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 L = 11 and D = 3, the result of 11–3 is 8, or the letter I. Should you need to decipher the letter A, it should wrap around to become X, as you saw in lesson 9.
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Table 11.1. Caesar cipher
| L | F | D | P | H | L | V | D | Z | L | F | R | Q | T | X | H | U | H | G |
| D | D | D | D | D | D | D | D | D | D | D | D | D | D | D | D | D | D | D |
The Caesar cipher and ROT13 are susceptible to what’s called frequency analysis. Letters that occur frequently in the English language, such as E, will occur frequently in the ciphered text as well. By looking for patterns in the ciphered text, the code can be cracked.