一、理論部分：
1、預備知識
1.1什么是數(shù)據(jù)校驗
通俗的說，就是為保證數(shù)據(jù)的完整性，用一種指定的算法對原始數(shù)據(jù)計算出的一個校驗值。接收方用同樣的算法計算一次校驗值，如果和隨數(shù)據(jù)提供的校驗值一樣，就說明數(shù)據(jù)是完整的。
1.2最簡單的檢驗
實現(xiàn)方法：最簡單的校驗就是把原始數(shù)據(jù)和待比較數(shù)據(jù)直接進行比較，看是否完全一樣這種方法是最安全最準確的。同時也是效率最低的。
適用范圍：簡單的數(shù)據(jù)量極小的通訊。
應用例子：龍珠cpu在線調(diào)試工具bbug.exe。它和龍珠cpu間通訊時，bbug發(fā)送一個字節(jié)cpu返回收到的字節(jié)，bbug確認是剛才發(fā)送字節(jié)后才繼續(xù)發(fā)送下一個字節(jié)的。
1.3奇偶校驗Parity Check
實現(xiàn)方法：在數(shù)據(jù)存儲和傳輸中，字節(jié)中額外增加一個比特位，用來檢驗錯誤。校驗位可以通過數(shù)據(jù)位異或計算出來。
應用例子：單片機串口通訊有一模式就是8位數(shù)據(jù)通訊，另加第9位用于放校驗值。
1.4 bcc異或校驗法(block check character)
實現(xiàn)方法：很多基于串口的通訊都用這種既簡單又相當準確的方法。它就是把所有數(shù)據(jù)都和一個指定的初始值（通常是0）異或一次，最后的結(jié)果就是校驗值，通常
把她附在通訊數(shù)據(jù)的最后一起發(fā)送出去。接收方收到數(shù)據(jù)后自己也計算一次異或和校驗值，如果和收到的校驗值一致就說明收到的數(shù)據(jù)是完整的。
校驗值計算的代碼類似于：
unsigned uCRC=0;//校驗初始值
for(int i=0;i<DataLenth;i++) uCRC^=Data[i];
適用范圍：適用于大多數(shù)要求不高的數(shù)據(jù)通訊。
應用例子：ic卡接口通訊、很多單片機系統(tǒng)的串口通訊都使用。
1.5 crc循環(huán)冗余校驗(Cyclic Redundancy Check)
實現(xiàn)方法：這是利用除法及余數(shù)的原理來進行錯誤檢測的.將接收到的碼組進行除法運算
，如果除盡，則說明傳輸無誤；如果未除盡，則表明傳輸出現(xiàn)差錯。crc校驗
具還有自動糾錯能力。
crc檢驗主要有計算法和查表法兩種方法，網(wǎng)上很多實現(xiàn)代碼。
適用范圍：CRC-12碼通常用來傳送6-bit字符串;CRC-16及CRC-CCITT碼則用是來傳送
8-bit字符。CRC-32：硬盤數(shù)據(jù)，網(wǎng)絡傳輸?shù)?br>應用例子：rar,以太網(wǎng)卡芯片、MPEG解碼芯片中
1.6 md5校驗和數(shù)字簽名
實現(xiàn)方法：主要有md5和des算法。
適用范圍：數(shù)據(jù)比較大或要求比較高的場合。如md5用于大量數(shù)據(jù)、文件校驗，des用于保密數(shù)據(jù)的校驗（數(shù)字簽名）等等。
應用例子：文件校驗、銀行系統(tǒng)的交易數(shù)據(jù)
2、具體的實現(xiàn)理論
2.1 算法概述
MD5算法是MD4算法的改進算法。Ron Rivest 于1990年提出MD4單向散列函數(shù)，MD表示消息摘要(Message Digest),對輸入消息，算法產(chǎn)生128位散列值。該算法首次公布之后，Bert den Boer和Antoon Bosselaers 對算法三輪中的后兩輪進行了成功的密碼分析。在一個不相關的分析結(jié)果中,Ralph MerKle成功地攻擊了前兩輪。盡管這些攻擊都沒有擴展到整個算法，但Rivest還是改進了其算法，結(jié)果就是MD5算法。
MD5算法是MD4的改進算法，它比MD4更復雜，但設計思想相似，輸入的消息可任意長，輸出結(jié)果也仍為128位，特別適用于高速軟件實現(xiàn)，是基于32-位操作數(shù)的一些簡單的位操作。
2.2 算法步驟
l 將輸入消息按512-位分組，最后要填充成為512位的整數(shù)倍，且最后一組的后64位用來填充消息長度(填充前)。填充方法為附一個1在消息后，后接所要求的多個0。這樣可以確保不同消息在填充后不相同。
l 由于留出64位用來表示消息長度，那么消息的長度最多可達264字節(jié),相當于4G×4G字節(jié)，文件的長度是不可能達到這么大，因此通常都是只采用64位中的低32位來表示消息長度，高32位填充0。
l 初始化MD變量。由于每輪輸出128位，這128位可用下面四個32位字A,B,C,D來表示。其初始值設為：
A=0x01234567
B=0x89ABCDEF
C=0xFEDCBA98
D=0x76543210
l 開始進入算法主循環(huán)，循環(huán)的次數(shù)是消息中512位消息分組的數(shù)目。先將上面A、B、C、D四個變量分別復制到另外四個變量a、b、c、d中去。主循環(huán)有四輪，每輪很相似。每輪進行16次操作，每次操作對a、b、c、d四個變量中的三個作一次非線性函數(shù)運算，然后將所得結(jié)果加上第四個變量，消息的一個子分組和一個常數(shù)。再將所得結(jié)果向右環(huán)移一個不定的數(shù)，并加上a,b,c或d中之一。最后用該結(jié)果取代a,b,c或d中之一。
以下是每次操作中用到的四個非線性函數(shù)（每輪一個）。
F(X,Y,Z)=(X∧Y)∨(( X)∧Z)
G(X,Y,Z)=(X∧Z)∨(Y∧( Z))
H(X,Y,Z)=X⊕Y⊕Z
I(X,Y,Z)=Y⊕(X∨( Z))
其中，⊕是異或，∧是與，∨是或， 是反符號。
這些函數(shù)是這樣設計的：如果X、Y和Z的對應位是獨立和均勻的，那么結(jié)果的每一位也應是獨立和均勻的。函數(shù)F是按逐位方式操作：如果X,那么Y,否則Z。函數(shù)H是逐位奇偶操作符。
設Mj表示消息的第j個子分組（從0到15），<<<s表示循環(huán)左移s，則四種操作為：
FF(a,b,c,d,Mj,s,ti)表示a = b+((a+F(b,c,d)+ Mj + ti)<<<s)
GG(a,b,c,d,Mj,s,ti)表示a = b+((a+G(b,c,d)+ Mj + ti)<<<s)
HH(a,b,c,d,Mj,s,ti)表示a = b+((a+H(b,c,d)+ Mj + ti)<<<s)
II(a,b,c,d,Mj,s,ti)表示a = b+((a+I(b,c,d)+ Mj + ti)<<<s)
四輪(64步)結(jié)果略。
注：常數(shù)ti的選擇：
第i步中，ti是232 ×abs (sin(i))的整數(shù)部分，i的單位是弧度。
所有這些完成之后，將A,B,C,D分別加上a,b,c,d。然后用下一分組數(shù)據(jù)繼續(xù)運行算法，最后的輸出是A,B,C和D的級聯(lián)。
l 最后得到的A,B,C,D就是輸出結(jié)果，A是低位，D為高位，DCBA組成128位輸出結(jié)果。
2.3 MD5的安全性
Ron Rivest概述了MD5安全性[8]：
l 與MD4相比，增加了第四輪。
l 每一步均有唯一的加法常數(shù)。
l 為減弱第二輪中函數(shù)G的對稱性從((X∧Y) ∨(X∧Z) ∨(Y∧Z))變?yōu)?(X∧Z) ∨(Y∧( Z)))。
l 每一步加上了上一步的結(jié)果，引起更快的雪崩效應。
l 改變了第二輪和第三輪中訪問消息子分組的次序，使其形式更不相似。
l 近似優(yōu)化了每一輪中的循環(huán)左移位移量以實現(xiàn)更快的雪崩效應。各輪的位移量互不相同。
從安全角度講，MD5的輸出為128位，若采用純強力攻擊尋找一個消息具有給定Hash值的計算困難性為2128,用每秒可試驗1 000 000 000個消息的計算機需時1.07×1022年。若采用生日攻擊法，尋找有相同Hash值的兩個消息需要試驗264個消息,用每秒可試驗1 000 000 000個消息的計算機需時585年。

二、實現(xiàn)方法
由于此處的文件校驗用到要求比較高的場合，故采用了方法6，md5校驗算法，從CodeGuru下載了一個md5校驗算法的實現(xiàn)模塊，加入自己要校驗的文件名，實現(xiàn)完成。下面具體描述一下實現(xiàn)過程：
1、創(chuàng)建一個簡單的對話框程序；
2、設置CString類型的變量m_filename和m_strFileChecksum以存放要校驗的文件名和校驗和；
3、在對話框類中創(chuàng)建ChecksumSelectedFile（）函數(shù)，調(diào)用md5校驗和類（附錄中有其實現(xiàn)文件）中的GetMD5計算文件校驗和。
4、使用定時器定時巡檢該文件的校驗和，一旦發(fā)現(xiàn)校驗和發(fā)生變化，立刻出現(xiàn)提示。
三、附錄（md5算法實現(xiàn)的源碼）
以下代碼實現(xiàn)均來自www.codeguru.com。
1、MD5ChecksumDefines.h（定義相關常量的頭文件）
//Magic initialization constants
#define MD5_INIT_STATE_0 0x67452301
#define MD5_INIT_STATE_1 0xefcdab89
#define MD5_INIT_STATE_2 0x98badcfe
#define MD5_INIT_STATE_3 0x10325476

//Constants for Transform routine.
#define MD5_S11  7
#define MD5_S12 12
#define MD5_S13 17
#define MD5_S14 22
#define MD5_S21  5
#define MD5_S22  9
#define MD5_S23 14
#define MD5_S24 20
#define MD5_S31  4
#define MD5_S32 11
#define MD5_S33 16
#define MD5_S34 23
#define MD5_S41  6
#define MD5_S42 10
#define MD5_S43 15
#define MD5_S44 21

//Transformation Constants - Round 1
#define MD5_T01  0xd76aa478 //Transformation Constant 1
#define MD5_T02  0xe8c7b756 //Transformation Constant 2
#define MD5_T03  0x242070db //Transformation Constant 3
#define MD5_T04  0xc1bdceee //Transformation Constant 4
#define MD5_T05  0xf57c0faf //Transformation Constant 5
#define MD5_T06  0x4787c62a //Transformation Constant 6
#define MD5_T07  0xa8304613 //Transformation Constant 7
#define MD5_T08  0xfd469501 //Transformation Constant 8
#define MD5_T09  0x698098d8 //Transformation Constant 9
#define MD5_T10  0x8b44f7af //Transformation Constant 10
#define MD5_T11  0xffff5bb1 //Transformation Constant 11
#define MD5_T12  0x895cd7be //Transformation Constant 12
#define MD5_T13  0x6b901122 //Transformation Constant 13
#define MD5_T14  0xfd987193 //Transformation Constant 14
#define MD5_T15  0xa679438e //Transformation Constant 15
#define MD5_T16  0x49b40821 //Transformation Constant 16

//Transformation Constants - Round 2
#define MD5_T17  0xf61e2562 //Transformation Constant 17
#define MD5_T18  0xc040b340 //Transformation Constant 18
#define MD5_T19  0x265e5a51 //Transformation Constant 19
#define MD5_T20  0xe9b6c7aa //Transformation Constant 20
#define MD5_T21  0xd62f105d //Transformation Constant 21
#define MD5_T22  0x02441453 //Transformation Constant 22
#define MD5_T23  0xd8a1e681 //Transformation Constant 23
#define MD5_T24  0xe7d3fbc8 //Transformation Constant 24
#define MD5_T25  0x21e1cde6 //Transformation Constant 25
#define MD5_T26  0xc33707d6 //Transformation Constant 26
#define MD5_T27  0xf4d50d87 //Transformation Constant 27
#define MD5_T28  0x455a14ed //Transformation Constant 28
#define MD5_T29  0xa9e3e905 //Transformation Constant 29
#define MD5_T30  0xfcefa3f8 //Transformation Constant 30
#define MD5_T31  0x676f02d9 //Transformation Constant 31
#define MD5_T32  0x8d2a4c8a //Transformation Constant 32

//Transformation Constants - Round 3
#define MD5_T33  0xfffa3942 //Transformation Constant 33
#define MD5_T34  0x8771f681 //Transformation Constant 34
#define MD5_T35  0x6d9d6122 //Transformation Constant 35
#define MD5_T36  0xfde5380c //Transformation Constant 36
#define MD5_T37  0xa4beea44 //Transformation Constant 37
#define MD5_T38  0x4bdecfa9 //Transformation Constant 38
#define MD5_T39  0xf6bb4b60 //Transformation Constant 39
#define MD5_T40  0xbebfbc70 //Transformation Constant 40
#define MD5_T41  0x289b7ec6 //Transformation Constant 41
#define MD5_T42  0xeaa127fa //Transformation Constant 42
#define MD5_T43  0xd4ef3085 //Transformation Constant 43
#define MD5_T44  0x04881d05 //Transformation Constant 44
#define MD5_T45  0xd9d4d039 //Transformation Constant 45
#define MD5_T46  0xe6db99e5 //Transformation Constant 46
#define MD5_T47  0x1fa27cf8 //Transformation Constant 47
#define MD5_T48  0xc4ac5665 //Transformation Constant 48

//Transformation Constants - Round 4
#define MD5_T49  0xf4292244 //Transformation Constant 49
#define MD5_T50  0x432aff97 //Transformation Constant 50
#define MD5_T51  0xab9423a7 //Transformation Constant 51
#define MD5_T52  0xfc93a039 //Transformation Constant 52
#define MD5_T53  0x655b59c3 //Transformation Constant 53
#define MD5_T54  0x8f0ccc92 //Transformation Constant 54
#define MD5_T55  0xffeff47d //Transformation Constant 55
#define MD5_T56  0x85845dd1 //Transformation Constant 56
#define MD5_T57  0x6fa87e4f //Transformation Constant 57
#define MD5_T58  0xfe2ce6e0 //Transformation Constant 58
#define MD5_T59  0xa3014314 //Transformation Constant 59
#define MD5_T60  0x4e0811a1 //Transformation Constant 60
#define MD5_T61  0xf7537e82 //Transformation Constant 61
#define MD5_T62  0xbd3af235 //Transformation Constant 62
#define MD5_T63  0x2ad7d2bb //Transformation Constant 63
#define MD5_T64  0xeb86d391 //Transformation Constant 64


//Null data (except for first BYTE) used to finalise the checksum calculation
static unsigned char PADDING[64] = {
  0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
};
2、CountChecksum.h（md5校驗和類的頭文件）
class CMD5Checksum  
{
public:
//interface functions for the RSA MD5 calculation
static CString GetMD5(BYTE* pBuf, UINT nLength);
static CString GetMD5(CFile& File);
static CString GetMD5(const CString& strFilePath);

protected:
//constructor/destructor
CMD5Checksum();
virtual ~CMD5Checksum() {};

//RSA MD5 implementation
void Transform(BYTE Block[64]);
void Update(BYTE* Input, ULONG nInputLen);
CString Final();
inline DWORD RotateLeft(DWORD x, int n);
inline void FF( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T);
inline void GG( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T);
inline void HH( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T);
inline void II( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T);

//utility functions
void DWordToByte(BYTE* Output, DWORD* Input, UINT nLength);
void ByteToDWord(DWORD* Output, BYTE* Input, UINT nLength);

private:
BYTE  m_lpszBuffer[64];  //input buffer
ULONG m_nCount[2];   //number of bits, modulo 2^64 (lsb first)
ULONG m_lMD5[4];   //MD5 checksum
};

#endif // !defined(AFX_MD5CHECKSUM_H__2BC7928E_4C15_11D3_B2EE_A4A60E20D2C3__INCLUDED_)
3、CountChecksum.cpp （md5校驗和類的實現(xiàn)文件）
/*****************************************************************************************
FUNCTION:  CMD5Checksum::GetMD5
DETAILS:  static, public
DESCRIPTION: Gets the MD5 checksum for a specified file
RETURNS:  CString : the hexadecimal MD5 checksum for the specified file
ARGUMENTS:  CString& strFilePath : the full pathname of the specified file
NOTES:   Provides an interface to the CMD5Checksum class. ''strFilePath'' name should
    hold the full pathname of the file, eg C:\My Documents\Arcticle.txt.
    NB. If any problems occur with opening or reading this file, a CFileException
    will be thrown; callers of this function should be ready to catch this
    exception.
*****************************************************************************************/
CString CMD5Checksum::GetMD5(const CString& strFilePath)
{
//open the file as a binary file in readonly mode, denying write access
CFile File(strFilePath, CFile::shareDenyNone);
//the file has been successfully opened, so now get and return its checksum
return GetMD5(File);
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::GetMD5
DETAILS:  static, public
DESCRIPTION: Gets the MD5 checksum for a specified file
RETURNS:  CString : the hexadecimal MD5 checksum for the specified file
ARGUMENTS:  CFile& File : the specified file
NOTES:   Provides an interface to the CMD5Checksum class. ''File'' should be open in
    binary readonly mode before calling this function.
    NB. Callers of this function should be ready to catch any CFileException
    thrown by the CFile functions
*****************************************************************************************/
CString CMD5Checksum::GetMD5(CFile& File)
{
try
{
  CMD5Checksum MD5Checksum;  //checksum object
  int nLength = 0;    //number of bytes read from the file
  const int nBufferSize = 1024; //checksum the file in blocks of 1024 bytes
  BYTE Buffer[nBufferSize];  //buffer for data read from the file

  //checksum the file in blocks of 1024 bytes
  while ((nLength = File.Read( Buffer, nBufferSize )) > 0 )
  {
   MD5Checksum.Update( Buffer, nLength );
  }

  //finalise the checksum and return it
  return MD5Checksum.Final();
}

//report any file exceptions in debug mode only
catch (CFileException* e )
{
  TRACE0("CMD5Checksum::GetMD5: CFileException caught");
  throw e;
}
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::GetMD5
DETAILS:  static, public
DESCRIPTION: Gets the MD5 checksum for data in a BYTE array
RETURNS:  CString : the hexadecimal MD5 checksum for the specified data
ARGUMENTS:  BYTE* pBuf  : pointer to the BYTE array
    UINT nLength : number of BYTEs of data to be checksumed
NOTES:   Provides an interface to the CMD5Checksum class. Any data that can
    be cast to a BYTE array of known length can be checksummed by this
    function. Typically, CString and char arrays will be checksumed,
    although this function can be used to check the integrity of any BYTE array.
    A buffer of zero length can be checksummed; all buffers of zero length
    will return the same checksum.
*****************************************************************************************/
CString CMD5Checksum::GetMD5(BYTE* pBuf, UINT nLength)
{
//entry invariants
AfxIsValidAddress(pBuf,nLength,FALSE);

//calculate and return the checksum
CMD5Checksum MD5Checksum;
MD5Checksum.Update( pBuf, nLength );
return MD5Checksum.Final();
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::RotateLeft
DETAILS:  private
DESCRIPTION: Rotates the bits in a 32 bit DWORD left by a specified amount
RETURNS:  The rotated DWORD
ARGUMENTS:  DWORD x : the value to be rotated
    int n   : the number of bits to rotate by
*****************************************************************************************/
DWORD CMD5Checksum::RotateLeft(DWORD x, int n)
{
//check that DWORD is 4 bytes long - true in Visual C++ 6 and 32 bit Windows
ASSERT( sizeof(x) == 4 );

//rotate and return x
return (x << n) (x >> (32-n));
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::FF
DETAILS:  protected
DESCRIPTION: Implementation of basic MD5 transformation algorithm
RETURNS:  none
ARGUMENTS:  DWORD &A, B, C, D : Current (partial) checksum
    DWORD X           : Input data
    DWORD S     : MD5_SXX Transformation constant
    DWORD T     : MD5_TXX Transformation constant
NOTES:   None
*****************************************************************************************/
void CMD5Checksum::FF( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T)
{
DWORD F = (B & C) (~B & D);
A += F + X + T;
A = RotateLeft(A, S);
A += B;
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::GG
DETAILS:  protected
DESCRIPTION: Implementation of basic MD5 transformation algorithm
RETURNS:  none
ARGUMENTS:  DWORD &A, B, C, D : Current (partial) checksum
    DWORD X           : Input data
    DWORD S     : MD5_SXX Transformation constant
    DWORD T     : MD5_TXX Transformation constant
NOTES:   None
*****************************************************************************************/
void CMD5Checksum::GG( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T)
{
DWORD G = (B & D) (C & ~D);
A += G + X + T;
A = RotateLeft(A, S);
A += B;
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::HH
DETAILS:  protected
DESCRIPTION: Implementation of basic MD5 transformation algorithm
RETURNS:  none
ARGUMENTS:  DWORD &A, B, C, D : Current (partial) checksum
    DWORD X           : Input data
    DWORD S     : MD5_SXX Transformation constant
    DWORD T     : MD5_TXX Transformation constant
NOTES:   None
*****************************************************************************************/
void CMD5Checksum::HH( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T)
{
DWORD H = (B ^ C ^ D);
A += H + X + T;
A = RotateLeft(A, S);
A += B;
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::II
DETAILS:  protected
DESCRIPTION: Implementation of basic MD5 transformation algorithm
RETURNS:  none
ARGUMENTS:  DWORD &A, B, C, D : Current (partial) checksum
    DWORD X           : Input data
    DWORD S     : MD5_SXX Transformation constant
    DWORD T     : MD5_TXX Transformation constant
NOTES:   None
*****************************************************************************************/
void CMD5Checksum::II( DWORD& A, DWORD B, DWORD C, DWORD D, DWORD X, DWORD S, DWORD T)
{
DWORD I = (C ^ (B ~D));
A += I + X + T;
A = RotateLeft(A, S);
A += B;
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::ByteToDWord
DETAILS:  private
DESCRIPTION: Transfers the data in an 8 bit array to a 32 bit array
RETURNS:  void
ARGUMENTS:  DWORD* Output : the 32 bit (unsigned long) destination array
    BYTE* Input   : the 8 bit (unsigned char) source array
    UINT nLength  : the number of 8 bit data items in the source array
NOTES:   Four BYTES from the input array are transferred to each DWORD entry
    of the output array. The first BYTE is transferred to the bits (0-7)
    of the output DWORD, the second BYTE to bits 8-15 etc.
    The algorithm assumes that the input array is a multiple of 4 bytes long
    so that there is a perfect fit into the array of 32 bit words.
*****************************************************************************************/
void CMD5Checksum::ByteToDWord(DWORD* Output, BYTE* Input, UINT nLength)
{
//entry invariants
ASSERT( nLength % 4 == 0 );
ASSERT( AfxIsValidAddress(Output, nLength/4, TRUE) );
ASSERT( AfxIsValidAddress(Input, nLength, FALSE) );

//initialisations
UINT i=0; //index to Output array
UINT j=0; //index to Input array

//transfer the data by shifting and copying
for ( ; j < nLength; i++, j += 4)
{
  Output[i] = (ULONG)Input[j]   
     (ULONG)Input[j+1] << 8
     (ULONG)Input[j+2] << 16
     (ULONG)Input[j+3] << 24;
}
}

/*****************************************************************************************
FUNCTION:  CMD5Checksum::Transform
DETAILS:  protected
DESCRIPTION: MD5 basic transformation algorithm;  transforms ''m_lMD5''
RETURNS:  void
ARGUMENTS:  BYTE Block[64]
NOTES:   An MD5 checksum is calculated by four rounds of ''Transformation''.
    The MD5 checksum currently held in m_lMD5 is merged by the
    transformation process with data passed in ''Block''.  
*****************************************************************************************/
void CMD5Checksum::Transform(BYTE Block[64])
{
//initialise local data with current checksum
ULONG a = m_lMD5[0];
ULONG b = m_lMD5[1];
ULONG c = m_lMD5[2];
ULONG d = m_lMD5[3];

//copy BYTES from input ''Block'' to an array of ULONGS ''X''
ULONG X[16];
ByteToDWord( X, Block, 64 );

//Perform Round 1 of the transformation
FF (a, b, c, d, X[ 0], MD5_S11, MD5_T01);
FF (d, a, b, c, X[ 1], MD5_S12, MD5_T02);
FF (c, d, a, b, X[ 2], MD5_S13, MD5_T03);
FF (b, c, d, a, X[ 3], MD5_S14, MD5_T04);
FF (a, b, c, d, X[ 4], MD5_S11, MD5_T05);
FF (d, a, b, c, X[ 5], MD5_S12, MD5_T06);
FF (c, d, a, b, X[ 6], MD5_S13, MD5_T07);
FF (b, c, d, a, X[ 7], MD5_S14, MD5_T08);
FF (a, b, c, d, X[ 8], MD5_S11, MD5_T09);
FF (d, a, b, c, X[ 9], MD5_S12, MD5_T10);
FF (c, d, a, b, X[10], MD5_S13, MD5_T11);
FF (b, c, d, a, X[11], MD5_S14, MD5_T12);
FF (a, b, c, d, X[12], MD5_S11, MD5_T13);
FF (d, a, b, c, X[13], MD5_S12, MD5_T14);
FF (c, d, a, b, X[14], MD5_S13, MD5_T15);
FF (b, c, d, a, X[15], MD5_S14, MD5_T16);

//Perform Round 2 of the transformation
GG (a, b, c, d, X[ 1], MD5_S21, MD5_T17);
GG (d, a, b, c, X[ 6], MD5_S22, MD5_T18);
GG (c, d, a, b, X[11], MD5_S23, MD5_T19);
GG (b, c, d, a, X[ 0], MD5_S24, MD5_T20);
GG (a, b, c, d, X[ 5], MD5_S21, MD5_T21);
GG (d, a, b, c, X[10], MD5_S22, MD5_T22);
GG (c, d, a, b, X[15], MD5_S23, MD5_T23);
GG (b, c, d, a, X[ 4], MD5_S24, MD5_T24);
GG (a, b, c, d, X[ 9], MD5_S21, MD5_T25);
GG (d, a, b, c, X[14], MD5_S22, MD5_T26);
GG (c, d, a, b, X[ 3], MD5_S23, MD5_T27);
GG (b, c, d, a, X[ 8], MD5_S24, MD5_T28);
GG (a, b, c, d, X[13], MD5_S21, MD5_T29);
GG (d, a, b, c, X[ 2], MD5_S22, MD5_T30);
GG (c, d, a, b, X[ 7], MD5_S23, MD5_T31);
GG (b, c, d, a, X[12], MD5_S24, MD5_T32);

//Perform Round 3 of the transformation
HH (a, b, c, d, X[ 5], MD5_S31, MD5_T33);
HH (d, a, b, c, X[ 8], MD5_S32, MD5_T34);
HH (c, d, a, b, X[11], MD5_S33, MD5_T35);
HH (b, c, d, a, X[14], MD5_S34, MD5_T36);
HH (a, b, c, d, X[ 1], MD5_S31, MD5_T37);
HH (d, a, b, c, X[ 4], MD5_S32, MD5_T38);
HH (c, d, a, b, X[ 7], MD5_S33, MD5_T39);
HH (b, c, d, a, X[10], MD5_S34, MD5_T40);
HH (a, b, c, d, X[13], MD5_S31, MD5_T41);
HH (d, a, b, c, X[ 0], MD5_S32, MD5_T42);
HH (c, d, a, b, X[ 3], MD5_S33, MD5_T43);
HH (b, c, d, a, X[ 6], MD5_S34, MD5_T44);
HH (a, b, c, d, X[ 9], MD5_S31, MD5_T45);
HH (d, a, b, c, X[12], MD5_S32, MD5_T46);
HH (c, d, a, b, X[15], MD5_S33, MD5_T47);
HH (b, c, d, a, X[ 2], MD5_S34, MD5_T48);

//Perform Round 4 of the transformation
II (a, b, c, d, X[ 0], MD5_S41, MD5_T49);
II (d, a, b, c, X[ 7], MD5_S42, MD5_T50);
II (c, d, a, b, X[14], MD5_S43, MD5_T51);
II (b, c, d, a, X[ 5], MD5_S44, MD5_T52);
II (a, b, c, d, X[12], MD5_S41, MD5_T53);
II (d, a, b, c, X[ 3], MD5_S42, MD5_T54);
II (c, d, a, b, X[10], MD5_S43, MD5_T55);
II (b, c, d, a, X[ 1], MD5_S44, MD5_T56);
II (a, b, c, d, X[ 8], MD5_S41, MD5_T57);
II (d, a, b, c, X[15], MD5_S42, MD5_T58);
II (c, d, a, b, X[ 6], MD5_S43, MD5_T59);
II (b, c, d, a, X[13], MD5_S44, MD5_T60);
II (a, b, c, d, X[ 4], MD5_S41, MD5_T61);
II (d, a, b, c, X[11], MD5_S42, MD5_T62);
II (c, d, a, b, X[ 2], MD5_S43, MD5_T63);
II (b, c, d, a, X[ 9], MD5_S44, MD5_T64);

//add the transformed values to the current checksum
m_lMD5[0] += a;
m_lMD5[1] += b;
m_lMD5[2] += c;
m_lMD5[3] += d;
}


/*****************************************************************************************
CONSTRUCTOR: CMD5Checksum
DESCRIPTION: Initialises member data
ARGUMENTS:  None
NOTES:   None
*****************************************************************************************/
CMD5Checksum::CMD5Checksum()
{
// zero members
memset( m_lpszBuffer, 0, 64 );
m_nCount[0] = m_nCount[1] = 0;

// Load magic state initialization constants
m_lMD5[0] = MD5_INIT_STATE_0;
m_lMD5[1] = MD5_INIT_STATE_1;
m_lMD5[2] = MD5_INIT_STATE_2;
m_lMD5[3] = MD5_INIT_STATE_3;
}

/*****************************************************************************************
FUNCTION:  CMD5Checksum::DWordToByte
DETAILS:  private
DESCRIPTION: Transfers the data in an 32 bit array to a 8 bit array
RETURNS:  void
ARGUMENTS:  BYTE* Output  : the 8 bit destination array
    DWORD* Input  : the 32 bit source array
    UINT nLength  : the number of 8 bit data items in the source array
NOTES:   One DWORD from the input array is transferred into four BYTES
    in the output array. The first (0-7) bits of the first DWORD are
    transferred to the first output BYTE, bits bits 8-15 are transferred from
    the second BYTE etc.
    
    The algorithm assumes that the output array is a multiple of 4 bytes long
    so that there is a perfect fit of 8 bit BYTES into the 32 bit DWORDs.
*****************************************************************************************/
void CMD5Checksum::DWordToByte(BYTE* Output, DWORD* Input, UINT nLength )
{
//entry invariants
ASSERT( nLength % 4 == 0 );
ASSERT( AfxIsValidAddress(Output, nLength, TRUE) );
ASSERT( AfxIsValidAddress(Input, nLength/4, FALSE) );

//transfer the data by shifting and copying
UINT i = 0;
UINT j = 0;
for ( ; j < nLength; i++, j += 4)
{
  Output[j] =   (UCHAR)(Input[i] & 0xff);
  Output[j+1] = (UCHAR)((Input[i] >> 8) & 0xff);
  Output[j+2] = (UCHAR)((Input[i] >> 16) & 0xff);
  Output[j+3] = (UCHAR)((Input[i] >> 24) & 0xff);
}
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::Final
DETAILS:  protected
DESCRIPTION: Implementation of main MD5 checksum algorithm; ends the checksum calculation.
RETURNS:  CString : the final hexadecimal MD5 checksum result
ARGUMENTS:  None
NOTES:   Performs the final MD5 checksum calculation (''Update'' does most of the work,
    this function just finishes the calculation.)
*****************************************************************************************/
CString CMD5Checksum::Final()
{
//Save number of bits
BYTE Bits[8];
DWordToByte( Bits, m_nCount, 8 );

//Pad out to 56 mod 64.
UINT nIndex = (UINT)((m_nCount[0] >> 3) & 0x3f);
UINT nPadLen = (nIndex < 56) ? (56 - nIndex) : (120 - nIndex);
Update( PADDING, nPadLen );

//Append length (before padding)
Update( Bits, 8 );

//Store final state in ''lpszMD5''
const int nMD5Size = 16;
unsigned char lpszMD5[ nMD5Size ];
DWordToByte( lpszMD5, m_lMD5, nMD5Size );

//Convert the hexadecimal checksum to a CString
CString strMD5;
for ( int i=0; i < nMD5Size; i++)
{
  CString Str;
  if (lpszMD5[i] == 0) {
   Str = CString("00");
  }
  else if (lpszMD5[i] <= 15)  {
   Str.Format("0%x",lpszMD5[i]);
  }
  else {
   Str.Format("%x",lpszMD5[i]);
  }

  ASSERT( Str.GetLength() == 2 );
  strMD5 += Str;
}
ASSERT( strMD5.GetLength() == 32 );
return strMD5;
}


/*****************************************************************************************
FUNCTION:  CMD5Checksum::Update
DETAILS:  protected
DESCRIPTION: Implementation of main MD5 checksum algorithm
RETURNS:  void
ARGUMENTS:  BYTE* Input    : input block
    UINT nInputLen : length of input block
NOTES:   Computes the partial MD5 checksum for ''nInputLen'' bytes of data in ''Input''
*****************************************************************************************/
void CMD5Checksum::Update( BYTE* Input, ULONG nInputLen )
{
//Compute number of bytes mod 64
UINT nIndex = (UINT)((m_nCount[0] >> 3) & 0x3F);

//Update number of bits
if ( ( m_nCount[0] += nInputLen << 3 )  <  ( nInputLen << 3) )
{
  m_nCount[1]++;
}
m_nCount[1] += (nInputLen >> 29);

//Transform as many times as possible.
UINT i=0;  
UINT nPartLen = 64 - nIndex;
if (nInputLen >= nPartLen)  
{
  memcpy( &m_lpszBuffer[nIndex], Input, nPartLen );
  Transform( m_lpszBuffer );
  for (i = nPartLen; i + 63 < nInputLen; i += 64)
  {
   Transform( &Input[i] );
  }
  nIndex = 0;
}
else
{
  i = 0;
}

// Buffer remaining input
memcpy( &m_lpszBuffer[nIndex], &Input[i], nInputLen-i);
}