[Cryptography] Secret Handshake problem.

[James A. Donald]

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.