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Buckle your seatbelt, encrypt your bits
Step 2: Generating your key pair
Now it's time to generate the key pair that will be used to encrypt and decrypt messages. (If this is your first time, it's not a bad idea to create a practice key in case you make any mistakes). To do so, open GPA, short for GNU Privacy Assistant. The first time the program is opened, it will open a window prompting you to generate a private key. This is exactly what you want to do, so click "Generate key now."
What you see the first time you open GPA
Most of the prompts are self-explanatory, but a few things are worth bearing in mind. First, be sure to pay close attention to the passphrase you choose. Choices such as "password" and "1234567" are clearly not acceptable. Better is a randomly generated password using a program like Password Safe. Even better still is use of a long phrase that's idiosyncratic enough that only you will know it. Whatever passphrase you use, be sure to remember it. Your key will be useless without it.
GPA will also ask you if you want to back up your private key. This is generally a good idea, because if you lose it, you will be unable to read encrypted messages sent to you. The best idea is to save the key to a USB thumb drive and then stash it in a secure lockbox (along with your passphrase written out). Be sure to enter a file name (e.g. mysecretkey.asc) in the backup dialog box, or GPA will give you a cryptic error message.
When you're done, the key you just created will appear in GPA's keyring editor. Notice that with the Details tab selected, GPA says that the key has both a private and public part.
Your key in the GPA keyring editor
For people to send you an encrypted email, they'll need your public key. You can get this by right-clicking on your key in the GPA keyring editor and choosing Copy. In theory, you should be able to paste the key into the body of an email message and send it to one or more of your contacts. In practice, GPA seems to add an extra carriage return to keys, which makes sending them in the body of an email problematic. To get around this, go to Start > All Programs > Accessories > Notepad, and paste the public key into the body. Then save using a file name such as mypublickey.asc and email it as an attachment to one or more contacts.
Your contacts, assuming they already know how to send encrypted email, now have what they need to send an encrypted email that you - and you alone - can decrypt. To make that easy, you'll need to install the Enigmail add-on to Thunderbird.