Reverse engineering CRC?

[Dave Angel]

Gregory Ewing wrote:
> <div class="moz-text-flowed" style="font-family: -moz-fixed">Dave 
> Angel wrote:
>> If you know so little about the value, how do you even know it's a 
>> CRC ?  Could it be a ones-complement sum, such as used in Ethernet?
>
> I'm going by the fact that the application reports a
> "CRC mismatch" when it's wrong. I can't be sure that what
> it calls a "CRC" is really a true CRC, but it's more than
> a simple sum, because changing one bit in the file results
> in a completely different value.
>
>> Is the problem really worth it?
>
> Probably not -- it looks like fixing the problem I'm trying
> to fix by hand will be faster in the long run. I just thought
> it might turn out to be a solved problem and someone could
> point me to an algorithm for it, but it seems not.
>
If you assume it's done in a single pass, and you know which byte is the 
end of the buffer, I'd think you could learn a lot by just tweaking that 
last byte.  But I still think you'd want to automate your testing, 
presumably by some keystroke-stuffer.

DaveA