Modern Cryptology

This repository documents my solutions to a series of cryptographic challenges presented as a multi-level, terminal-based game. Each level required analyzing and breaking a different cipher to extract a password, which was the key to unlocking the next stage of the game.

Assignment 1: Substitution Cipher

Conducted a frequency analysis on the ciphertext to identify it as a simple substitution cipher. Successfully deciphered the message by mapping the most frequent ciphertext characters to their common English language equivalents.

Assignment 2: Vigenère Cipher

Applied the Kasiski examination to identify repeating patterns in the ciphertext, which revealed the length of the encryption key. Subsequently, frequency analysis was performed on the corresponding sub-alphabets to recover the full key and decrypt the message.

Assignment 3: Substitution-Permutation Network (SPN)

Analyzed repeating sequences and their intervals within the ciphertext to determine the cipher's block size. Following this, frequency analysis was employed to deduce the character mappings used in the substitution layer, leading to a successful cryptanalysis of the network.

Assignment 4: 6-Round Data Encryption Standard (DES)

Executed a chosen-plaintext attack using efficient bash scripts to automate the process. The core of the attack involved differential cryptanalysis, where high-probability differential characteristics were exploited to systematically recover the subkeys for the 6-round DES variant.

Assignment 5: Advanced Encryption Standard (AES) Variant

Performed a chosen-plaintext attack by encrypting specially crafted plaintexts to probe the cipher's internal structure. This technique revealed a vulnerability, specifically the lower-triangular property of the MixColumns matrix, which was then exploited to efficiently break the encryption. Bash scripts were used to automate the attack.

Directory Structure

├── Assignment 1
│   ├── cipher.txt
│   ├── plain.txt
│   └── substitution.ipynb
├── Assignment 2
│   ├── cipher.txt
│   ├── plain.txt
│   └── vigenere.ipynb
├── Assignment 3
│   ├── cipher.txt
│   ├── combined cipher.ipynb
│   └── plain.txt
├── Assignment 4
│   ├── README.md
│   ├── brute_force.ipynb
│   ├── constants.py
│   ├── cryptanalysis.py
│   ├── decode.ipynb
│   ├── des_functions.py
│   ├── letter_pair.ipynb
│   ├── level4.sh
│   ├── supporting_functions.py
│   ├── theirdes_crack631.ipynb
│   └── theirdes_crack632.ipynb
├── Assignment 5
│   ├── README.md
│   ├── constants.py
│   ├── cryptanalysis.py
│   ├── decoding.ipynb
│   ├── find_e_diag.ipynb
│   ├── find_mat.ipynb
│   ├── level5.sh
│   ├── mat_inv.ipynb
│   └── supporting_functions.py
└── README.md