sha1:
          openssl提供了sha1的库，在安装openssl之后可以直接调用sha1。
 
MD5：
          函数原型见附件，其中static void MDFile (filename)是对文件进行MD5校验的，static void MDString (inString)是对字符串进行MD5校验的。可以直接使用，也可以封装成库后在调用，值得注意的是，需要将源码中函数定义中的static去掉。
 
hash：
        hash算法见附件。
 
TCP/IP/UDP/ICMP中的checksum：
        /*计算校验和*/
       USHORT   checksum(USHORT   *buffer,   int   size)  
      {  
                unsigned   long   cksum=0;  
                while(size   >1)  
               {  
                       cksum   +=   *buffer++;  
                       size   -=   sizeof(USHORT);  
                }   
                if(size)  
               {   
                        cksum   +=   *(UCHAR*)buffer;  
               }  
               cksum   =   (cksum   >>   16)   +   (cksum   &   0xffff);  
               cksum   +=   (cksum   >>   16);  
               return   (USHORT)(~cksum);  
      }
 
 
CRC校验：
          int file_crc32(const char *filename, unsigned int *crc)
         {
                  unsigned char buffer[MAX_BUFFER_SIZE];
                  unsigned int vcrc = 0xffffffff;
                  unsigned int read = 0;
                  unsigned int filesize = 0;
                  FILE *fp = NULL;
                  struct stat fst;
 
                 if(stat(filename, &fst))
                {
                            printf("get file info failed\n");
                            return -1;
                 }
                /* unsigned long may denote the file size */
                if((filesize = fst.st_size) == 0)
                         return -1;
 
                 /* open file */
                 if((fp = fopen(filename, "r")) == NULL)
                {
                         printf("open the file failed\n");
                 }
                while(filesize)
                {
                          read = filesize > MAX_BUFFER_SIZE ? MAX_BUFFER_SIZE:filesize;
                          if((read = fread(buffer, 1, read, fp)) == 0) break;
                         /* CRC */
                         crc32(buffer, read, &vcrc);
                         filesize -= read;
                 }
               *crc = ~vcrc;
                fclose(fp);
                 return 0;
         }

        void crc32(const unsigned char* byte, unsigned int length, unsigned int *vcrc)
        {
                 unsigned int i = 0;
                 for(i = 0; i < length; i++)
               *vcrc = ((*vcrc) >> 8) ^ crc32table[byte[i] ^ ((*vcrc) & 0x000000FF)];
        }