For the record.

[Winterton.HISCAN%his-phoenix-multics at BRL-VGR.ARPA]

In fact, both GCOS 3 and GCOS 8 have a neat facility called the intercom
file that work on the basis you describe.  The mechanism is:
          Hello, this is :: anybody want to tell me their program number?
          hi there ::, my program number is nn, what's yours?
          mm.  OH, fine, now we can talk securely....