Sharing enctypted files with other Ubuntu users is straightforward enough, but real-world situations are rarely so simple. For one thing, it seems a little pointless encrypting files if you attach them to email messages that pretty much anyone can read and, for another, most people don't use Ubuntu (hard to believe, I know!).
So, the question is, can you exchange encrypted files and email with Windows users? The answer is most assuredly, yes!
The embedded email client in Ubuntu is Evolution and it supports "signing and encrypting mail via GPG (GNU Privacy Guard) and S/MIME" out of the box. However if, like me, you have migrated to Thunderbird, you can extend the security capabilities of the client by adding the Enigmail plugin. Using both of these applications is a simple affair: draft your email and, before you click Send, click on the Security toolbar button or menu option in the message window. The plugin will prompt you for the relevant key and handles the encryption for you.
For Windows users looking for PGP support, there is Gpg4win. I've tested this on my Vista machine with Microsoft® Outlook 2003 and it works - eventually. The Windows OS being what it is, I found that multiple reboots were required to get the plugin working and that importing my key proved to be problematic: however, when I did get it to work, it performs excellently and there is a superb manual that goes a long way to helping novices get the installation right.
Let's not get paranoid - it's unlikely that anyone is terribly interested in the bulk of email messages and attachments that we send off into the ether: however, it is nice to know that you can share confidential information with your colleagues, friends, and family without letting the whole world take a peak!
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