You never have the private key of your contact. Private keys are private. But I get what you're saying. The system is trust on first use, so your client will ac

You never have the private key of your contact. Private keys are private. But I get what you're saying. The system is trust on first use, so your client will accept whatever the server gives you as the identity. But that's how it is with any system. Even with PKI you're just trusting a different server. At least it's possible for you to check your contact's fingerprint out of band if you're really concerned. You need the TextSecure server because without it you're not going to be able to connect to your contact at all. When everyone's moving between different connections to the internet and is almost always behind NAT then communication is only practical with a server. There's nothing stopping anyone from exchanging messages directly over TCP sockets. It would totally work. But how practical is that? How will anyone initiate a connection to your cell phone?