Move (nonwritten) discussion of path selection

To extend the circuit to a third node or beyond, Alice

proceeds as above, always telling the last node in the circuit to

extend one hop further.

% XXX Briefly mention path selection and path length.

This circuit-level handshake protocol achieves unilateral entity

authentication (Alice knows she's handshaking with the OR, but

%[XXX Choosing paths and path lengths: I'm not writing this bit till

%  Arma's pathselection stuff is in. -NM]

%Alice always chooses her path to contain at least

%three nodes unrelated to herself and her destination, choosing the

%number of nodes beyond the third from a geometric distribution;

%explain why. -NM

%%%% Roger said that he'd put a path selection paragraph into section

%%%% 4 that would replace this.