深入了解crc32算法
由于项目需要，解决一个流媒体文件的crc32校验码。网上查了很多的资料，发现了此校验码和生成多项式以及算法本身都有关系。对于不同类型的文件所使用的多项式以及算法不同，对于不同的生成多项式所生成的crc32表不同，不同的算法也会产生不同的结果。下面分类比较两种不同用途的crc32校验码的计算方法。
1、普通文件（如压缩文件）的crc32校验码计算方法：
生成多项式采用0xEDB88320，所生成的表为：
/*
This polynomial ( 0xEDB88320L) DOES generate the same CRC values as ZMODEM and PKZIP
*/
static const u_int32_t crc32tab[] = {
0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL,
0x076dc419L, 0x706af48fL, 0xe963a535L, 0x9e6495a3L,
0x0edb8832L, 0x79dcb8a4L, 0xe0d5e91eL, 0x97d2d988L,
0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L, 0x90bf1d91L,
0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL,
0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L,
0x136c9856L, 0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL,
0x14015c4fL, 0x63066cd9L, 0xfa0f3d63L, 0x8d080df5L,
0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L, 0xa2677172L,
0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL,
0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L,
0x32d86ce3L, 0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L,
0x26d930acL, 0x51de003aL, 0xc8d75180L, 0xbfd06116L,
0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L, 0xb8bda50fL,
0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L,
0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL,
0x76dc4190L, 0x01db7106L, 0x98d220bcL, 0xefd5102aL,
0x71b18589L, 0x06b6b51fL, 0x9fbfe4a5L, 0xe8b8d433L,
0x7807c9a2L, 0x0f00f934L, 0x9609a88eL, 0xe10e9818L,
0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L,
0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL,
0x6c0695edL, 0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L,
0x65b0d9c6L, 0x12b7e950L, 0x8bbeb8eaL, 0xfcb9887cL,
0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L, 0xfbd44c65L,
0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L,
0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL,
0x4369e96aL, 0x346ed9fcL, 0xad678846L, 0xda60b8d0L,
0x44042d73L, 0x33031de5L, 0xaa0a4c5fL, 0xdd0d7cc9L,
0x5005713cL, 0x270241aaL, 0xbe0b1010L, 0xc90c2086L,
0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL,
0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L,
0x59b33d17L, 0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL,
0xedb88320L, 0x9abfb3b6L, 0x03b6e20cL, 0x74b1d29aL,
0xead54739L, 0x9dd277afL, 0x04db2615L, 0x73dc1683L,
0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L,
0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L,
0xf00f9344L, 0x8708a3d2L, 0x1e01f268L, 0x6906c2feL,
0xf762575dL, 0x806567cbL, 0x196c3671L, 0x6e6b06e7L,
0xfed41b76L, 0x89d32be0L, 0x10da7a5aL, 0x67dd4accL,
0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L,
0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L,
0xd1bb67f1L, 0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL,
0xd80d2bdaL, 0xaf0a1b4cL, 0x36034af6L, 0x41047a60L,
0xdf60efc3L, 0xa867df55L, 0x316e8eefL, 0x4669be79L,
0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L,
0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL,
0xc5ba3bbeL, 0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L,
0xc2d7ffa7L, 0xb5d0cf31L, 0x2cd99e8bL, 0x5bdeae1dL,
0x9b64c2b0L, 0xec63f226L, 0x756aa39cL, 0x026d930aL,
0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L,
0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L,
0x92d28e9bL, 0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L,
0x86d3d2d4L, 0xf1d4e242L, 0x68ddb3f8L, 0x1fda836eL,
0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L, 0x18b74777L,
0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL,
0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L,
0xa00ae278L, 0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L,
0xa7672661L, 0xd06016f7L, 0x4969474dL, 0x3e6e77dbL,
0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L, 0x37d83bf0L,
0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L,
0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L,
0xbad03605L, 0xcdd70693L, 0x54de5729L, 0x23d967bfL,
0xb3667a2eL, 0xc4614ab8L, 0x5d681b02L, 0x2a6f2b94L,
0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL, 0x2d02ef8dL
};
所使用的算法：
u_int32_t ssh_crc32(const u_char *buf, u_int32_t size)
{
u_int32_t i, crc;
crc = 0xFFFFFFFF;
for (i = 0; i < size; i++)
crc = crc32tab[(crc ^ buf[i]) & 0xff] ^ (crc >> 8);
return crc^0xFFFFFFFF;
}
2、媒体文件（如mpeg）的crc32校验码计算方法：
生成多项式采用0x04c11db7，所生成的表为：
/*
This polynomial (0x04c11db7) is used at: AUTODIN II, Ethernet, & FDDI
*/
static u_int32_t crc32table[256] = {
0x00000000, 0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6, 0x2b4bcb61,
0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd, 0x4c11db70, 0x48d0c6c7,
0x4593e01e, 0x4152fda9, 0x5f15adac, 0x5bd4b01b, 0x569796c2, 0x52568b75,
0x6a1936c8, 0x6ed82b7f, 0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3,
0x709f7b7a, 0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58, 0xbaea46ef,
0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033, 0xa4ad16ea, 0xa06c0b5d,
0xd4326d90, 0xd0f37027, 0xddb056fe, 0xd9714b49, 0xc7361b4c, 0xc3f706fb,
0xceb42022, 0xca753d95, 0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1,
0xe13ef6f4, 0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0,
0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5, 0x2ac12072,
0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16, 0x018aeb13, 0x054bf6a4,
0x0808d07d, 0x0cc9cdca, 0x7897ab07, 0x7c56b6b0, 0x71159069, 0x75d48dde,
0x6b93dddb, 0x6f52c06c, 0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08,
0x571d7dd1, 0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b, 0xbb60adfc,
0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698, 0x832f1041, 0x87ee0df6,
0x99a95df3, 0x9d684044, 0x902b669d, 0x94ea7b2a, 0xe0b41de7, 0xe4750050,
0xe9362689, 0xedf73b3e, 0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2,
0xc6bcf05f, 0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34,
0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80, 0x644fc637,
0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb, 0x4f040d56, 0x4bc510e1,
0x46863638, 0x42472b8f, 0x5c007b8a, 0x58c1663d, 0x558240e4, 0x51435d53,
0x251d3b9e, 0x21dc2629, 0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5,
0x3f9b762c, 0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e, 0xf5ee4bb9,
0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65, 0xeba91bbc, 0xef68060b,
0xd727bbb6, 0xd3e6a601, 0xdea580d8, 0xda649d6f, 0xc423cd6a, 0xc0e2d0dd,
0xcda1f604, 0xc960ebb3, 0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7,
0xae3afba2, 0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74, 0x857130c3,
0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640, 0x4e8ee645, 0x4a4ffbf2,
0x470cdd2b, 0x43cdc09c, 0x7b827d21, 0x7f436096, 0x7200464f, 0x76c15bf8,
0x68860bfd, 0x6c47164a, 0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e,
0x18197087, 0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d, 0x2056cd3a,
0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce, 0xcc2b1d17, 0xc8ea00a0,
0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb, 0xdbee767c, 0xe3a1cbc1, 0xe760d676,
0xea23f0af, 0xeee2ed18, 0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4,
0x89b8fd09, 0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf, 0xa2f33668,
0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
};
所使用的算法是：
u_int32_t mpeg_crc32(const u_char *data, int len){
int i;
u_int32_t crc = 0xFFFFFFFF;
for(i = 0; i < len; i++)
crc = (crc << 8) ^ crc32table[((crc >> 24) ^ *data++) & 0xFF];
return crc;
}