Through the use of an encryption library also developed by ACagliano, when you log in to your game account from your calculator, your session authentication is secure. A login token for your account is saved into an Application Variable within your "Web Deck" account. Once you've put that on your calculator, TI-Trek will import it the next time it runs.
Upon connecting to the server, an RSA key will be exchanged with the server in order to set up an AES session. Your login token will then be sent encrypted with AES.