See http://www.drh-consultancy.demon.co.uk/pkcs12faq.html and http://www.openssl.org/docs/apps/pkcs12.html for more information.

This, http://www.ripe.net/ripencc/pub-services/db/mail_client_tests.html, may also be of interest.

If you have a certificate from a certificate authority, you can skip the first two steps.

PKCS12, Personal Information Exchange Syntax Standard, certificates can be used for things such as email signing and file signing. They are different from other certificates in that rather than being only the public or private certificate, they are a combination of both plus the root certificate. This means the person they are made for only has to worry with one file.

Apache, imap, and exim all support ssl connections. Because I want webmail connections to be encrypted, so that passwords are not sent over the internet in plain text, I needed to create a key and a certificate for apache. For testing purposes I am going to be my own certificate authority. This will most likely be changed for the production server. You don't have to use any certificate authority - see http://www.tldp.org/HOWTO/SSL-RedHat-HOWTO.html

Web servers, imap servers, smtp servers can be configured to use ssl connections and there are many other uses for ssl certificates, such as encrypting email or digitally signing documents. You don't have to pay a certificate authority, such as Verisign, because you can use the OpenSSL package to create your own certificates. I do not cover the installation of OpenSSL here and I assume you know at least how to change directories, move files, use an editor, and other basics from the command-line.

With AutoCAD 2004 and newer, including LT, you can use digital certificates to "sign" dwg files.