CheckSum.cs :  » Aspect-Oriented-Frameworks » Runtime-Assembly-Instrumentation-Library » Mono » PEToolkit » C# / CSharp Open Source

/*
 * Copyright (c) 2002 Sergey Chaban <serge@wildwestsoftware.com>
 */

// see http://cvs.winehq.com/cvsweb/wine/dlls/imagehlp/modify.c
// starting from Revision 1.8

using System;
using System.IO;

namespace Mono.PEToolkit{

  public sealed class CheckSum {

    private CheckSum()
    {
      // Never instantiated.
    }


    public static uint Calc(string peFile)
    {
      uint res = 0;

      FileInfo pe = new FileInfo(peFile);
      if (!pe.Exists) {
        throw new Exception("CheckSum : Invalid file path.");
      }

      using (BinaryReader reader = new BinaryReader(pe.OpenRead())) {
        if (!reader.BaseStream.CanSeek) {
          throw new Exception("Can't seek.");
        }

        DOSHeader dosHdr = new DOSHeader();
        COFFHeader coffHdr = new COFFHeader();
        PEHeader peHdr = new PEHeader();

        dosHdr.Read (reader);
        reader.BaseStream.Position = dosHdr.Lfanew;
        ExeSignature peSig = (ExeSignature) reader.ReadUInt16();
        if (peSig != ExeSignature.NT) {
          throw new BadImageException("Checksum : Invalid image format, cannot find PE signature.");
        }
  
        peSig = (ExeSignature) reader.ReadUInt16();
        if (peSig != ExeSignature.NT2) {
          throw new BadImageException("Checksum : Invalid image format, cannot find PE signature.");
        }

        coffHdr.Read(reader);
        peHdr.Read(reader);

        uint oldSum = peHdr.CheckSum;
        reader.BaseStream.Position = 0;
        long len = pe.Length;
        long whole = len >> 1;
        uint sum = 0;
        uint hi, lo;
        for (long i = whole; --i >= 0;) {
          sum += reader.ReadUInt16();
          hi = sum >> 16;
          if (hi != 0) {
            sum = hi + (sum & 0xFFFF);
          }
        }
        if ((len & 1L) != 0) {
          sum += (uint) reader.ReadByte();
          hi = sum >> 16;
          if (hi != 0) {
            sum = hi + (sum & 0xFFFF);
          }
        }

        // fix low word of checksum
        lo = oldSum & 0xFFFF;
        if ((sum & 0xFFFF) >= lo) {
          sum -= lo;
        } else {
          sum = (((sum & 0xFFFF) - lo) & 0xFFFF) - 1;
        }

        // fix high word of checksum
        hi = oldSum >> 16;
        if ((sum & 0xFFFF) >= hi) {
          sum -= hi;
        } else {
          sum = (((sum & 0xFFFF) - hi) & 0xFFFF) - 1;
        }
      }

      return res;
    }
  }

}