Classical Cryptanalysis: Vigenère Cipher

Background

The Vigenère cipher is a polyalphabetic substitution cipher defined as:

Ci = (Pi + Ki) mod 26

Its security depends on the secrecy of the key length and key characters.

Key Length Detection

Two statistical techniques are used:

Index of Coincidence (IC)

The IC measures the probability that two randomly selected letters are identical. English text has a higher IC than random text. By computing IC for different assumed key lengths, the correct key length can be estimated.

Kasiski Examination

Repeated ciphertext patterns reveal periodic spacing. The greatest common divisor of these spacings provides candidate key lengths.

Frequency Analysis

After determining the key length, the ciphertext is divided into columns. Each column behaves like a Caesar cipher. By comparing letter frequency distributions with English frequencies, the shift for each column is determined and the key is reconstructed.

Results

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Outcome

  • Successful recovery of the key
  • Demonstration of classical statistical cryptanalysis
  • Baseline comparison for modern machine learning based attacks