Add modulo 256 in LUA?

Question

Programming

I have a byte string: 0x40,0x04,0x07,0x20,0x00,0x1C,0x04,0x01,0xFE,0xB0,0x00,0x70,0x07,0x00,0x00,0x81,0x00,0x00
I have a checksum calculation description:

Byte #19 is the checksum. The checksum is the sum (addition) of the previous 18 bytes modulo 256.

After a few tests it appears that the checksum is for the payload part only (not including the introduction), and is the result of the simple sum of all bytes, keeping only one byte of the result (ignoring overflows). The trick is, of course, that this checksum is made by a Big Endian system, so every byte has to be reversed before being summed, then the result reversed again.

I have a checksum for this set of bytes: 0x9B
Q: how to calculate this checksum using lua?
Tried in different ways:

result = 0
bytes = {0x40,0x04,0x07,0x20,0x00,0x1C,0x04,0x01,0xFE,0xB0,0x00,0x70,0x07,0x00,0x00,0x81,0x00,0x00}
for i = 1, #bytes do 
  current_byte = bytes[i]
  for i = 1, 8 do 
    result = current_byte + result
  end
end

result = bit.bxor(0x40,0x04,0x07,0x20,0x00,0x1C,0x04,0x01,0xFE,0xB0,0x00,0x70,0x07,0x00,0x00,0x81,0x00,0x00)

result = bit.bxor(0x00,0x00,0x81,0x00,0x00,0x07,0x70,0x00,0xB0,0xFE,0x01,0x04,0x1C,0x00,0x20,0x07,0x04,0x40)

The checksum does not match. How can you calculate it anyway?

Answer 1

I asked myself, I answered myself (well, not myself, 0leGG helped on Twitter):

function HashBE(bytes)
  function rot(byte)
      result = 0
      for i = 0, 7 do
          result = result + bit.lshift(bit.band(bit.rshift(byte, i), 1), 7 - i)
      end
      return bit.band(result, 0xFF)
  end
    hash = 0
    for i = 1, #bytes do
        hash = bit.band(hash + rot(bytes[i]), 0xFF)
    end
    return rot(hash)
end