crackme4

Platform: rev-kids20.forkbomb.ru | Category: Reversing | Type: Challenge | Difficulty: Easy | OS: NA | Author: XESXOR | Date: 2026-03-09 | Status: Solved Techniques: keygen_implementation, xor_swap_analysis

Summary

Language: Python. Source code provided. Generate serial for name 'SPb_CTF_2017'.

crackme4 — Reverse Kids 2.0

Description

Language: Python. Source code provided. Generate serial for name 'SPb_CTF_2017'.

Analysis

 python read-only • copy allowed Copy
def check(name, serial):
    name = bytes(name, 'utf-8')
    valid = [
        int.from_bytes(name[:4], 'big'),
        int.from_bytes(name[4:8], 'big'),
        int.from_bytes(name[8:], 'big')
    ]
    valid[0] ^= valid[2]  # XOR swap
    valid[2] ^= valid[0]
    valid[0] ^= valid[2]
    return serial == '{:08x}-{:08x}-{:08x}'.format(*valid)

The name is split into 3 blocks of 4 bytes each, then the first and third blocks are swapped using XOR swap. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Solution

 python read-only • copy allowed Copy
name = 'SPb_CTF_2017'
name_bytes = bytes(name, 'utf-8')
valid = [
    int.from_bytes(name_bytes[:4], 'big'),   # SPb_ = 0x5350625f
    int.from_bytes(name_bytes[4:8], 'big'),  # CTF_ = 0x4354465f
    int.from_bytes(name_bytes[8:], 'big')    # 2017 = 0x32303137
]
# XOR swap: valid[0] and valid[2] are swapped
valid[0], valid[2] = valid[2], valid[0]
serial = '{:08x}-{:08x}-{:08x}'.format(*valid)
# REDACTED

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍