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)

#ifndef MD5_H  
#define MD5_H  
 
typedef struct 
{ 
  unsigned int count[2]; 
  unsigned int state[4]; 
  unsigned char buffer[64]; 
}MD5_CTX; 
 
 
#define F(x,y,z) ((x & y) | (~x & z))  
#define G(x,y,z) ((x & z) | (y & ~z))  
#define H(x,y,z) (x^y^z)  
#define I(x,y,z) (y ^ (x | ~z))  
#define ROTATE_LEFT(x,n) ((x << n) | (x >> (32-n)))  
 
#define FF(a,b,c,d,x,s,ac) { \ 
a += F(b, c, d) + x + ac; \ 
a = ROTATE_LEFT(a, s); \ 
a += b; \ 
} 
 
#define GG(a,b,c,d,x,s,ac) { \ 
  a += G(b, c, d) + x + ac; \ 
  a = ROTATE_LEFT(a, s); \ 
  a += b; \ 
} 
 
#define HH(a,b,c,d,x,s,ac) { \ 
  a += H(b, c, d) + x + ac; \ 
  a = ROTATE_LEFT(a, s); \ 
  a += b; \ 
} 
#define II(a,b,c,d,x,s,ac) { \ 
  a += I(b, c, d) + x + ac; \ 
  a = ROTATE_LEFT(a, s); \ 
  a += b; \ 
} 
 
 
void MD5Init(MD5_CTX *context); 
void MD5Update(MD5_CTX *context, unsigned char *input, unsigned int inputlen); 
void MD5Final(MD5_CTX *context, unsigned char digest[16]); 
void MD5Transform(unsigned int state[4], unsigned char block[64]); 
void MD5Encode(unsigned char *output, unsigned int *input, unsigned int len); 
void MD5Decode(unsigned int *output, unsigned char *input, unsigned int len); 
 
#endif

unsigned char PADDING[] = {  
  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 }; 
 
//在逆向代码的时候，需要关注下面的特征值 
void MD5Init(MD5_CTX *context) 
{ 
  context->count[0] = 0; 
  context->count[1] = 0; 
  context->state[0] = 0x67452301; 
  context->state[1] = 0xEFCDAB89; 
  context->state[2] = 0x98BADCFE; 
  context->state[3] = 0x10325476; 
} 
 
void MD5Update(MD5_CTX *context, unsigned char *input, unsigned int inputlen) 
{ 
  unsigned int i = 0, index = 0, partlen = 0; 
  index = (context->count[0] >> 3) & 0x3F; 
  partlen = 64 - index; 
  context->count[0] += inputlen << 3; 
  if (context->count[0] < (inputlen << 3)) 
    context->count[1]++; 
  context->count[1] += inputlen >> 29; 
 
  if (inputlen >= partlen) 
  { 
    memcpy(&context->buffer[index], input, partlen); 
    MD5Transform(context->state, context->buffer); 
    for (i = partlen; i + 64 <= inputlen; i += 64) 
      MD5Transform(context->state, &input[i]); 
    index = 0; 
  } 
  else 
  { 
    i = 0; 
  } 
  memcpy(&context->buffer[index], &input[i], inputlen - i); 
} 
 
void MD5Final(MD5_CTX *context, unsigned char digest[16]) 
{ 
  unsigned int index = 0, padlen = 0; 
  unsigned char bits[8]; 
  index = (context->count[0] >> 3) & 0x3F; 
  padlen = (index < 56) ? (56 - index) : (120 - index); 
  MD5Encode(bits, context->count, 8); 
  MD5Update(context, PADDING, padlen); 
  MD5Update(context, bits, 8); 
  MD5Encode(digest, context->state, 16); 
} 
 
void MD5Encode(unsigned char *output, unsigned int *input, unsigned int len) 
{ 
  unsigned int i = 0, j = 0; 
  while (j < len) 
  { 
    output[j] = input[i] & 0xFF; 
    output[j + 1] = (input[i] >> 8) & 0xFF; 
    output[j + 2] = (input[i] >> 16) & 0xFF; 
    output[j + 3] = (input[i] >> 24) & 0xFF; 
    i++; 
    j += 4; 
  } 
} 
 
void MD5Decode(unsigned int *output, unsigned char *input, unsigned int len) 
{ 
  unsigned int i = 0, j = 0; 
  while (j < len) 
  { 
    output[i] = (input[j]) | 
      (input[j + 1] << 8) | 
      (input[j + 2] << 16) | 
      (input[j + 3] << 24); 
    i++; 
    j += 4; 
  } 
} 
 
void MD5Transform(unsigned int state[4], unsigned char block[64]) 
{ 
  unsigned int a = state[0]; 
  unsigned int b = state[1]; 
  unsigned int c = state[2]; 
  unsigned int d = state[3]; 
  unsigned int x[64]; 
 
  MD5Decode(x, block, 64); 
  FF(a, b, c, d, x[0], 7, 0xd76aa478); 
  FF(d, a, b, c, x[1], 12, 0xe8c7b756); 
  FF(c, d, a, b, x[2], 17, 0x242070db); 
  FF(b, c, d, a, x[3], 22, 0xc1bdceee); 
  FF(a, b, c, d, x[4], 7, 0xf57c0faf); 
  FF(d, a, b, c, x[5], 12, 0x4787c62a); 
  FF(c, d, a, b, x[6], 17, 0xa8304613); 
  FF(b, c, d, a, x[7], 22, 0xfd469501); 
  FF(a, b, c, d, x[8], 7, 0x698098d8); 
  FF(d, a, b, c, x[9], 12, 0x8b44f7af); 
  FF(c, d, a, b, x[10], 17, 0xffff5bb1); 
  FF(b, c, d, a, x[11], 22, 0x895cd7be); 
  FF(a, b, c, d, x[12], 7, 0x6b901122); 
  FF(d, a, b, c, x[13], 12, 0xfd987193); 
  FF(c, d, a, b, x[14], 17, 0xa679438e); 
  FF(b, c, d, a, x[15], 22, 0x49b40821); 
 
 
  GG(a, b, c, d, x[1], 5, 0xf61e2562); 
  GG(d, a, b, c, x[6], 9, 0xc040b340); 
  GG(c, d, a, b, x[11], 14, 0x265e5a51); 
  GG(b, c, d, a, x[0], 20, 0xe9b6c7aa); 
  GG(a, b, c, d, x[5], 5, 0xd62f105d); 
  GG(d, a, b, c, x[10], 9, 0x2441453); 
  GG(c, d, a, b, x[15], 14, 0xd8a1e681); 
  GG(b, c, d, a, x[4], 20, 0xe7d3fbc8); 
  GG(a, b, c, d, x[9], 5, 0x21e1cde6); 
  GG(d, a, b, c, x[14], 9, 0xc33707d6); 
  GG(c, d, a, b, x[3], 14, 0xf4d50d87); 
  GG(b, c, d, a, x[8], 20, 0x455a14ed); 
  GG(a, b, c, d, x[13], 5, 0xa9e3e905); 
  GG(d, a, b, c, x[2], 9, 0xfcefa3f8); 
  GG(c, d, a, b, x[7], 14, 0x676f02d9); 
  GG(b, c, d, a, x[12], 20, 0x8d2a4c8a); 
 
 
  HH(a, b, c, d, x[5], 4, 0xfffa3942); 
  HH(d, a, b, c, x[8], 11, 0x8771f681); 
  HH(c, d, a, b, x[11], 16, 0x6d9d6122); 
  HH(b, c, d, a, x[14], 23, 0xfde5380c); 
  HH(a, b, c, d, x[1], 4, 0xa4beea44); 
  HH(d, a, b, c, x[4], 11, 0x4bdecfa9); 
  HH(c, d, a, b, x[7], 16, 0xf6bb4b60); 
  HH(b, c, d, a, x[10], 23, 0xbebfbc70); 
  HH(a, b, c, d, x[13], 4, 0x289b7ec6); 
  HH(d, a, b, c, x[0], 11, 0xeaa127fa); 
  HH(c, d, a, b, x[3], 16, 0xd4ef3085); 
  HH(b, c, d, a, x[6], 23, 0x4881d05); 
  HH(a, b, c, d, x[9], 4, 0xd9d4d039); 
  HH(d, a, b, c, x[12], 11, 0xe6db99e5); 
  HH(c, d, a, b, x[15], 16, 0x1fa27cf8); 
  HH(b, c, d, a, x[2], 23, 0xc4ac5665); 
 
 
  II(a, b, c, d, x[0], 6, 0xf4292244); 
  II(d, a, b, c, x[7], 10, 0x432aff97); 
  II(c, d, a, b, x[14], 15, 0xab9423a7); 
  II(b, c, d, a, x[5], 21, 0xfc93a039); 
  II(a, b, c, d, x[12], 6, 0x655b59c3); 
  II(d, a, b, c, x[3], 10, 0x8f0ccc92); 
  II(c, d, a, b, x[10], 15, 0xffeff47d); 
  II(b, c, d, a, x[1], 21, 0x85845dd1); 
  II(a, b, c, d, x[8], 6, 0x6fa87e4f); 
  II(d, a, b, c, x[15], 10, 0xfe2ce6e0); 
  II(c, d, a, b, x[6], 15, 0xa3014314); 
  II(b, c, d, a, x[13], 21, 0x4e0811a1); 
  II(a, b, c, d, x[4], 6, 0xf7537e82); 
  II(d, a, b, c, x[11], 10, 0xbd3af235); 
  II(c, d, a, b, x[2], 15, 0x2ad7d2bb); 
  II(b, c, d, a, x[9], 21, 0xeb86d391); 
  state[0] += a; 
  state[1] += b; 
  state[2] += c; 
  state[3] += d; 
} 

int _tmain(int argc, _TCHAR* argv[]) 
{ 
 
  int i; 
  unsigned char encrypt[] = "admin";//21232f297a57a5a743894a0e4a801fc3  
  unsigned char decrypt[16]; 
 
  MD5_CTX md5; 
 
  MD5Init(&md5); 
  MD5Update(&md5, encrypt, strlen((char *)encrypt)); 
  MD5Final(&md5, decrypt); 
 
  //Md5加密后的32位结果 
  printf("加密前:%s\n加密后16位:", encrypt); 
  for (i = 4; i<12; i++) 
  { 
    printf("x", decrypt[i]);  
  } 
 
  //Md5加密后的32位结果 
  printf("\n加密前:%s\n加密后32位:", encrypt); 
  for (i = 0; i<16; i++) 
  { 
    printf("x", decrypt[i]);  
  } 
 
  getchar(); 
 
  return 0; 
}