The first invisible ink in recorded history was discovered by Pliny the Elder. He was a Roman general born in 23AD who was perhaps most famous for saying "Fortune favours the brave" ...just before sailing to Mount Vesuvius as it was erupting, and (quite ironically) dying of smoke inhalation.
Before his death, he discovered that the milk from the thithymallus plant could be used to make marks on paper which were invisible to the eye, but which could subsequently be developed to show secret messages.
Generally speaking, there are three kinds of invisible ink:
1) those developed by heat
2) those developed by light
3) those developed by chemical reactions
Decrypting secret messages takes a multitude of different forms. Even in our time now there's a new method that's just been created!
And who isn't curious about deciphering a secret message?