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02 - Ultimate Minesweeper

f1802 intro

Ultimate Minesweeper is a ~1MB executable. CFF tags it as a 32-bit .NET executable with some very amusing properties. Time to break out dotNetSpy.

f1802 cff


  • CFF: CFF Explorer (Explorer Suite) - Graphical PE file parser with built-in Resource and Hex Editors
  • dotNetSpy: .NET reverse engineering framework with built-in debugger and much more

Xtreme Gaming

Running the executable (you're doing this in a VM, right?) presents us with the following Minesweeper grid:

f1802 main screen

A 30x30 grid with 897 mines. That's pretty ... extreme. We definitely need to peek under the hood if we are to have any chance of finding the three mine-free squares, dotNetSpy to the rescue:

f1802 dnSpy

Ultimate Minesweeper is not heavily obfuscated, it consists of six classes with all the interesting code being in MainForm. The MineField class is responsible for tracking the state of each cell in the game grid. The weird thing about it is that it has two identical accessors for the grid:

// Token: 0x17000003 RID: 3
// (get) Token: 0x06000014 RID: 20 RVA: 0x00002AE4 File Offset: 0x00000CE4
// (set) Token: 0x06000015 RID: 21 RVA: 0x00002AEC File Offset: 0x00000CEC
public bool[,] MinesPresent
	get { return this.minesPresent; }
	set	 { this.minesPresent = value; }

// Token: 0x17000004 RID: 4
// (get) Token: 0x06000016 RID: 22 RVA: 0x00002AE4 File Offset: 0x00000CE4
// (set) Token: 0x06000017 RID: 23 RVA: 0x00002AEC File Offset: 0x00000CEC
public bool[,] GarbageCollect
	get { return this.minesPresent;  }
	set { this.minesPresent = value; }

The GarbageCollect property is just a misleading alias for the MinePresent property. Going through the constructor code (shown in the dotNetSpy image of) brings us to a similarly misleading method called AllocateMemory which uses the GarbageCollect property to initialize the minefield. The constant VALLOC_NODE_LIMIT is the grid size (30) and the list called VALLOC_TYPES contains the obfuscated locations of the only cells in the game that do not have mines (flag is set to false when the mine location is contained the the VALLOC_TYPES list). Location obfuscation is performed by the function DeriveVallocType(uint r, uint c) which converts a 1-based grid location to a 32-bit unsigned by taking the binary complement of r * NUM_COLUMNS + c. The VALLOC_TYPES array is initialized on construction as follows:

// Token: 0x04000007 RID: 7
private static uint VALLOC_TYPE_HEADER_PAGE = 4294966400u;

// Token: 0x04000008 RID: 8
private static uint VALLOC_TYPE_HEADER_POOL = 4294966657u;

// Token: 0x04000009 RID: 9
private static uint VALLOC_TYPE_HEADER_RESERVED = 4294967026u;

// Token: 0x0400000A RID: 10
private uint[] VALLOC_TYPES = new uint[]

We can find the locations of the mine-free squares with a couple of lines of python:

>>> def decm(k):
    k = ~k & 0xffffffff
    return (k/30, k%30)

>>> map(decm,[4294966400,4294966657,4294967026])
[(29L, 25L), (21L, 8L), (8L, 29L)]

Once the game is won, the GetKey function is responsible for decoding the flag:

// Token: 0x0600000D RID: 13 RVA: 0x000023E4 File Offset: 0x000005E4
private string GetKey(List<uint> revealedCells)
	Random random = new Random(Convert.ToInt32(revealedCells[0] << 20 | revealedCells[1] << 10 | revealedCells[2]));
	byte[] array = new byte[32];
	byte[] array2 = new byte[]
	{ 245, 75, 65, 142, 68, 71, 100, 185, 74, 127, 62, 130, 231, 129, 254, 243, 28, 58,
	  103, 179, 60, 91, 195, 215, 102, 145, 154, 27, 57, 231, 241, 86
	uint num = 0u;
	while ((ulong)num < (ulong)((long)array2.Length))
		byte[] expr_5B_cp_0 = array2;
		uint expr_5B_cp_1 = num;
		expr_5B_cp_0[(int)expr_5B_cp_1] = (expr_5B_cp_0[(int)expr_5B_cp_1] ^ array[(int)num]);
		num += 1u;
	return Encoding.ASCII.GetString(array2);

The variable array2 contains the encrypted flag which GetKey decrypts by xor-ing with a byte stream generated from the .NET random number generator (class Random) seeded by the sorted list of the (0-based) locations of mine-free squares (the revealedCells list). We can write a bit of C# [code][./Program.cs] that uses the cell locations we've discovered to decrypt the flag, but it's just easier to just run the game and click on the cells:

f1802 win