[C#] oembios.dat decryption in C#

Code:

enum KeyType
{
  WindowsXP,
  Windows2003
}

void DecryptOembios(byte[] data, KeyType keyType)
{
  // select keys
  byte[] key0;
  uint[] key1;

  if (keyType == KeyType.WindowsXP)
  {
    key0 = new byte[] { 0x65, 0x48, 0x73, 0xb3, 0x19, 0x81, 0x67, 0xca, 0xbf, 0x02, 0xfd, 0x2e, 0xb7, 0xe3, 0x3d, 0x17 };
    key1 = new uint[] { 0x6bef9812, 0xe4ccc512, 0x08714298, 0x3ed2ee96 };
  }
  else
  {
    key0 = new byte[] { 0x76, 0xd0, 0xe7, 0x70, 0xf6, 0xb4, 0xd1, 0x76, 0x13, 0xcd, 0x6e, 0x06, 0xd7, 0x28, 0x2c, 0xce };
    key1 = new uint[] { 0xf3462a5d, 0xc605e0d6, 0x0e524b90, 0x80649049 };
  }

  // initialize keystream (using key0)
  byte[] keystream = new byte[256];
  for (int i = 0; i < 256; ++i)
    keystream = (byte)i;

  byte swapIndex = 0;
  for (int i = 0; i < 256; ++i)
  {
    swapIndex += (byte)(keystream + key0[i & 0xf]);
    byte swap = keystream;
    keystream = keystream[swapIndex];
    keystream[swapIndex] = swap;
  }

  // first step (using keystream)
  byte keystreamIndex0 = 0, keystreamIndex1 = 0;
  for (int offset = 0; offset < data.Length; ++offset)
  {
    keystreamIndex1 += keystream[++keystreamIndex0];
    byte swap = keystream[keystreamIndex1];
    keystream[keystreamIndex1] = keystream[keystreamIndex0];
    keystream[keystreamIndex0] = swap;
    data[offset] ^= keystream[(byte)(keystream[keystreamIndex0] + keystream[keystreamIndex1])];
  }

  // second step (using key1)
  for (int offset = data.Length - 8; offset >= 0; --offset)
  {
    uint data0 = BitConverter.ToUInt32(data, offset);
    uint data1 = BitConverter.ToUInt32(data, offset + 4);
    uint tmp = 0xc6ef3720;
    for (int i = 0; i < 32; ++i)
    {
      data1 -= tmp + key1[tmp >> 11 & 3] ^ data0 + (data0 << 4 ^ data0 >> 5);
      tmp += 0x61c88647;
      data0 -= tmp + key1[tmp & 3] ^ data1 + (data1 << 4 ^ data1 >> 5);
    }
    BitConverter.GetBytes(data0).CopyTo(data, offset);
    BitConverter.GetBytes(data1).CopyTo(data, offset + 4);
  }
}

 

Code:

byte[] oembios = System.IO.File.ReadAllBytes(@"C:\oembios.dat");
DecryptOembios(oembios, KeyType.WindowsXP);
System.IO.File.WriteAllBytes(@"C:\oembios_decrypted.txt", oembios);

 

Code:

using System;
using System.IO;

namespace OembiosDecrypt
{
  class Program
  {
    enum KeyType
    {
      WindowsXP,
      Windows2003
    }

    private static void DecryptOembios(byte[] data, KeyType keyType)
    {
      // select keys
      byte[] key0;
      uint[] key1;

      if (keyType == KeyType.WindowsXP)
      {
        key0 = new byte[] { 0x65, 0x48, 0x73, 0xb3, 0x19, 0x81, 0x67, 0xca, 0xbf, 0x02, 0xfd, 0x2e, 0xb7, 0xe3, 0x3d, 0x17 };
        key1 = new uint[] { 0x6bef9812, 0xe4ccc512, 0x08714298, 0x3ed2ee96 };
      }
      else
      {
        key0 = new byte[] { 0x76, 0xd0, 0xe7, 0x70, 0xf6, 0xb4, 0xd1, 0x76, 0x13, 0xcd, 0x6e, 0x06, 0xd7, 0x28, 0x2c, 0xce };
        key1 = new uint[] { 0xf3462a5d, 0xc605e0d6, 0x0e524b90, 0x80649049 };
      }

      // initialize keystream (using key0)
      byte[] keystream = new byte[256];
      for (int i = 0; i < 256; ++i)
        keystream = (byte)i;

      byte swapIndex = 0;
      for (int i = 0; i < 256; ++i)
      {
        swapIndex += (byte)(keystream + key0[i & 0xf]);
        byte swap = keystream;
        keystream = keystream[swapIndex];
        keystream[swapIndex] = swap;
      }

      // first step (using keystream)
      byte keystreamIndex0 = 0, keystreamIndex1 = 0;
      for (int offset = 0; offset < data.Length; ++offset)
      {
        keystreamIndex1 += keystream[++keystreamIndex0];
        byte swap = keystream[keystreamIndex1];
        keystream[keystreamIndex1] = keystream[keystreamIndex0];
        keystream[keystreamIndex0] = swap;
        data[offset] ^= keystream[(byte)(keystream[keystreamIndex0] + keystream[keystreamIndex1])];
      }

      // second step (using key1)
      for (int offset = data.Length - 8; offset >= 0; --offset)
      {
        uint data0 = BitConverter.ToUInt32(data, offset);
        uint data1 = BitConverter.ToUInt32(data, offset + 4);
        uint tmp = 0xc6ef3720;
        for (int i = 0; i < 32; ++i)
        {
          data1 -= tmp + key1[tmp >> 11 & 3] ^ data0 + (data0 << 4 ^ data0 >> 5);
          tmp += 0x61c88647;
          data0 -= tmp + key1[tmp & 3] ^ data1 + (data1 << 4 ^ data1 >> 5);
        }
        BitConverter.GetBytes(data0).CopyTo(data, offset);
        BitConverter.GetBytes(data1).CopyTo(data, offset + 4);
      }
    }

    static void Main(string[] args)
    {
      if (args.Length < 2 || args.Length > 3) goto ShowUsage;
      
      KeyType keyType;

      if (args[0] == "dx")
        keyType = KeyType.WindowsXP;
      else if (args[0] == "d3")
        keyType = KeyType.Windows2003;
      else
        goto ShowUsage;

      try
      {
        byte[] oembios = File.ReadAllBytes(args[1]);
        DecryptOembios(oembios, keyType);
        if (args.Length == 3)
          File.WriteAllBytes(args[2], oembios);
        else
          using (Stream stdout = Console.OpenStandardOutput())
            stdout.Write(oembios, 0, oembios.Length);
      }
      catch (Exception ex)
      {
        Console.Error.WriteLine("Error ({0}): {1}", ex.GetType(), ex.Message);
      }

      return;

    ShowUsage:
      Console.WriteLine("Usage: oembiosdecrypt.exe <command> <input file> [output file]");
      Console.WriteLine("Commands:");
      Console.WriteLine("\tdx\tDecrypt using XP key");
      Console.WriteLine("\td3\tDecrypt using 2003 key");
    }
  }
}