TheCodeBreakers
the censors. Presumably they were carrying contraband information and
so should be banned. But, in the absence of solution, no proof of this
existed, and so the letter could not be mutilated. 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
annaabi.ee 
