CRC32檔案檢查

透過CRC32可以讓檔案產生一組驗證碼，這組驗證碼用來檢驗下載的檔案是否跟原作者提供的檔案一樣。
程式碼

using System;
using System.Collections;
using System.IO;
using System.Security.Cryptography;

namespace CodeProject
{
/// 

///
/// 
public class CRC32 : HashAlgorithm
{
protected static uint AllOnes = 0xffffffff;
protected static Hashtable cachedCRC32Tables;
protected static bool autoCache;

protected uint[] crc32Table;
private uint m_crc;

/// 

/// Returns the default polynomial (used in WinZip, Ethernet, etc)
/// 
public static uint DefaultPolynomial
{
get { return 0x04C11DB7; }
}

/// 

/// Gets or sets the auto-cache setting of this class.
/// 
public static bool AutoCache
{
get { return autoCache; }
set { autoCache = value; }
}

/// 

/// Initialize the cache
/// 
static CRC32()
{
cachedCRC32Tables = Hashtable.Synchronized( new Hashtable() );
autoCache = true;
}

public static void ClearCache()
{
cachedCRC32Tables.Clear();
}

/// 

/// Builds a crc32 table given a polynomial
/// 
///

/// 
protected static uint[] BuildCRC32Table( uint ulPolynomial )
{
uint dwCrc;
uint[] table = new uint[256];

// 256 values representing ASCII character codes.
for (int i = 0; i < 256; i++)                         {                                 dwCrc = (uint)i;                                 for (int j = 8; j > 0; j--)
{
if((dwCrc & 1) == 1)
dwCrc = (dwCrc >> 1) ^ ulPolynomial;
else
dwCrc >>= 1;
}
table[i] = dwCrc;
}

return table;
}

/// 

/// Creates a CRC32 object using the DefaultPolynomial
/// 
public CRC32() : this(DefaultPolynomial)
{
}

/// 

/// Creates a CRC32 object using the specified Creates a CRC32 object
/// 
public CRC32(uint aPolynomial) : this(aPolynomial, CRC32.AutoCache)
{
}

/// 

/// Construct the
/// 
public CRC32(uint aPolynomial, bool cacheTable)
{
this.HashSizeValue = 32;

crc32Table = (uint []) cachedCRC32Tables[aPolynomial];
if ( crc32Table == null )
{
crc32Table = CRC32.BuildCRC32Table(aPolynomial);
if ( cacheTable )
cachedCRC32Tables.Add( aPolynomial, crc32Table );
}
Initialize();
}

/// 

/// Initializes an implementation of HashAlgorithm.
/// 
public override void Initialize()
{
m_crc = AllOnes;
}

/// 

///
/// 
///

///

///

protected override void HashCore(byte[] buffer, int offset, int count)
{
// Save the text in the buffer.
for (int i = offset; i < count; i++)                         {                                 ulong tabPtr = (m_crc & 0xFF) ^ buffer[i];                                 m_crc >>= 8;
m_crc ^= crc32Table[tabPtr];
}
}

/// 

///
/// 
/// 
protected override byte[] HashFinal()
{
byte [] finalHash = new byte [ 4 ];
ulong finalCRC = m_crc ^ AllOnes;

finalHash[0] = (byte) ((finalCRC >> 24) & 0xFF);
finalHash[1] = (byte) ((finalCRC >> 16) & 0xFF);
finalHash[2] = (byte) ((finalCRC >>  8) & 0xFF);
finalHash[3] = (byte) ((finalCRC >>  0) & 0xFF);

return finalHash;
}

/// 

/// Computes the hash value for the specified Stream.
/// 
new public byte[] ComputeHash(Stream inputStream)
{
byte [] buffer = new byte [4096];
int bytesRead;
while ( (bytesRead = inputStream.Read(buffer, 0, 4096)) > 0 )
{
HashCore(buffer, 0, bytesRead);
}
return HashFinal();
}

/// 

/// Overloaded. Computes the hash value for the input data.
/// 
new public byte[] ComputeHash(byte[] buffer)
{
return ComputeHash(buffer, 0, buffer.Length);
}

/// 

/// Overloaded. Computes the hash value for the input data.
/// 
///

///

///

/// 
new public byte[] ComputeHash( byte[] buffer, int offset, int count )
{
HashCore(buffer, offset, count);
return HashFinal();
}
}

}