What are the secret writing techniques of private bodyguards' intelligence transmission skills?

Such scenes can often be seen in movies. A wizard holding a piece of white paper can show his handwriting after processing it in some ways. In fact, the principle is very simple. Some use substances containing carbon based elements, and some use chemical reactions. Steganography is one of the basic methods for private bodyguards to transmit information. In fact, the Chinese invented "alum steganography" very early. The words written in alum water can't be seen on the paper after drying, but if they are immersed in water, the handwriting will appear. Modern chemical steganography can generally be traced back to the first World War, and various chemists showed their powers. A few years ago, the CIA announced the formula of stealth ink, which is the production method of spy stealth ink used by the US Naval Intelligence Agency in 1917 and 1918. In the first World War, the use of invisible ink was one of the important information transmission technologies. Today, all kinds of steganography are still used in the intelligence transmission skills of private bodyguards: first, primary steganography: it is to use the invisible ink everywhere in life to complete the intelligence transmission. For example, sugar water, add a lot of sugar to a cup of hot water, melt it and write on white paper. When reading information, baking slowly on the fire will show yellowish brown handwriting. This is the principle that the carbon element in sugar water will be scorched by preheating. For example, rice soup, milk, lemonade and so on can be used as steganography tools. 2、 CIA World War I steganography: in 2011, the U.S. Central Intelligence Agency (CIA) recently disclosed the oldest confidential documents of the U.S. federal government, which were dated from 1917 to 1918 and were in the middle and late stage of World War I; The decrypted document reveals the highest information transmission technology used by spies, generals and diplomats - invisible ink formula. Now it seems that these things are not very clever. But we must know that everything has the limitations of the times, and we can't look at the past with current technology. For example, using ferric sulfate solution as invisible ink, the developer is potassium ferrocyanide solution. This is because iron ions appear blue when they encounter ferrous cyanide ions. There are many such steganography that uses chemical reactions to complete information transmission. 3、 Other steganography: for example, seaweed steganography. In 2015, someone invented a magical ink for invisible writing and painting. This magic ink is made of algae. The inventors Scott and Steve are not agents, but crazy scientists in the algae world. This steganography uses seaweed raw materials. This kind of ink shows the color when writing, but when you finish the book, the handwriting will disappear after a period of time. The original intention of the invention is to make a time developing art, that is, after painting a picture, mount it in the picture frame and give it to people for a few days. After a few days of sufficient light and water, the painting will naturally appear. What other fluorescent molecular steganography uses the principle that fluorescent molecules can be sensitized only under the irradiation of ultraviolet lamps. This steganography is made into an invisible pen and is sold in many prop stores on the market.