一合再離三發一知
Joined Once, Split in Three, Sent Thrice, Known Once
武王問太公曰：「引兵深入諸侯之地，主將欲合兵行無窮之變，圖不測之利，其事繁多，符不能明，相去遼遠，言語不通，為之奈何？」太公曰：「諸有陰事大慮，當用書，不用符，主以書遺將，將以書問主，書皆一合而再離，三發而一知。再離者，分書為三部；三發而一知者，言三人，人操一分，相參而不知情也。此謂陰書，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 commander wishes to coordinate forces and execute limitless variations, planning for unforeseen advantages — the matters are numerous and complex, tallies cannot convey them clearly, the distances are vast, and verbal messages cannot get through. What should be done?' Taigong said: 'For all secret affairs and weighty plans, use letters rather than tallies. The ruler sends letters to the general, and the general sends letters to the ruler. Each letter is first composed as a whole, then split into three parts, and dispatched by three messengers so that only when reassembled can the message be known. "Split in three" means dividing the letter into three portions. "Sent thrice, known once" means that three men each carry one portion, and though they can cross-reference, none of them knows the full content. This is called the secret letter. Even if the enemy possesses sagely wisdom, he cannot decipher it.' King Wu said: 'Excellent!'
Notes
The secret letter (陰書) system is an early form of secret-sharing — a concept rediscovered in modern cryptography. The message is split into three parts sent by separate couriers, so that capturing any one courier reveals nothing.