ok, so the key is that you get to encrypt it in one place, using any arbitrary encryption you like, and then you use the same thing to decrypt on the other side. Because you have control this becomes pretty straight-forward.
Firstly, it's worth noting that you may want to, for aesthetic reasons, convert the encrypted data (which is binary) to base 64 before using it on the link. This will look a bit "neater". StringTheory has methods for converting to Base64, so obviously you can use that.
Encryption itself is outside the scope of NetTalk itself, so typically you'd use a separate encryption library to encrypt and decrypt the data. (Cryptonite is obviously the one I'd use, but any good encryption library would work.)
If you need more then just ask - but that should get you going in the right direction.
cheers
Bruce