Secret of the 'invisibility' letter of spies

The US Central Intelligence Agency (CIA) recently released classified documents showing that World War I spies had printed messages on their toenails and used lemonade to write stealth letters.

The World War I spies used lemonade to write stealth letters. Photo: CTV.

The BBC reported that six newly declassified documents were among the old top secret documents that the CIA still holds to this day. These nearly 100-year-old records reveal a lot of espionage techniques by former intelligence agents, including "skeptical and checking everything possible" guidelines .

According to a document, soaking a handkerchief or any other hardening material in nitrates, soda and starch, will create a portable stealth ink solution. Embedding this specially treated handkerchief in the water will help spies write secret messages on it.

Another document, in 1914, in French, introduced a secret secret formula of German ink to make secret and claim that French spies found a way to crack the enemy's cryptography.

In a guide written by a handwriting expert in California (USA), the person proposed printing invisible messages to the human body. " To make messages appear, it is necessary to use an appropriate reagent sprayer , " the author wrote.

The document also warns of " other methods, often used by spies and smugglers, according to the skills and educational background of criminals", for example "printing messages and believe in the nails ".

The CIA said recent advances in technology have allowed them to publish a series of confidential documents. For example, some of the secrets in old profile pages have become obsolete by the rapid growth in stealth chemistry and the lighting methods used to detect them.

Last year, the CIA conducted more than one million historical documents. They are still publicly available on the agency's website.