Cryptanalysis of Vigenere Cipher

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Presentation transcript:

Cryptanalysis of Vigenere Cipher Day 11

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

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.

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.