[Cryptography] Secret Handshake problem.

[John Denker]

On 03/04/2017 06:00 PM, James A. Donald wrote:

> A is an identity that is a member of a club.
> 
> C and D are identities that are not members of this club.
> 
> C wants to reveal to D that C and A are the same, that C is secretly a member of the club, if and only if D is secretly a member of the club under a different identity.
> 
> I seem to recollect that this is a solved problem, but do not remember the solution.
> 
> Also known as the "Are you also a Soviet Spy?" problem.

There is an eeeenormous literature on this under the heading of IFF --
Identification Friend or Foe.

Not just the membership status, but the very existence of C is concealed
unless D is a member of the clube.