Works on any website/forum/email/IM/etc:
- Install OpenPGP
- Create a public and a private key
- Give your public key to anyone who wants to write to you
- Get the public key of people you wish to write to
- Encrypt your text using their public key
- Decrypt their text using your private key
- Enjoy privacy
Public key = only used for encryption
Private key = only used for decryption
Example:
I send you my public key, you encrypt your message for me using that, and send the encrypted message over to me.
Then I decrypt it using my private key.
I do not need to send over my private key to you, so the man-in-the-middle cannot decrypt your message to me.
All he can do is writing encrypted messages for me, by intercepting my public key.
But you can "sign" your messages so I can be assured that they are indeed written by you, and are not fake.