Question

I’m not understanding how this result can be zero. This was presented to me has an example to validate a checksum of a message.

ED(12+01+ED=0)

How can this result be zero?

“1201 is the message” ED is the checksum, my question is more on, how can I determine the checksum?

Thank you for any help.

Best regards,
FR

Answer 1

How can this result be zero?

The checksum is presumably represented by a byte.

A byte can store 256 different values, so the calculation is probably done module 256.

Since 0x12 + 0x01 + 0xED = 256, the result becomes 0.

how can I determine the checksum?

The checksum is the specific byte value B that makes the sum of the bytes in the message + B = 0 (modulo 256).

So, as @LanceH says in the comment, to figure out the checksum B, you…

1. add up the values of the bytes in the message (say it adds up to M)
2. compute M' = M % 256
3. Now, the checksum B is computed as 256 - M'.