Lesson 11. Capstone: The Vigenère cipher

The Vigenère cipher (see en.wikipedia.org/wiki/Vigenere_cipher) is a 16th century variant of the Caesar cipher. For this challenge, you will write a program to decipher text using a keyword.

Before describing the Vigenère cipher, allow us to reframe the Caesar cipher, which you’ve already worked with. With the Caesar cipher, a plain text message is ciphered by shifting each letter ahead by three. The direction is reversed to decipher the resulting message.

Assign each English letter a numeric value, where A = 0, B = 1, all the way to Z = 25. With this in mind, a shift by 3 can be represented by the letter D (D = 3).

To decipher the text in table 11.1, start with the letter L and shift it by D. Because ...

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