Abstract
On a moonless night the spy returns to the castle after a reconnoitering
mission to the enemy camp. As he nears the gate a voice whispers, "What's
the password?" But is it a friend or foe who whispers? How can the spy
show he knows the password without actually revealing it to a possible
imposter?
The spy's dilemma is commonplace now with the widespread use of telecom-
munications. When your automatic teller machine communicates with your bank,
each must be assured that the other is legitimate; the electronic "passwords"
must be unforgeable and must be of no use to imposters and eavesdroppers.
