1. Classical Polyalphabetic Ciphers
Day 8

2. Vigenere Cipher
Choose a keyword and write it repeatedly over the plaintext.
The pairing determine the resulting ciphertext character by the Vigenere table.
Key
Plaintext
Ciphertext

3. Index of Coincidence f is the number of ’s in the ciphertext
Key Length IC 1
.0660 2
.0520 3
.0473 4
.0450 5
.0436 6
.0427 7
.0420 8
.0415 9
.0411 10
.0408 11
.0405 12
.0403
f is the number of ’s in the ciphertext
N is the number of characters in the text

4. Steps to breaking Vigenere 1
Find repetitions in the ciphertext.
Count the number of characters between the repetitions.
Find the factors of the numbers discovered in Step 2.
The most common factor is likely to be the keyword length.
Break the ciphertext into chunks according to the keyword length and perform frequency analysis on each chunk.

5. Steps to breaking Vigenere 2
Perform shifts on each segment and calculate the percentage of j’s, k’s, q’s, x’s, and z’s. These should only account for about 2% of the total.
Determine the keyword.