Alien: Isolation

"
Security:
-> Custom SHA1
   -> Offset: 8
   -> Calculation Start: 32
   -> Calculation Length: See Code

Additional:
The SHA1 hash is moved to a buffer with the size of 24 bytes.
After that the 3 UInt64 parts of the buffer are combined using XOR.
"

byte[] data = File.ReadAllBytes(fileName);

using (var r = new Reader(data, Endian.BIG)) {
    r.Position = 24;
    int cLen = r.ReadInt32() + r.ReadInt32() - 32;

    byte[] hash = new byte[24];
    using (var sha = new SHA1Managed()) {
        sha.TransformFinalBlock(data, 32, cLen);
        Buffer.BlockCopy(sha.Hash, 0, hash, 0, 20);
    }

    long checksum = 0;
    fixed (byte* x = hash) {
        long* hashP = (long*) x;
        for (int i = 0; i < 3; ++i, ++hashP)
            checksum ^= *hashP;
    }
}