根据别人的MD5源码封装的一个类[2]

/* md5 block update operation. continues an md5 message-digest
  operation, processing another message block, and updating the
  context.
 */
void cmd5::md5update (md5_ctx *context, unsigned char *input, unsigned int inputlen)
{
  unsigned int i, index, partlen;

  /* compute number of bytes mod 64 */
  index = (unsigned int)((context->count[0] >> 3) & 0x3f);

  /* update number of bits */
  if ((context->count[0] += ((uint4)inputlen << 3))
   < ((uint4)inputlen << 3))
 context->count[1]++;
  context->count[1] += ((uint4)inputlen >> 29);

  partlen = 64 - index;

  /* transform as many times as possible.
*/
  if (inputlen >= partlen) {
 md5_memcpy
   ((pointer)&context->buffer[index], (pointer)input, partlen);
 md5transform (context->state, context->buffer);

 for (i = partlen; i + 63 < inputlen; i += 64)
   md5transform (context->state, &input[i]);

 index = 0;
  }
  else
 i = 0;

  /* buffer remaining input */
  md5_memcpy
 ((pointer)&context->buffer[index], (pointer)&input[i],
  inputlen-i);
}

/* md5 finalization. ends an md5 message-digest operation, writing the
  the message digest and zeroizing the context.
 */
void cmd5::md5final (unsigned char digest[16], md5_ctx *context)
{
  unsigned char bits[8];
  unsigned int index, padlen;

  /* save number of bits */
  encode (bits, context->count, 8);

  /* pad out to 56 mod 64.
*/
  index = (unsigned int)((context->count[0] >> 3) & 0x3f);
  padlen = (index < 56) ? (56 - index) : (120 - index);
  md5update (context, padding, padlen);

  /* append length (before padding) */
  md5update (context, bits, 8);

  /* store state in digest */
  encode (digest, context->state, 16);

  /* zeroize sensitive information.
*/
  md5_memset ((pointer)context, 0, sizeof (*context));
}

/* md5 basic transformation. transforms state based on block.
 */
static void encode (unsigned char *output, uint4 *input, unsigned int len)
{
  unsigned int i, j;

  for (i = 0, j = 0; j < len; i++, j += 4) {
 output[j] = (unsigned char)(input[i] & 0xff);
 output[j+1] = (unsigned char)((input[i] >> 8) & 0xff);
 output[j+2] = (unsigned char)((input[i] >> 16) & 0xff);
 output[j+3] = (unsigned char)((input[i] >> 24) & 0xff);
  }
}

/* decodes input (unsigned char) into output (uint4). assumes len is
  a multiple of 4.
 */
static void decode (uint4 *output, unsigned char *input, unsigned int len)
{
  unsigned int i, j;

  for (i = 0, j = 0; j < len; i++, j += 4)
 output[i] = ((uint4)input[j]) | (((uint4)input[j+1]) << 8) |
   (((uint4)input[j+2]) << 16) | (((uint4)input[j+3]) << 24);
}

/* note: replace "for loop" with standard memcpy if possible.
 */

static void md5_memcpy (pointer output, pointer input, unsigned int len)
{
  unsigned int i;

  for (i = 0; i < len; i++)
    output[i] = input[i];
}

/* note: replace "for loop" with standard memset if possible.
 */
static void md5_memset (pointer output, int value, unsigned int len)
{
  unsigned int i;

  for (i = 0; i < len; i++)
 ((char *)output)[i] = (char)value;
}

void cmd5::md5transform (uint4 state[4], unsigned char block[64])
{
  uint4 a = state[0], b = state[1], c = state[2], d = state[3], x[16];

  decode (x, block, 64);