem to work well. I have yet to check out if Slackware even *has* a /etc/kde directory...

start of gpg-agent
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#!/bin/sh

GPG_AGENT=/usr/bin/gpg-agent
## Run gpg-agent only if not already running, and available
if [ -x "${GPG_AGENT}" ] ; then

# check validity of GPG_SOCKET (in case of session crash)
GPG_AGENT_INFO_FILE=${HOME}/.gpg-agent-info
if [ -f "${GPG_AGENT_INFO_FILE}" ]; then
GPG_AGENT_PID=`cat ${GPG_AGENT_INFO_FILE} | cut -f2 -d:`
GPG_PID_NAME=`ps -p ${GPG_AGENT_PID} -o comm=`
if [ ! "x${GPG_PID_NAME}" = "xgpg-agent" ]; then
rm -f "${GPG_AGENT_INFO_FILE}" 2>&1 >/dev/null
else
GPG_SOCKET=`cat "${GPG_AGENT_INFO_FILE}" | cut -f1 -d: | cut -f2 -d=`
if ! test -S "${GPG_SOCKET}" -a -O "${GPG_SOCKET}" ; then
rm -f "${GPG_AGENT_INFO_FILE}" 2>&1 >/dev/null
fi
fi
unset GPG_AGENT_PID GPG_SOCKET GPG_PID_NAME
fi

if [ -f "${GPG_AGENT_INFO_FILE}" ]; then
eval "$(cat \"${GPG_AGENT_INFO_FILE}\")"
eval "$(cut -d= -f 1 < \"${GPG_AGENT_INFO_FILE}\" | xargs echo export)"
export GPG_TTY=$(tty)
else
eval "$(${GPG_AGENT} -s --daemon --write-env-file ${GPG_OPTIONS})"
fi

fi
-----------------------------8<-------------------------------------

stop of gpg-agent
-----------------------------8<-------------------------------------
#!/bin/sh

## The nice way
if test -n "${GPG_AGENT_INFO}"; then
GPG_AGENT_PID=`echo ${GPG_AGENT_INFO} | cut -d: -f2` && kill ${GPG_AGENT_PID} ||:
unset GPG_AGENT_INFO
fi

## The not so nice way
## NOTE: a root login will kill *all* users' gpg-agents
#killall gpg-agent

## clean/remove .gpg-agent-info
rm -f $HOME/.gpg-agent-info
-----------------------------8<-------------------------------------