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"

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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.

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