What This Code Does

This code is designed to crack a ciphertext that was encrypted using the RC2 algorithm. It uses a technique called brute-force, where it tries every possible key within a certain range until it finds the correct one. Here's the core functionality:

1. Target: The code is looking for a valid credit card number. It assumes the original plaintext is a 16-digit number that passes the Luhn algorithm check (a common credit card validation method).
2. Key Generation: It systematically generates possible keys within a specified range. Each key is 8 bytes (64 bits) long.
3. Decryption: Each generated key is used to decrypt the ciphertext. The result is checked to see if it matches the pattern of a valid credit card number.
4. Parallel Processing: To speed things up, the code uses goroutines (lightweight threads) to perform key generation and decryption in parallel.
5. Early Termination: Once a valid credit card number is found, the program stops and reports the decrypted plaintext and the key used to decrypt it.

Thought Process: How to Write This Code

1. Goal: Your objective is to recover a credit card number from encrypted data. You know the encryption algorithm (RC2) but not the key.

2. Brute-Force Approach: Since you don't have any hints about the key, brute-force is a logical choice. You'll try all possible keys systematically.

3. Key Space: Decide on a reasonable range for the keys you'll test. In this case, the code uses the full range of 64-bit values.

4. Parallelism: To make the brute-force process faster, use goroutines to split the work. You can have multiple goroutines generating keys and others decrypting with those keys.

5. Data Structures:

   • CryptoData: A struct to hold a decrypted block of data and the corresponding key used for decryption.
   • Channels (work, done): Channels are used to communicate between goroutines. The work channel sends CryptoData objects from key generators to decryptors. The done channel signals when a valid key is found.

6. Functions:

   • generate: This function produces keys within a given range and sends them on the work channel.
   • decrypt: This function receives keys from the work channel, decrypts the ciphertext, and checks if the result is a valid credit card.

7. Main Function:

   • Set up the ciphertext you want to decrypt.
   • Initialize goroutines for key generation and decryption.
   • Wait for the brute-force process to finish.
   • Close channels and clean up.

Simplified Explanation (ELI5)

Imagine you have a locked treasure chest (ciphertext). You know the type of lock (RC2), but you don't have the key. Here's what the code does:

1. Helpers: You have friends (goroutines) who can help you.
2. Trying Keys: Your friends start trying every possible key combination (brute-force). They pass each key to you.
3. Checking: You try each key to unlock the chest. If it opens, you check if the treasure inside is a real credit card (Luhn check).
4. Success: If you find a key that works and reveals a valid credit card, you shout "Eureka!" and everyone stops.

Important Notes:

• Insecurity of RC2: RC2 is now considered insecure for serious use due to vulnerabilities.
• Realistic Scenario: This is a simplified example. Real-world ciphertext cracking often involves more complex algorithms and requires significant computing power.