Actionscript Three Asymmetric Encryption

Question

I cant seem to find a reliable asymmetric encryption solution to secure data between a python based server application and a client over an open data channel.
I need some way for my client to prevent a man in the middle attack over an open data channel, my current exchange has me sending my clients a token they use to verify they are talking to my server application by checking the token is valid with a php script on my site. This is far from ideal and could easily be compromised by waiting to be sent the token and passing it off to another user.
I have tried as3crypto's rsa encryption but it is an old implementation that is not supported by many libraries as well as having a known vulnerability.
I would really like a solution that lets me hard code public/private keys into both the client and server to prevent something like this from happening.