А вот 5 строк, принёсшие Криске Касперски мировую известность.

int main()
{
    unsigned long c, c2, p2, pol = 0xEDB88320;
    long n, k;
    {
        printf("CRC32 Adjuster (c) 2001 by RElf @ HHT/2\n");
        printf("Length of data: "); scanf_s("%ld", &n);
        printf("Offset to patch: "); scanf_s("%ld", &k);
        n = (n - k) << 3;
        printf("Current CRC32: 0x"); scanf_s("%x", &c);
        printf("Desired CRC32: 0x"); scanf_s("%x", &c2);
        c ^= c2;
        p2 = (pol << 1) | 1;
        while (n--) if (c & 0x80000000) c = (c << 1) ^ p2; else c <<= 1;
        printf("XOR masks:%02X%02X%02X%02X\n", c & 0xff, (c >> 8) & 0xff, (c >> 16) & 0xff, c >> 24);
    }
    return 0;
}