On Mon, 5 Jun 2017 16:18:55 +1000 "James A. Donald" <jamesd@echeque.com> wrote:
They are "potential" in that person A does not know that person B is actual fellow member of the ingroup.
so to make this a bit more clear, person A is a 'newbie' and person B is a grand wizard...
Thus, for person B, person B is an actual member of the ingroup, and person A is a potential member
but for person A, person B is a potential member of the ingroup and person A is a potential member.
ok
The problem is that we want person A to be able to find out that person B is a member of the ingroup, without person C finding out that person B is a member of the ingroup.
that's just a matter of A and B using a 'secure' channel. Like...tor!! Apart from that, I'm not seeing anything novel about the problem or the solution. And of course the other age old problem is that A, or B, or both may indeed be spies, so their knowing a 'secret' handshake wouldn't necessarily prove that they are bona fide actors instead of spies.