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i am struggling since a long time to determine the checksum of the following sequences.

I have two examples.

Ex1: Have the same checksum and one have a payload incrementation by 0x0A
at the same position (Byte 8).

Ex2: This one have the same checksum (9.byte).

I am sure this are the checksum and is always 1 Byte.
What I noticed is that the values of checksum is not bigger than 0x7F.
I have many expamples but i see that it never grows up to 0x80.
It can also not 0xFF because 0xFF marks the sequenze as the end.

Example with paylod increment of 0x0A on the 8.Byte.
01: ED 0 1 0 1 0 0 00 29
02: ED 0 1 0 1 0 0 0A 19
03: ED 0 1 0 1 0 0 14 49
04: ED 0 1 0 1 0 0 1E 79
05: ED 0 1 0 1 0 0 28 4A
06: ED 0 1 0 1 0 0 32 44

This examples has the same checksum (0x53):
01: E4 09 01 02 26 50 00 53
02: E4 10 01 01 00 0F 00 53
03: E4 14 01 09 42 09 41 53
04: E4 15 01 14 48 08 03 53
05: E4 01 02 21 02 50 00 53
05: E4 01 02 22 22 52 00 53
06: E4 01 02 22 31 05 00 53
07: E4 02 02 02 10 0D 05 53

I have tested various algorythms. Fletcher, Adler32, BSD 8Bit are one of them.
Xor8 an some tests with shiftet bits and various modulus.
I have xored the values but nothing of worth comes out.
Reveng gave me some results but not for all sequences.
I am also sure that it is not a CRC but only a simple checksum.

I have read many threads if there was a hint how i can reverse the checksum.
But i see nothing wich handles 7bit. I also look at the bits but i dont know on how to begin.

I see some participations but dont see the relation.
In the first block I see that the checksum is ending in 0x09. (Set 1 - 4)
If the payload increases by 0x0A the checksum also increments by 0x30 (Set 2-4) until a specific point (Set 5).

I am sure it is very simple. In german i would say "ich sehe den Wald vor lauter Bäumem nicht".
Don't know the correct translation in english. ;-)

It would very appreciate if someone can point me the right direction.

With best regards
Chris

Edit: - replaced the word "crc" in the text by "checksum" for better understanding.
Tnx to JimmyB for the hint.

  • your expression would be "I cannot see the wood for the trees" in English. – Florian Castellane Jul 28 '17 at 7:38
  • I find it confusing that you're referring to the unknown checksum as "CRC" throughout your question. – JimmyB Jul 28 '17 at 10:02
  • You are right. I have not considered when writing. I think in this case at first it makes no difference but I already know that a crc is not checksum. ;-) – smith007 Jul 28 '17 at 18:24

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