Strangeness (Странность)

Platform: CUSTOM | Category: Reversing | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: static_analysis, xor_decryption

Summary

Walkthrough of the CUSTOM Reversing Strangeness (Странность). Task: Binary checks input for palindrome and outputs encrypted flag. Solution: Enter any palindrome to pass check, then XOR encrypted bytes with 0x41 to decrypt flag.

Description

The program asks for a string input and checks it for "strangeness". If the condition is met, it outputs the flag.

Analysis

When analyzing the strangeness binary (ELF 64-bit, not stripped), the following interesting points were discovered:

1. main function:

   • Takes user input.
   • Calls a check function (likely a palindrome check).
   • If the check passes, the decrypt function is called.

2. Check logic:

   • The program compares the string with its reversed copy. Thus, to get the flag, you need to enter any palindrome (e.g., aba or racecar). ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

3. decrypt function:

   • The .rodata section contains a byte array encrypted_flag.
   • The function iterates through this array and performs an XOR operation on each byte with the constant 0x41.

Solution

Extract the encrypted bytes from the binary and write a simple decryption script.

 python read-only • copy allowed Copy
#!/usr/bin/env python3

encrypted_flag = [
    0x05, 0x14, 0x02, 0x0a, 0x04, 0x13, 0x1b, 0x3a, 
    0x12, 0x71, 0x0c, 0x72, 0x1e, 0x12, 0x70, 0x33, 
    0x20, 0x2f, 0x26, 0x72, 0x1e, 0x07, 0x34, 0x2f, 
    0x22, 0x70, 0x28, 0x71, 0x2f, 0x3c
]
‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

key = 0x41
flag = "".join(chr(b ^ key) for b in encrypted_flag)

print(f"Flag: {flag}")

Running the script outputs the flag. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Key Takeaways / Lessons

• Mastering static_analysis, xor_decryption was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-01-09 • CUSTOM • Strangeness (Странность) • Difficulty: Easy