Cipher's Secret Message - TryHackMe writeup

Task 1. Crypto Cipher's Secret Message

Answer the questions below

What is the flag?

THM{a_sm4ll_crypt0_message_to_st4rt_with_THM_cracks}

Writeup

Decrypt a custom Caesar cipher with Python and uncover the flag**
Reverse a Caesar-style encryption by analyzing Python code in this beginner-friendly TryHackMe challenge.

🧩 Overview

In this TryHackMe room, Cipher's Secret Message, you’re handed a jumbled string and the encryption function that produced it. Your mission? Figure out how the cipher works, reverse it, and grab the flag.

Here’s what you’re given:

• 🔐 An encrypted message
• 🧠 The encryption logic (written in Python)
• 🏁 A hint that the result should be wrapped in THM{}

If you’re just getting into cryptography CTFs, this is a perfect warm-up.

📜 The Encrypted Message

Here’s the message you need to decode:

a\_up4qr\_kaiaf0\_bujktaz\_qm\_su4ux\_cpbq\_ETZ\_rhrudm

Looks messy, right? But there’s a pattern hiding in there — let’s break it down.

🔍 The Encryption Logic

You’re given the Python function that was used to scramble the original flag. Here's what it looks like:

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)
    )

🧠 How it Works:

• The code loops through each character in the plaintext along with its index i.
• If the character is a letter, it gets shifted forward by i positions in the alphabet.
• Uppercase and lowercase are handled separately.
• Non-alphabetic characters (_, 4, 0, etc.) stay the same.

In short: it’s a position-based Caesar cipher — the further into the string, the more the letter shifts.

🔓 The Decryption Script

To crack this, we just need to reverse the logic: instead of shifting letters forward by their index, we’ll shift them backward.

Here’s the Python code to do exactly that:

cipher = "a_up4qr_kaiaf0_bujktaz_qm_su4ux_cpbq_ETZ_rhrudm"

def dec(ciphertext):
    result = ""
    for i, c in enumerate(ciphertext):
        if c.isalpha():
            base = ord('A') if c.isupper() else ord('a')
            result += chr((ord(c) - base - i) % 26 + base)
        else:
            result += c
    return result

print(dec(cipher))

🧾 Output

When you run the script, you’ll get this decoded message:

a_sm4ll_crypt0_message_to_st4rt_with_THM_cracks

Nice! That’s the original string before encryption.

🏁 Final Flag

To complete the challenge, wrap the decrypted message in THM{}:

THM{a_sm4ll_crypt0_message_to_st4rt_with_THM_cracks}

Submit this on TryHackMe, and you’re done ✅

✅ Summary

This was a simple but fun challenge that walked through:

• 🔍 Understanding a custom Caesar cipher
• 🐍 Using Python to reverse-engineer the encryption
• 🏁 Extracting a clean flag for submission

Great for practicing basic cryptanalysis, string manipulation, and Python logic.

💡 Pro Tips for CTFs

• Caesar-style ciphers with variable shifts show up often in beginner CTFs.
• Use Python’s ord() and chr() to convert between letters and numbers.
• enumerate() is perfect for handling index-based transformations.

The more you see these patterns, the faster you'll crack future challenges.

💬 Let’s Connect!

If you found this helpful, feel free to follow me for more hands-on walkthroughs in:

• 🔐 Cryptography and CTFs
• 🐍 Python scripting for security
• 🛠️ Practical reverse engineering

Stay curious and keep hacking! 🚀