Bandit Level 11 to 12: Cracking the Caesar Cipher (ROT13)

Introduction

After decoding Base64, we now face our first real cipher in Bandit Level 11 to 12. The password isn’t encoded for data transport; it’s been deliberately obfuscated using a simple character-shifting algorithm.

This level introduces the Caesar cipher, one of the oldest and most well-known forms of encryption. We’ll learn how to crack it with tr, a command-line utility for translating characters. 📜

The Challenge: Level 11 Goal

The level’s objective is described as follows:

The password for the next level is stored in the file data.txt, where all lowercase (a-z) and uppercase (A-Z) letters have been rotated by 13 positions.

This specific cipher, where letters are rotated by 13 places, is famously known as ROT13.

Step-by-Step Walkthrough

Let’s reverse time on this ancient cipher.

Step 1: Log into bandit11

Use the decoded Base64 password from the last level to SSH into bandit11.

ssh [email protected] -p 2220

Step 2: Examine the Enciphered Text

Let’s look at the contents of data.txt.

cat data.txt

The output will look like nonsense, but you might notice that it retains the structure of words and sentences. This is a hallmark of a simple substitution cipher.

Gur cnffjbeq vf PVTeKys4T5P-GUYf-fMRf-c0s-g2-m-L-f-

Step 3: The Solution - The tr Command

The tr command is perfect for this. It “translates” or substitutes characters from one set to another. To reverse ROT13, we need to map every letter to the letter 13 places away from it in the alphabet.

The clever trick with ROT13 is that applying it twice gets you back to the original text. So, we just need to apply the same ROT13 transformation.

• The first half of the alphabet (A-M and a-m) maps to the second half (N-Z and n-z).
• The second half (N-Z and n-z) maps back to the first (A-M and a-m).

The tr command to do this is:

tr 'INPUT_SET' 'OUTPUT_SET'

Our input set is all the letters: A-Za-z Our output set is the rotated version: N-ZA-Mn-za-m

Notice how N-ZA-M correctly maps A-M to N-Z and N-Z back to A-M.

Step 4: Execute the Command

We will pipe the contents of the file into our tr command to perform the translation.

cat data.txt | tr 'A-Za-z' 'N-ZA-Mn-za-m'

The command will process the ROT13 text and print the original, readable English text, revealing the password for bandit12.

The password is 7x16WNeHIi5YkIhWsfFIqoognUTyj9Q4

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Key Concepts Learned

1. Caesar Cipher/ROT13: We learned what a simple substitution cipher is. ROT13 is often used in online forums to hide spoilers or punchlines, not for serious security.
2. tr Command: This is a powerful utility for performing character-based translations on streams of text. It’s a fundamental tool for text manipulation on the command line.
3. Character Sets and Ranges: We used character ranges like A-Z and a-m to build our translation sets, a common feature in many command-line tools.

Conclusion

You’ve successfully cracked your first cryptographic cipher! While ROT13 is simple, the principles of analyzing and reversing transformations are fundamental to cybersecurity.

Save the password for bandit12 and prepare for the next challenge!