[Cryptography] Secret Handshake problem.

[Alfie John]

> On 5 Mar 2017, at 1:15 pm, John Denker <jsd at av8n.com> wrote:
> 
> 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 club.

I was thinking this could be done with group signatures, but even that’s complicating it… why not just use a shared secret?

Alfie