Delete comment from: Ken Shirriff's blog
When you compute sumpart, you basically compute some 'x' from input bits, then sumpart = (-x -1) & 0xf. -x - 1 is equivalent to ~x. I guess that is how it was implemented
Dec 8, 2017, 8:55:40 AM
Posted to Decoding an air conditioner control's checksum with differential cryptanalysis