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Vigenère ciphers work like the Caesar method, except that an entire block of 2,
3, 4, or 5 letters is shifted by a keyword.
For example, if the keyword is "CAR'' and attending to the cipher each of these
three letters correspond to, then the first letter of the message is shifted by the
letter 2 places after it, the second letter is shifted by the same (0 places after it),
the third by the one 17 places away, the fourth by the one 2 away again (here
we return to the beginning of the keyword), the fifth is again not replaced, and
so on.
Example : If the key is "CAR" then the message "HELLO WORLD" would turn into "JECNO NQRCF"
Along with mathematical concepts functions, inverse functions, cyclic groups,
modular addition this topic introduces the students to ideas from other fields,
such as statistics and languages. In addition, it provides a natural leading to a
discussion of onetime pads, which are Vigenère ciphers of ``infinite'' keyword
length and are the only cryptosystem with perfect security.