That is what traffic analysis is for. They will use traffic analysis to trivially link your IP to apples server , from there they get the username associated with that IP address from apple, from there they intercept communications going through apples server in plaintext. It is a shitty solution and if you value your security at all you will just immediately stop thinking about using imessage to defeat the feds, lol. Get an XMPP client that supports OTR, run it on an android smart phone routed through Tor, and use OTR to client to client encrypt the messages. That is a solution the feds cannot get around. TLS to apples servers being the end of federal wiretaps is just a bad joke, obviously intentionally planted by the feds to get people thinking they are secure using a totally, entirely, fundamentally insecure solution.