Module 256 checksums

checksums.lua

data = "\x00\x00\x02\x02\x00"

local function modulo( data )
  local checksum = 0
  for _, byte in ipairs( table.pack( data:byte( 1, #data ) ) ) do
    checksum = ( checksum + byte ) % 256
  end
  return string.char( checksum )
end

local function twosComplimentModulo( data )
  local checksum = 0
  for _, byte in ipairs( table.pack( data:byte( 1, #data ) ) ) do
    checksum = ( checksum + byte ) % 256
  end
  return string.char( (256 - checksum) % 256 )
end

-- Use example:

checksum = twosComplimentModulo( data )
-- or
checksum = twosComplimentModulo( "This is my string!" )

-- The functions all return a 'char', so to get a byte you need to do a string.byte.
-- This will print a 'hex' representation.
print( string.format( "\\x%02X", string.byte( twosComplimentModulo( data ) ) ) )
print( string.format( "\\x%02X", string.byte( modulo( data ) ) ) )