MD5 and Crc__algorithm

MD5 and CRC are some sort of information digest algorithms.

MD5 can generate information summaries (126bits,16bytes) of information (which can be as large as a video file, or as small as a password). This information digest is equivalent to the digital signature of this information, which prevents the information from being tampered with.

CRC generates a summary of the length of the information 4bytes, and the source information is limited in size (less than 4GBytes).

CRC (cyclic redundancy check) cyclic redundancy check code

http://blog.csdn.net/liyuanbhu/article/details/7882789

CRC calibration Core is to specify a divisor, the implementation of borrow division operations, and the remainder as a checksum code. The so-called borrow Division refers to normal division, but the addition and subtraction of modulo 2 operation (XOR or).

MD5 Library Download Address http://download.csdn.net/detail/huang_yx005/9724535

/* Md5 number length/#define SMART_MD5NUMBER_SIZE                     16//Calculate the MD5 value of a piece of information from the first address as memory, the size is memorysize, and the result is stored in md5number static int calc_md5number (const char *memory, UINT Memorysize, Uchar *md5number) {if (NULL = = Memory | | NULL = = Md5number | |
  Memorysize = = 0) return 0;
  /* Do MD5 purity versification * * Md5_ctx mdcontext;
  Md5init (&mdcontext);
  Md5update (&mdcontext, (UCHAR *) memory, memorysize);
  Md5final (&mdcontext);
  memcpy (Md5number, Mdcontext.digest, smart_md5number_size);
return 1;
  }//Compare two MD5 values equal static int Equal_md5number (Uchar *md51, Uchar *md52) {UINT i = 0; if (NULL = = Md51 | |
  NULL = = md52) return 0;
  while (I < smart_md5number_size && Md51[i] = = Md52[i]) i++;
return i = = smart_md5number_size;
}//another example int bytes;
unsigned char data[1024];
Md5_ctx Tmp_mdcontext;
Md5init (&tmp_mdcontext); while ((bytes = fread (data, 1, 1024, fp_outtmp))!= 0)//From FileRead 1024 bytes of data in fp_outtmp to {md5update (&tmp_mdcontext, data, bytes);//update MD5 value fwrite (data, sizeof (unsigned char), b Ytes, fp_out);
Writes data to Fp_out} md5final (&tmp_mdcontext);
Write check code fprintf (fp_out, "md5\n");
Fwrite (tmp_mdcontext.digest, sizeof (unsigned char), fp_out);
Write version number fprintf (Fp_out, "ver\n");
 Fwrite (&version, sizeof (unsigned int), 1, fp_out);