Fixing a Checksum-32 checksum.

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I have a 4428472 byte-long file. I want my program to be able to figure out the Sum32 checksum for all the bytes from offset 28 onwards.
The code I'm using to read the file is:

C#

System.IO.File.ReadAllBytes(filename)

This is a winforms application.
Thanks.

EDIT: This program is basically going to take user input, and according to that input, is simply going to write an 0x01 at specified offset(one of them is offset 1276), because this file has been edited, the checksum is no longer valid, so, I need to figure out how to generate this checksum, which I'm stuck on.

Posted 12-Sep-13 22:10pm

Miztah Raza

Updated 12-Sep-13 22:23pm

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OriginalGriff 13-Sep-13 4:13am

And?
You have read the file, but what have you done with it to generate your checksum?
Please, show us what you have tried, and where you are stuck.

Miztah Raza 13-Sep-13 4:23am

That's actually where I'm stuck. At generating the checksum, I've tried googling for information on fixing Sum32Checksums, but I've not been successful. The only thing I know is that the file I'm working with needs to have Sum32 checksum fixed in order to be a valid save file(this is for a game). The hex editor HxD has this function built-in. I've also edited the original question to provide more information about what my program does and why it needs the checksum fixed. Thanks

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CPallini · Answer 1 · 2013-09-12T22:37:00

Solution 1

Basically you have two options.

(the lucky one) if you can get relevant info on the exact algorithm used for the checksum then find its source code or implement it yourself.
(the not-so-lucky-one) Try (that is obtain or code yourself) different 'checksum 32' algorithms (e.g. the Fletcher-32) and compare their results with the one you have.

Posted 12-Sep-13 22:37pm

CPallini

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Miztah Raza 13-Sep-13 4:58am

Alright, thanks, dude.

CPallini 13-Sep-13 5:31am

You are welcome.