You can use kmail and gpg to send encrypted or digitally signed mail. To do this, there are the buttons "sign" and "encrypt" in the kmail compose window. It appears if you click on "File|New|New message".

Overview: GPG

A web of trust are a lot of persons who trust each other. To show their trust, they have signed each other's keys. gpg allows you to do this.

The web of trust is extended in key signing parties that go like this:

you (me@home.org) meet John Foo (foo@bar.org) who is already a member of the web of trust
you install gpg
you have a private/public key pair or generate it with

gpg --gen-key

Your private key is stored in secring.gpg, your public one is stored in pubring.gpg. You keep your private key and are free to distribute your public key.

you find out your key's name

gpg --list-keys
/home/me/.gnupg/pubring.gpg
----------------------------------
pub   1024D/45E377BB 2008-02-03
[...]

you upload your public key to your key server

gpg --send-key

John downloads your key from the key server

gpg --search-key me@home

John controls your passport and signs your key

gpg --sign-key 45E377BB

John uploads your key again

gpg --send-key 45E377BB

You are now part of the web of trust and your public key is on the key server.

How to...

Import a saved keyring

If you have a keyring saved to your disk, reload ("import") it like this:

cat secring.skr | gpg --import   #yours may be called 'secring.pub'

Then call kgpg and set your trust on your private key to ultimate.

TroubleShooting

Whatever problems arise, first make sure your gpg is properly set up:

List your keys

gpg --list-keys

Has to deliver something like a sensible list of keys.

Check the output of

gpg --update-trustdb

In case of a problem look at the below.

no ultimately trusted keys found

If

gpg --update-trustdb

delivers

gpg: no ultimately trusted keys found

Call kgpg and set the trust of your private key to ultimate.

"Gpg does not seem to be running..."

You get an error message like this whenever you start up? You need scripts to make it start and stop cleanly. You can get scripts here:

https://fedorahosted.org/kde-settings/browser/trunk/etc/kde/env/gpg-agent-startup.sh

https://fedorahosted.org/kde-settings/browser/trunk/etc/kde/shutdown/gpg-agent-shutdown.sh

Copy them into your ~/.kde/env and ~/.kde/shutdown folders, check permissions, and make sure they are executable.