In the Southern Song Dynasty, the Tang dynasty was ordered by the Privy Council to secretly go to Xingren and give the information written by alum to Cao, the general of the Song Dynasty. Cao Jiang, this piece of white paper is soaked in water, and it shows intellectual handwriting.
12 16 years, powerful Mongolian soldiers besieged Taiyuan city, and Gu Wu Li Lun, the ambassador of Fu Xuan, Taiyuan, also wrote an emergency message in alum water, and sent spies to Beijing for emergency.
The famous alum case in Qing Dynasty is a typical espionage case using this method. 1675, Kang Xili's second son Yin was the Crown Prince. Yin, on the other hand, greatly expanded his power and even competed with Kangxi. Kangxi was afraid that the crown prince who was engaged in espionage would be killed by conspiracy means of being killed today and being killed tomorrow. He was accused of gathering partisans and spying on my daily life with a dove, thus deposing the crown prince and imprisoning him.
17 15 In the spring, Wang Ce Alabotan of Junggar invaded Hami. Kangxi sent troops to crusade. After learning this information, Yin wanted to take this opportunity to get rid of house arrest. Yin paid a large sum of money to a doctor named He Meng, who often treated Yin Fujin, making him a spy who sent information to the outside world. Yin wrote a secret letter with alum water, sent him out of the palace to the Manchu ruler of Zhenghongqi, and sent him to Kangxi to intercede for himself, recommending him as the general of the crusade for Allah Botan, and let him take the opportunity to restore the status of the Crown Prince. But this secret was discovered by lord protector's assistant Abring, and immediately reported to Kangxi, which completely shattered Yin's attempt to restore the crown prince's status.