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Reversing a Checksum Algorithm?

Posted by Cliff on from the hope-you've-brushed-up-on-your-math dept.

Todd asks: "Does anyone have good suggestions or advice regarding techniques for determining the checksum of a serial data protocol? I am currently working on a fun little project using a digital display sign which uses an unknown serial protocol. I cannot move on with the project until I figure this dreaded checksum out."

1 of 35 comments (clear)

  1. Solved it by sepulcrum · · Score: 5, Interesting

    I put sign.exe thru IDA and identified the checksum algorithm. I found out that the only thing that goes thru the checksum is 35 35 00 then 0E DA is skipped and then the rest is put thru. The algorithm is a simple crc alike algo that adds the chars xors with the length and rotates some bits. You can find a perl program i wrote to calculate the checksum for a given range at: this location.

    Good luck with your project,
    Gijs