Qwe

Platform: CUSTOM | Category: Reversing | Type: Challenge | Difficulty: Medium | OS: N/A | **Author: XESXOR Techniques: java_bytecode_analysis, linear_system_solving, big_integer_arithmetic

Summary

Walkthrough of the CUSTOM Reversing Qwe. "Disassembly is ready, mylord!" Download: Qwe.java.dis (Java disassembly).

Description

"Disassembly is ready, mylord!" Download: Qwe.java.dis (Java disassembly).

Analysis

The Java disassembly contains a system of 3 linear equations:

5a + 4b + 4c = 11
4a + 5b + 4c = 8
4a + 4b + 5c = 7

Solution: a=3, b=0, c=-1.

Then the following is computed: 0xcafebabedeadbeef × (b-1) × a × c

Solution

 python read-only • copy allowed Copy
a, b, c = 3, 0, -1
base = 0xcafebabedeadbeef
result = base * (b - 1) * a * c
# = 0xcafebabedeadbeef * (-1) * 3 * (-1)
# = 0xcafebabedeadbeef * 3
# = REDACTED
flag = f"spbctf{{{result}}}"

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Key Takeaways / Lessons

• Mastering java_bytecode_analysis, linear_system_solving, big_integer_arithmetic was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-03-09 • CUSTOM • Qwe • Difficulty: Medium