八等陰符
The Eight Grades of Secret Tallies
武王問太公曰：「引兵深入諸侯之地，三軍卒有緩急，或利或害，吾將以近通遠，從中應外，以給三軍之用，為之奈何？」太公曰：「主與將有陰符凡八等：凡大勝克敵之符，長一尺；破軍殺將之符，長九寸；降城得邑之符，長八寸；卻敵報遠之符，長七寸；警眾堅守之符，長六寸；請糧益兵之符，長五寸。敗軍亡將之符，長四寸；失利亡士之符，長三寸。諸奉使行符，稽留者，若符事泄，聞者、告者，皆誅之。八符者，主將秘聞，所以陰通言語不泄，中外相知之術。敵雖聖智，莫之能識。」武王曰：「善哉。」
King Wu asked Taigong: 'When leading the army deep into the territory of the feudal lords, the three armies suddenly face crises — sometimes favorable, sometimes perilous. I wish to communicate between near and far, coordinate the center with the flanks, and supply the needs of the three armies. How should this be done?' Taigong said: 'The ruler and the general share a system of secret tallies in eight grades: The tally for a great victory over the enemy is one foot long. The tally for smashing an army and killing its general is nine inches long. The tally for forcing a city to surrender and capturing a town is eight inches long. The tally for driving back the enemy and reporting from afar is seven inches long. The tally for alerting the troops and strengthening the defense is six inches long. The tally for requesting grain and reinforcements is five inches long. The tally for a defeated army and a fallen general is four inches long. The tally for a lost advantage and fallen soldiers is three inches long. Any envoy who carries a tally and is delayed, or if the tally's message is leaked — both those who hear it and those who reveal it shall be executed. These eight tallies are the secret communication between ruler and general, the art of transmitting messages in code without leaking them, so that those within and those without can know each other's situation. Even if the enemy possesses sagely wisdom, he cannot decipher them.' King Wu said: 'Excellent!'
Notes
The secret tally (陰符) system described here is one of the earliest known military communication protocols. The different lengths of the tallies encode different messages, creating a simple but effective cipher that requires no written text.