TheCodeBreakers
was hidden beneath the awkward or illiterate or misspelled writing. And
even if he felt certain, solution often eluded him. He usually had only one
message to work on, and no probable words. Early in the war, censorship
practice even forbade working on a suspected cryptogram more than half
an hour, on the theory that if the cryptanalyst hadn't gotten it by then,
he'd never get it. These unsolved messages posed a difficult problem to
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
annaabi.ee 
