Encryption: simulate SSL in javascript and python
You can't stop the men in the middle from trapping your packets/messages, especially if they don't really care if you find out. What you can do is encrypt your messages so that trapping them does not enable them to read what you're sending and receiving. In theory that's fine, but in practice you can't do modern crypto by hand even with the keys: you need to transfer some software too, and that's where it gets much more awkward.
You want to have the client's side of the crypto software locally, or at least enough to be able to check whether a digital signature of the crypto software is correct. Digital signatures are very difficult to forge. Deliver signed code, check its signature, and if the signature validates against a public key that you trust (alas, you'll have to transfer that out of band) then you know that the code (plus any CA certificates – trust roots – sent along with it) can be trusted to work as desired. The packets can then go over plain HTTP; they'll either get to where they're meant to or be intercepted, but either way nobody but the intended recipient will be able to read them. The only advantage of SSL is that it builds virtually all of this stuff for you and makes it easy.
I have no idea how practical it is to do this all in Javascript. Obviously it can do it – it's a Turing-complete language, it has access to all the requisite syscalls – but it could be stupidly expensive. It might be easier to think in terms of using GPG…
(Hiding the fact from the government that you are communicating at all is a different problem entirely.)
Because of china Great Firewall has blocked google appengine's https port. So I want to simulate a Secure Socket Layer by javascript and python to protect my users information will not be capture by those ISP and GFW.
My plan: 

Shake hands:

Browser request server, server generate a encrypt key k1, and decrypt key k2, send k1 to browser.
Browser generate a encrypt key k3, and decrypt key k4, send k3 to server.

Browse:

During the session, browser encrypt data with k1 and send to server, server decrypt with k2. server encrypt data with k3 and response to browser, browser decrypt with k4.
Please figure out my mistake.
If it's right, my question is 

how to generate a key pair in
javascript and python, are there
some libraries?
how to encrypt and decrypt data in
javascript and python , are there
some libraries?