Roulette Simulator Lab

A complete writeup and automated solver for the XorShift Roulette Java crackme by Rodrigo Teixeira. This challenge involves reverse-engineering a custom 16-bit PRNG and exploiting a 32-bit integer overflow.

🖥️ Machine Specifications

• Author: RodrigoTeixeira
• Platform: Multiplatform
• Architecture: Java
• Language: Java
• Difficulty: 🟢 3.0/6.0
• Quality: ⭐ 4.0/5.0
• Download: https://crackmes.one/download/crackme/693c48822d267f28f69b8518
• Password: crackmes.one

📌 Overview

• Platform: Java SE 19 (Class version 63.0).
• Objective: Make your balance go NEGATIVE to win.
• Constraint: Bets are capped to your current balance, making a negative balance through "honest" play impossible.
• Solution: Identify the 16-bit XorShift seed through observation and use Java's silent integer wraparound to flip the balance.

🛠️ Technical Analysis

1. The PRNG (XorShift-16)

Bytecode analysis of the rand() method reveals a 3-tap XorShift algorithm:

• Algorithm:
  • seed ^= (seed << 7) & 0xFFFF
  • seed ^= (seed >> 9)
  • seed ^= (seed << 8) & 0xFFFF
• Seed Source: System.nanoTime() & 0xFFFF.
• State Space: 65,535 possible values with a single cycle length of 65,535.

2. The Vulnerability

The game loop uses the iadd instruction for winning bets without performing overflow checks.

• Max Value: $2,147,483,647$ (32-bit signed integer).
• Exploit: If money + bet exceeds the maximum integer value, it wraps to a negative value.
• Threshold: You must reach a balance of at least $1,073,741,824$ so that an "all-in" win triggers the overflow.

🚀 Usage

Phase 1: Probe (Seed Recovery)

1. Start the crackme and the Python solver script.
2. Bet 1 in the game for approximately 20-30 rounds.
3. Enter the outcome (W for Win, L for Loss) into the script for each round.
4. The script compares your sequence against a precomputed map to identify the exact seed.

Phase 2: Exploit (Overflow)

1. Once the seed is identified, follow the script's betting instructions.
2. On Win rounds: Bet your ENTIRE balance to achieve exponential growth.
3. On Loss rounds: Bet 1 to minimize losses.
4. After ~50-70 total rounds, the balance will overflow to a negative number, triggering the WIN condition.

📊 Summary

Feature	Detail
Language	Java (Compiled .class)
PRNG Type	16-bit XorShift
Seed Space	65,535 (fully enumerable)
Vulnerability	Unchecked iadd integer overflow
Rounds to Solve	~50 total rounds

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This writeup is for educational purposes only, focusing on reverse engineering and PRNG cryptanalysis.