"Throughout most of history, secure message exchange has been
based on some form of shared secret. The secret might take the form
of a code, in which the parties to the exchange have an agreed-upon
set of substitutions for phrases or actions. Or it could be a
cipher, in which text is converted into other text using some
algorithm. The secret could even take other forms, such as a
language unknown to other parties who might have access to the
messages. The shared secret made the message exchange secure. If
some other party discovered the secret, though, the message
exchange would be compromised, with potentially devastating results
for the parties to the exchange. (Think Enigma and the German
military communications in World War II, for example.)"
