View source for Packet:CRC

CRC stands for Cycle Redundency Check. The specific algorithm used in SWG is a seeded CRC32 algorithm. 
The algorithm calculates a unique checksum of all the data in a packet in exception of the 
last 2 bytes of the footer which are used to store a portion of this checksum. 

The process begins by generating a standard CRC32 checksum using a seeded value. 
This seed is determined by the server and can be any randomly generated number.
It can be located in a Session Reply as part of the SOE Protocol.

The checksum is an unsigned 32bit integer (4 bytes in size).

The next part is to append part of it to the end of the SWG packet. 
SWG only wants 2 values for the CRC, and the bytes to append are the 2 lower value bytes.

So if the CRC value generated is 0x04030201 or 04 03 02 01

the 2 bytes to be appended are 0x0201 or 02 01

so the packet would look like this:
(00 05 AA BB CC DD 00 06 00) (02 01)
 data to be CRC'd        appended portion

ALL packets that contain a footer need to be CRC'd unless otherwise noted.
CRC is appended AFTER Encryption, which comes AFTER Compression.

The following are the algorithms used by SWGEmu:

<pre>
//CRC TABLE

static const unsigned int g_nCrcTable[256] =
{
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
};

//FUNCTION PROTOTYPES

unsigned int GenerateCrc(char *pData,unsigned short nLength,unsigned int nCrcSeed);
bool CrcTest(char *pData,unsigned short nLength,unsigned int nCrcSeed, short nCrcLength=2);
void AppendCRC(char *pData,unsigned short nLength,unsigned int nCrcSeed, short nCrcLength=2);


// GENERATE CRC

unsigned int GenerateCrc(char* pData,unsigned short nLength,unsigned int nCrcSeed)
{
    unsigned int nCrc = g_nCrcTable[(~nCrcSeed) & 0xFF];
    nCrc ^= 0x00FFFFFF;
    unsigned int nIndex = (nCrcSeed >> 8) ^ nCrc;
    nCrc = (nCrc >> 8) & 0x00FFFFFF;
    nCrc ^= g_nCrcTable[nIndex & 0xFF];
    nIndex = (nCrcSeed >> 16) ^ nCrc;
    nCrc = (nCrc >> 8) & 0x00FFFFFF;
    nCrc ^= g_nCrcTable[nIndex & 0xFF];
    nIndex = (nCrcSeed >> 24) ^ nCrc;
    nCrc = (nCrc >> 8) &0x00FFFFFF;
    nCrc ^= g_nCrcTable[nIndex & 0xFF];

    for( short i = 0; i < nLength; i++ )
    {
        nIndex = (pData[i]) ^ nCrc;
        nCrc = (nCrc >> 8) & 0x00FFFFFF;
        nCrc ^= g_nCrcTable[nIndex & 0xFF];
    }
    return ~nCrc;
}


//CRCTEST

bool CrcTest(char *pData, unsigned short nLength,unsigned int nCrcSeed, short nCrcLength)
{
    bool crctest = true;
    if(nCrcLength > 0)
    {
        unsigned int p_crc = GenerateCrc(pData,(nLength-nCrcLength),nCrcSeed);
        unsigned int  crc = 0;
        unsigned int mask = 0;
        unsigned int pullbyte = 0;
        pData = pData + (nLength-nCrcLength);
        for( short i = 0; i < nCrcLength; i++ )
        {
            pullbyte = (unsigned char)pData[i];
            crc |=(pullbyte << (((nCrcLength - 1) - i) * 8));
            mask <<= 8;
            mask |= 0xFF;
        }
        p_crc &= mask;
        if(p_crc != crc)
            crctest = false;
    }
    return crctest;
}


//APPEND CRC

void AppendCRC(char *pData, unsigned short nLength, unsigned int nCrcSeed, short nCrcLength)
{
    if (nCrcLength > 0)
    {
        unsigned int crc = GenerateCrc(pData,(nLength-nCrcLength),nCrcSeed);
        pData += (nLength-nCrcLength);
        for( short i = 0; i < nCrcLength; i++ )
        {
            pData[(nCrcLength - 1) - i] = (char)((crc >> (8 * i)) & 0xFF);
        }
    }

}
</pre>