... to get that key, Charlie has to actively step between Alice and Bob and capture their pieces, while pretending to be the other person. If both use some signature mechanism, Charlie is SOL.
What has Charlie done with Eve?
I didn't read TFA, let alone finish reading TFS... After all, and I'm assuming the service is hosted in the US, the White House has access to any keys which are transmitted to and from Silent Circle's systems.
From TFA: "Silent Circle stresses that their product offers secure communications within the networks and only uses Canadian servers that are outside of U.S. government control."
...though his invention worked superbly -- his theory was a crock of sewage from beginning to end. -- Vernor Vinge, "The Peace War"