Dead Rising

"
Security:
-> Custom Checksum(s)
   -> Offset: 16, 36, 56, ..., EOF - 4
   -> Calculation Start: 0, 20, 40, ..., EOF - 20
   -> Calculation Length: 16

Additional:
You can calculate the checksum count n with Size / 20.
Each checksum consist of two separate Int16 checksums, that are calculated with the following formulas.

1: x[0] + x[1] + x[2] + x[3] + x[4] + x[5] + ...
2: x[0] - x[1] + x[2] - x[3] + x[4] - x[5] + ...
"

public void FixChecksums(byte[] data, int offset, int size) {
    fixed (byte* x = data) {
        byte* e = x + offset + size;

        ushort sumL, sumH;
        for (byte* s = e - size; s < e; s += 4) {

            sumL = sumH = 0;
            for (int n = 0; n < 16; ++n, ++s) {
                sumL += *s;
                sumH += (ushort) ((n % 2) == 0 ? *s : -*s);
            }

            *(ushort*) s = sumL.Swap();
            *(ushort*) (s + 2) = sumH.Swap();
        }
    }
}