Function decide_bind 

pub fn decide_bind(addr: &SocketAddr, token: Option<B3Hash>) -> BindVerdict

Decides whether the listener may bind addr, and which face it then presents.

Pure. Four cells, one of which refuses: an address reachable from outside this machine with no pairing token configured. The refusal happens before the socket exists, so there is no window in which the port is open and unauthenticated - and the credential the exposed face demands comes back inside the verdict, so there is no second way to learn it.