Invisible ink?

During the war agents would communicate by writing harmless letters to friend, but they write special messages on the envelope. There are generally three kinds of invisible ink: those developed by heat, those developed by light, and those developed by chemical reactions. Axis powers responded by developing 'dry ink' and hiding packets of it in care packages. Upon discovery of the packets with another round of creative chemistry to develop new papers with sensitivity to fluid and dry inks. Products used to make the ink included Food like honey, milk, sugar solutions, and clear colas were also developed by heat, and could be used by civilians. Even soapy water worked.