This script decrypts a Vigenère cipher using frequency analysis. It includes functions to estimate the key length, perform frequency analysis, and decrypt the ciphertext.

• Kasiski Examination: Estimates the length of the Vigenère key.
• Frequency Analysis: Analyzes letter frequencies to determine the most likely key.
• Decryption: Uses the derived key to decrypt the ciphertext.

1. Calculate Index of Coincidence (IC):
   • The calculate_ic function computes the IC for a given text segment to help identify the key length.
2. Kasiski Examination:
   • The kasiski_examination function calculates the IC for various assumed key lengths to identify the most probable key lengths.
3. Frequency Analysis:
   • The frequency_analysis function analyzes the frequency of each letter in segments of the ciphertext to determine the most likely shift for each segment.
4. Decrypt Vigenère Cipher:
   • The decrypt_vigenere function uses the derived key to decrypt the entire ciphertext.

1. Set the Ciphertext: Replace the ciphertext variable with your Vigenère encrypted message.

2. Estimate Key Length:

   • The script uses Kasiski examination to estimate the key length and prints the top 5 key length estimates.

3. Decrypt the Ciphertext:

   • The script performs frequency analysis for each estimated key length, derives the key, and decrypts the ciphertext.