CRC32加密算法的识别

Posted 2020-07-22

CRC32有如下几个特征：

1. 一张静态的CRC32表或者动态生成一张CRC32的表，表的元素个数为256，元素大小为4字节

2. 在计算CRC32时，会进行查表操作，然后异或上一次CRC32的结果右移8位

3. 最终生成4字节CRC32值

 

1. 静态CRC32表

static const DWORD g_dwCrc32Table[] =
{
        0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
        0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
        0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2,
        0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
        0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
        0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
        0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c,
        0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
        0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
        0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
        0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106,
        0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
        0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d,
        0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
        0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
        0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
        0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7,
        0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
        0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa,
        0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
        0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81,
        0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
        0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84,
        0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
        0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
        0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
        0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e,
        0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
        0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55,
        0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
        0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28,
        0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
        0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f,
        0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
        0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
        0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
        0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69,
        0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
        0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc,
        0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
        0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693,
        0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
        0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
};

2. 在计算CRC32时，会进行查表操作，然后异或上一次CRC32的结果右移8位

00401017  |> /33D2          /XOR     EDX, EDX
00401019  |. |8BD8          |MOV     EBX, EAX                                 ;  //上次CRC32结果
0040101B  |. |8A1439        |MOV     DL, BYTE PTR DS:[ECX+EDI]                ;  //取1字节数据
0040101E  |. |81E3 FF000000 |AND     EBX, 0xFF                                ;  //ebx & 0xff
00401024  |. |33D3          |XOR     EDX, EBX                                 ;  //edx ^ (ebx & 0xff)
00401026  |. |C1E8 08       |SHR     EAX, 0x8                                 ;  //eax >> 8
00401029  |. |8B1495 A05040>|MOV     EDX, DWORD PTR DS:[EDX*4+0x4050A0]       ;  //查表  0x4050a0[edx ^ (ebx & 0xff)]
00401030  |. |33C2          |XOR     EAX, EDX                                 ;  //(eax >> 8) ^ 0x4050a0[edx ^ (ebx & 0xff)]
00401032  |. |41            |INC     ECX
00401033  |. |3BCE          |CMP     ECX, ESI
00401035  |.^\\72 E0         \\JB      SHORT crc32.00401017

 

也有可能会动态生成CRC32表：

00401070  /$  56            PUSH    ESI
00401071  |.  33D2          XOR     EDX, EDX
00401073  |.  B9 D0844000   MOV     ECX, crc32.004084D0               ;  //全局表首地址
00401078  |>  8BC2          /MOV     EAX, EDX                         ;  //0开始
0040107A  |.  BE 08000000   |MOV     ESI, 0x8
0040107F  |>  A8 01         |/TEST    AL, 0x1                         ;  //输出为1时，有异或门的位置，数据被翻转向右移向下一位置
00401081  |.  74 09         ||JE      SHORT crc32.0040108C            ;  //没有异或门的位置，数直接向右移向下一位置。
00401083  |.  D1E8          ||SHR     EAX, 1
00401085  |.  35 2083B8ED   ||XOR     EAX, 0xEDB88320                 ;  //(eax >> 1) & 0xEDB88320        这也可以当作动态生成的特征
0040108A  |.  EB 02         ||JMP     SHORT crc32.0040108E
0040108C  |>  D1E8          ||SHR     EAX, 1
0040108E  |>  4E            ||DEC     ESI
0040108F  |.^ 75 EE         |\\JNZ     SHORT crc32.0040107F
00401091  |.  8901          |MOV     DWORD PTR DS:[ECX], EAX          ;  //初始化的值填入到全局CRC32表中
00401093  |.  83C1 04       |ADD     ECX, 0x4                         ;  //4字节
00401096  |.  42            |INC     EDX
00401097  |.  81F9 D0884000 |CMP     ECX, crc32.004088D0
0040109D  |.^ 7C D9         \\JL      SHORT crc32.00401078
0040109F  |.  5E            POP     ESI
004010A0  \\.  C3            RETN