TheCodeBreakers
Sometimes this was
done anyway, to destroy the suspected code.
Technological steganography early in the war consisted almost
exclusively of invisible inks. This is truly an ancient device. Pliny the
Elder, in his Natural History, written in the first century A.D., told how
the "milk" of the tithy-mallus plant could be used as a secret ink. Ovid
referred to secret ink in his Art of Love. A Greek military scientist, Philo
of Byzantium, described the use of a kind of ink made from gall nuts
(gallotannic acid), which could be made visible by a solution of what is
now called copper sulfate. Qalqashandi described several kinds of
invisible ink in his Subh al-a' sha. Alberti mentions them. The
Renaissance employed them in diplomatic correspondence. About 1530 a
book was printed with panels in invisible ink; if these pages were dipped
in water, the message would appear; this could be repeated three or four
times.
The common inks are of two kinds: organic fluids and sympathetic
chemicals
annaabi.ee 
