"what is the reason for the AND and subtraction?"

To make sure you have a 1-byte value. If you sum 16 bytes you could have a sum as large as 4095. You can't subtract that from 0x100. But if you take 4095 AND 255, you get 255--and that can be subtracted from 0x100.

Basically it just means sum all the values and subtract the least-significant-byte of the sum from 0x100.

Regards,
Craig Steiner