Breaking Vigenère Cipher

Cryptography · Axiom Academy

EXAMPLE Breaking the Vigenère Cipher Step-by-step cryptanalysis using Kasiski examination and frequency analysis Excellent work! You've successfully broken a Vigenère cipher. Here's what we learned: Kasiski Examination: Finding repeated sequences in ciphertext reveals information about key length, since identical plaintext encrypted with the same key position produces identical ciphertext. GCD Analysis: The greatest common divisor of distances between repetitions indicates the most likely key length, as repetitions occur at multiples of the key length. Divide and Conquer: By splitting the ciphertext into groups by key position, we reduce the problem to solving multiple simple Caesar ciphers. Frequency Analysis: Each position's letter frequencies reveal the shift value, since the most common ciphertext letter likely corresponds to 'E' (the most common English letter). Verification: Always decrypt with your recovered key to verify the plaintext makes sense - this confirms your cryptanalysis was successful. The Vigenère cipher was once considered unbreakable, but systematic frequency analysis defeats it. This demonstrates why modern cryptography requires much more sophisticated techniques!

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