Indeed, it is not unknown for modern spies who have run out of standard-issue invisible ink to improvise by using their own urine.Īlthough cryptography and steganography are independent, it is possible to both scramble and hide a message to maximise security. Many organic fluids behave in a similar way, because they are rich in carbon and therefore char easily.

Although transparent after drying, gentle heating chars the ink and turns it brown. As far back as the first century AD, Pliny the Elder explained how the ‘milk’ of the thithymalus plant could be used as an invisible ink. Steganography also includes the practice of writing in invisible ink. The solution penetrates the porous shell, and leaves a message on the surface of the hardened egg albumen, which can be read only when the shell is removed. In the sixteenth century, the Italian scientist Giovanni Porta described how to conceal a message within a hard-boiled egg by making an ink from a mixture of one ounce of alum and a pint of vinegar, and then using it to write on the shell. The messenger would then swallow the ball of wax.

For example, the ancient Chinese wrote messages on fine silk, which was then scrunched into a tiny ball and covered in wax.

In the two thousand years since Herodotus, various forms of steganography have been used throughout the world. Secret communication achieved by hiding the existence of a message is known as steganography, derived from the Greek words steganos, meaning ‘covered’, and graphein, meaning ‘to write’.

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