using System;
using System.IO;

// Used to obtain the file offset and the size of the strong name signature
// and the strong name data directory
public class StrongNameSignature
{
   // Locations and sizes of various things in the PE file
   const int pePos = 0x003c;
   const int numSectOffset = 0x02;
   const int peIdentSize = 0x04;
   const int coffHeaderSize = 0x14;
   const int dataDirectoryOffset = 0x60;
   const int dataDirectoryLength = 0x08;
   const int clrHeaderIndex = 0x0e;
   const int strongNameSigOffset = 0x20;
   const int sectionHeaderSize = 0x28;
   const int sectionSizeOffSet = 0x08;
   const int sectionRVAOffset = 0x0c;
   const int sectionRawOffset = 0x14;

   int strongNameSigSize = 0;
   // Returns the size of the strong name signature
   public int Size
   {
      get{return strongNameSigSize;}
   }

   int strongNameSig = 0;
   // Returns the offset in the file of the strong name signature
   public int Address
   {
      get{return strongNameSig;}
   }

   int strongNameDataDirectory = 0;
   // Returns the offset of the strong name data directory
   public int StrongNameDataDirectory
   {
      get{return strongNameDataDirectory;}
   }

   FileStream fs;
   // Passed an open file of a .NET assembly
   public StrongNameSignature(FileStream file)
   {
      fs = file;

      // Find the pointer to the PE identifier before the COFF header 
      int coffHeader = GetInt(pePos);
      // Get the number of sections from the COFF header
      short numSections = GetWord(coffHeader + peIdentSize + numSectOffset);

      // Calculate the location of the PE header
      int peHeader = coffHeader + coffHeaderSize + peIdentSize;
      // Determine the location of the data directories
      int dataDirectories = peHeader + dataDirectoryOffset;
      // Determine the location of the CLR directory
      int clrHeaderDD = dataDirectories + (dataDirectoryLength * clrHeaderIndex);

      // Read the RVA of the CLR header
      int clrHeader = GetInt(clrHeaderDD);
      // Read the size of the CLR header
      int clrHeaderSize = GetInt(clrHeaderDD + 4);

      // Note that CLR header is stored in the .text section, so read this section
      // so that we can convert between RVA and actual file offsets
      int textRVA = 0;
      int textRaw = 0;
      int textSize = 0;
      // Determine the location of the section headers
      int sections = clrHeaderDD + (2 * dataDirectoryLength);
      // Iterate through the section headers until we find the header for the 
      // .text section
      byte[] bText = {0x2e, 0x74, 0x65, 0x78, 0x74, 0x00, 0x00, 0x00};
      for (int idx = 0; idx < numSections; ++idx)
      {
         // Read the first eight bytes which will have the name of the section
         // Note that this is NOT a NUL terminated string, the section name
         // can be 8 characters.
         int sectionStart = sections + (idx * sectionHeaderSize);
         byte[] eightBuf = GetEightBytes(sectionStart);
         if (!Compare(eightBuf, bText)) continue;

         // We have the right section so get the values
         textSize = GetInt(sectionStart + sectionSizeOffSet);
         textRVA = GetInt(sectionStart + sectionRVAOffset);
         textRaw = GetInt(sectionStart + sectionRawOffset);
      }

      // Convert RVA to file offset
      clrHeader = clrHeader - textRVA + textRaw;
      // Calculate the file offset of the strong name data directory 
      strongNameDataDirectory = clrHeader + strongNameSigOffset;
      // Get the RVA of the strong name signature
      strongNameSig = GetInt(clrHeader + strongNameSigOffset);
      // Convert RVA to file offset
      strongNameSig = strongNameSig - textRVA + textRaw;
      // Get the size of the strong name signature
      strongNameSigSize = GetInt(clrHeader + strongNameSigOffset + 4);
   }

   // Do a character by character comparison
   private static bool Compare(byte[] lhs, byte[] rhs)
   {
      int size = (lhs.Length > rhs.Length) ? rhs.Length : lhs.Length;
      for (int idx = 0; idx < size; ++idx)
      {
         if (lhs[idx] != rhs[idx]) return false;
      }
      return true;
   }
   // Read a 16-bit value from the file
   private short GetWord(int pos)
   {
      byte[] wordBuf = new byte[2];
      fs.Seek(pos, SeekOrigin.Begin);
      fs.Read(wordBuf, 0, wordBuf.Length);
      return BitConverter.ToInt16(wordBuf, 0);
   }
   // Read a 32-bit value from the file
   private int GetInt(int pos)
   {
      byte[] intBuf = new byte[4];
      fs.Seek(pos, SeekOrigin.Begin);
      fs.Read(intBuf, 0, intBuf.Length);
      return BitConverter.ToInt32(intBuf, 0);
   }
   // Read an eight byte array from the file
   private byte[] GetEightBytes(int pos)
   {
      byte[] eightBuf = new byte[8];
      fs.Seek(pos, SeekOrigin.Begin);
      fs.Read(eightBuf, 0, eightBuf.Length);
      return eightBuf;
   }
}

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