🧩 TryHackMe: Cipher's Secret Message – Detailed Walkthrough

🗂️ Room Overview

• Room Name: Cipher's Secret Message
• Category: Crypto
• Difficulty: Easy
• Objective: Decrypt a secret message encrypted with a shifting Caesar cipher and retrieve the flag.

🧠 Challenge Prompt

We are given:

• An encrypted message:

a_up4qr_kaiaf0_bujktaz_qm_su4ux_cpbq_ETZ_rhrudm

The encryption logic:

from secret import FLAG
 
def enc(plaintext):
    return "".join(
        chr((ord(c) - (base := ord('A') if c.isupper() else ord('a')) + i) % 26 + base) 
        if c.isalpha() else c
        for i, c in enumerate(plaintext)
    )
 
with open("message.txt", "w") as f:
    f.write(enc(FLAG))

• Instruction: Wrap the decoded message in the flag format THM{}.

🔍 Step 1: Understand the Algorithm

The function enc() describes a modified Caesar cipher:

• It loops through each character in the message (enumerate gives both index i and character c).
• If the character is alphabetical:
  • Determine the base ('A' for uppercase, 'a' for lowercase).
  • Add the index i to the alphabetical offset of the character.
  • Take modulo 26 to wrap around the alphabet.
  • Convert back to a character.
• If it's not a letter (like a digit or underscore), it remains unchanged.

📌 Implication:

This cipher shifts each letter forward by its index position.

🔁 Step 2: Reversing the Cipher (Decryption Logic)

To decode:

• For every character at index i, subtract i from its position in the alphabet.
• Use modulo 26 to handle negative wraparound.
• Preserve non-alphabetic characters.

✅ Decryption Function in Python:

def decrypt(ciphertext):
    result = []
    for i, c in enumerate(ciphertext):
        if c.isalpha():
            base = ord('A') if c.isupper() else ord('a')
            decoded_char = chr((ord(c) - base - i) % 26 + base)
            result.append(decoded_char)
        else:
            result.append(c)
    return ''.join(result)

🧪 Step 3: Run the Decryption

🔐 Input:

a_up4qr_kaiaf0_bujktaz_qm_su4ux_cpbq_ETZ_rhrudm

🧾 Output:

a_sm4ll_crypt0_message_to_st4rt_with_THM_cracks

🏁 Step 4: Format the Flag

As per the instructions, wrap the result in THM{}:

🎯 Final Flag:

THM{a_sm4ll_crypt0_********************************}

✅ Summary

This walkthrough demonstrates how analyzing Python logic in challenges can help decode obfuscated or encrypted messages. Happy hacking!