Decrypting-Ciphertexts

This exercise involves decrypting two different pieces of ciphertext using two different methods to find the encryption keys and the original plaintext messages.

How To Run

To run the Java version:

cd <project_dir>

javac rotor96Crypto/*.java

java rotor96Crypto.KPA # to run the Known Plaintext Attack
java rotor96Crypto.COA # to run the Ciphertext Only Attack

To run the Python version:

cd <project_dir/python>

python KPA.py # to run the Known Plaintext Attack
python COA.py # to run the Ciphertext Only Attack

Problem Statement

1. All of the secret messages have been encyrpted by an 8-rotor encryption machine called Rotor96Crypto.
2. All keys have been taken from a file of common passwords called 'passwords'.
3. The ciphertexts are stored in 'ciphertext1.txt' and 'ciphertext2.txt'.
4. The first two letters of the plaintext are 'We' and it's stored in 'plaintext_start_chars.txt'.
5. Two methods are used to decrypt the ciphertexts:
   • Known Plaintext Attach (KPA): The first two letters of the plaintext are known.
   • Ciphertext Only Attack (COA): Only the ciphertext is known.

Rotor96 Encryption & Decryption Equations

1. Normalisation

Convert input character to a byte value in range [0, 95]:

$$ B = \text{ord}(P) - 32 $$

2. Encryption Process

For each rotor ( k ) (from 0 to 7):

$$ B = R_k(B) = \text{rotor}[k] \left( (B + O_k) \mod 96 \right) $$

After transformation, update rotor offset:

$$ O_k = (O_k + I_k) \mod 96 $$

Final ASCII conversion:

$$ C = B + 32 $$

3. Decryption Process

For each rotor ( k ) (from 7 to 0):

$$ B = R_k^{-1}(B) = (\text{rotor}[k][B] + 96 - O_k) \mod 96 $$

Update rotor offset:

$$ O_k = (O_k + I_k) \mod 96 $$

Final ASCII conversion:

$$ P = B + 32 $$

4. Overall Encryption Equation

$$ C_i = R_7(R_6(R_5(R_4(R_3(R_2(R_1(R_0(B_i)))))))) $$

5. Overall Decryption Equation

$$ P_i = R_0^{-1}(R_1^{-1}(R_2^{-1}(R_3^{-1}(R_4^{-1}(R_5^{-1}(R_6^{-1}(R_7^{-1}(B_i)))))))) $$

Each rotor transformation introduces a key-dependent substitution, making the encryption scheme a polyalphabetic cipher similar to classical rotor-based encryption machines.

Known Plaintext Attack (KPA)

Given a chunk of ciphertext with the first two characters of the plaintext, perform a dictionary attach to find the key. Use this key to decode the rest of the message.

Ciphertext Only Attack (COA)

Given another chunk of ciphertext but with no information about the plaintext, perform a dictionary attack but this time decide whether a key produces the correct plaintext given that the plaintext is an English message.