Some of the generals are traitors, can the loyal ones still agree on a plan?
Several divisions of an army, each led by a general, surround an enemy city. They must all attack together or all retreat together to avoid disaster, communicating only by messenger, and some of the generals are traitors who will send conflicting orders to sabotage any agreement. Is there a way for the loyal generals to always reach the same decision, whatever the traitors do?
Reveal the answer
Yes, but only under limits: using verbal, forgeable messages, loyal generals can reach agreement only if fewer than a third of all generals are traitors. Using unforgeable signed messages, agreement is always possible regardless of how many are traitors. Proven by Leslie Lamport, Robert Shostak, and Marshall Pease in 'The Byzantine Generals Problem,' 1982, now the foundation of fault-tolerant distributed computing and blockchain consensus.