It’s pretty straightforward to setup PGP signing and encryption in OS X’s Mail program:
- install PGP on your machine
- install the PGP keychain access app
- create yourself a public/private key pair with the above app – I chose the default 1024 bit encyption and it took a few seconds to generate, Tom chose 4096 and it took > 5 mins on his Quad Mac Pro 2.66 Ghz
- install the Mail.app PGP plugin and restart the program
After that you get a box to tick if you want to sign your messages, which prompts you for your passphrase if you don’t set your Preferences to remember it. And for any addressee whose public key you’ve imported into your keychain access app, you can send an encrypted message.
UPDATE: I regret recommending this plugin, as reported by Wes in the comments I also found unread message began to report incorrectly for my IMAP account. Removing the plugin solved the problem. From the author’s FAQ
To uninstall GPGMail, you only need to delete the file GPGMail.mailbundle located in $HOME/Library/Mail/Bundles.