关于python实现CRC32的应用和总结
目前使用的Crc计算包含Crc32和Crc32mpeg2两种计算方式。
循环冗余检验 CRC 差错检测技术能够证明数据是完整的，是无差错的（只是非常近似的认为是无差错的）。
保证数据可靠性传输的方法包含如下：
- 检验和
用循环冗余码Crc检验接收到的数据完整性，Crc计算结果完全一致表示数据是完整的，没有发生错位。 - 确认应答机制（ACK）
TCP通过确认应答机制实现可靠的数据传输。在TCP的首部中有一个标志位——ACK，此标志位表示确认号是否有效。接收方对于按序到达的数据会进行确认，当标志位ACK=1时确认首部的确认字段有效。进行确认时，确认字段值表示这个值之前的数据都已经按序到达了。而发送方如果收到了已发送的数据的确认报文，则继续传输下一部分数据；而如果等待了一定时间还没有收到确认报文就会启动重传机制。 - 超时重传机制
当数据发出后在一定的时间内未收到接收方的确认，发送方就会进行重传（通常是在发出报文段后设定一个特定的时间间隔，到点了还没有收到应答则进行重传）。
用python实现查表法计算Crc32的过程如下。
- 定义crc的table
origin_crc32_table = [
#/* CRC32生成多项式采用0x04C11DB7 */
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]- 编写创建crc32_table的函数。
def generate_crc32_table(_poly):global custom_crc32_table
for i in range(256):c = i << 24
for j in range(8):if (c & 0x80000000):c = (c << 1) ^ _polyelse:c = c << 1
custom_crc32_table[i] = c & 0xffffffff- 编写计算Crc32的函数。
def getCrc32(bytes_arr):length = len(bytes_arr)
if bytes_arr != None:crc = 0xffffffff
for i in range(0, length):#print i, getReverse(bytes_arr[i], 8)crc = (crc << 8) ^ custom_crc32_table[(getReverse(bytes_arr[i], 8) ^ (crc >> 24)) & 0xff]else:crc = 0xffffffff
# /*- 返回计算的CRC值 */crc = getReverse(crc ^ 0xffffffff, 32)return crc
def getReverse(tempData, byte_length):reverseData = 0for i in range(0, byte_length):reverseData += ((tempData>>i)&1)<<(byte_length-1-i)
return reverseData当以上函数编写完成后，用如下代码进行检验
if __name__ == '__main__':a = 20b = 400s = struct.pack('>ii', a, b)print(s, type(s))
s = struct.pack('<ii', a, b)print(s, type(s))
print '反转方式LSB First，异或值为0xFFFFFFFF, test:'s = struct.pack('>i', 400)print ' 当前CRC输入初始值：', (s, type(s))test = binascii.crc32(s) & 0xffffffffprint '算出来的CRC值:', '0x'+"{:0>8s}".format(str('%x'%test))test = zlib.crc32(s) & 0xffffffffprint '算出来的CRC值:', '0x'+"{:0>8s}".format(str('%x'%test))
crc32mpeg2_poly = 0x04C11DB7#buf = [0x31, 0x32, 0x33, 0x34, 0x35, 0x36]#buf = [0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x30, 0x31, 0x32]#buf = [0x31, 0x32, 0x33]#buf = [0x31, 0x32]#buf = [0x31]buf_s = [0x00, 0x00, 0x01, 0x90]#buf_s = [0x90, 0x01, 0x00, 0x00]
generate_crc32_table(crc32mpeg2_poly)print 'create_crc32_table:', " ".join('0x'+"{:0>8s}".format(str('%x'%i)) for i in custom_crc32_table)print 'origin_crc32_table:', " ".join('0x'+"{:0>8s}".format(str('%x'%i)) for i in origin_crc32_table)crc_stm = getCrc32(bytearray(buf_s)) & 0xffffffffprint ' 算出来的CRC值：', '0x' + "{:0>8s}".format(str('%x' % crc_stm))
