I mentioned this a while back, and am finally getting ’round to it. I want to wipe out my iBook, and this was the only thing I couldn’t remember how to recreate, so bear with me, while I do it here. It is, as I remember, easier and more publicly well known that the whole SSI thing.
Pre-first thing is: you must, must, must have installed the BSD subsystem when you installed OS X. End of story. It comes on your OS X install disks, so no purchase is necessary. And it allows you to run oh-so-many wonderful unix tools. You will also need to be running system 10.3 or later.
Next: Download GPG (GNU Privacy Guard). It’s easy to install. You won’t see any new programs anywhere. It’s one of those ‘invisible’ unix apps that you access from the command line.
I already had a set of PGP keys, created with the official PGP software. If you don’t already have your own, you can use GPG to walk you through the process of creating and publishing your own keys. If you do have PGP keys, you will need to import them into your ‘GPG keyring’. First step is to export a copy from PGP, (File > Export). I then had to ‘clean up’ my old PGP keys, because they were generated long ago in a galaxy far, far away, under OS9. You need to swap the end-of-line characters to something Unix-compatible. There is an application at the above site that can do it for you, or you can do it from the command line with:

tr -d '\r' ‹ myMacOS9ExportedKeyring › myMacOSXImportableKeyring

You’re then ready to import. Now, in Terminal, you will tell GPG to import the cleaned-up keys. Type:

gpg --import --allow-secret-key-import < keypair.asc

With "keypair.asc" being the name/location of your cleaned keys.
You're done in terminal.
The program on the front end, for OSX Mail is Sen:te's GPGMail. This software will install a 'bundle' in Mail, allowing you to set your preferences, (under "Preferences"), and to encrypt and specify recipients and keys, all from a new message that you're composing. Very easy to install. Restart Mail once you're done.
This should all now work. I glossed over some of the details. But if you didn't understand any of it, you should probably stick with the standard PGP package. If and when I wipe out my iBook, I will update this article if need be based on that experience.