Vigenère Cipher Cryptanalysis

This project is a Python-based tool designed to perform cryptanalysis on text encrypted with the Vigenère cipher.

A comprehensive explanation of the methodology can be found in the accompanying IEEE report
(which was structured following the )

Description

The code implements techniques to analyze and break the Vigenère cipher, a classical polyalphabetic substitution cipher. To deduce the key length and retrieve the original message, it uses:

• Kasiski Examination – to identify repeated sequences and estimate possible key lengths.
• Index of Coincidence (IC) – to measure the probability of letter repetition and refine the key length guess.
• Chi-Square Test (χ²) – to compare the frequency of letters in decrypted segments with the expected frequency of letters in Portuguese. This helps estimate the most likely key.

A separate file contains the expected frequency distribution of letters in Portuguese, which serves as the statistical basis for the chi-square comparison.

Requirements

• Python 3.x

How to Run

1. Open a terminal.

2. Navigate to the folder containing both the Python scripts and the encrypted text file (ciphertext.txt, or equivalent).

3. Run the program using:

   python VigenereDecipher.py