Dining cryptographers protocol

The method is as follows: At least three cryptographers arrange themselves around a circular dinner table, with menus hiding the interaction of each pair of adjacent cryptographers from the rest. Each adjacent pair picks a random number in private. Then each cryptographer announces publicly the difference between the number on his right and the number on his left, adding a message if he wants to transmit one. All cryptographers then add up the publicly announced numbers. If the sum is 0, no one sent a message. If the sum is a valid message, one cryptographer transmitted a message. If the sum is invalid, more than one cryptographer tried to transmit a message; they wait a random time and try again.

see also cryptography