Reverse engineering CRC?

[Steve Howell]

On Mar 7, 7:09 pm, Gregory Ewing <greg.ew... at canterbury.ac.nz> wrote:
> Given some known data/crc pairs, how feasible is it to
> figure out the polynomial being used to generate the crc?
>
> In the case I'm looking at, it appears that the crc
> size may be at least 24 bits, so just trying all possible
> polynomials probably isn't doable.
>
> An article I found hints at the possibility of using
> GCDs to make the search more efficient, but doesn't go
> into any details. Anyone know of any literature about
> this?
>
> If it helps, I have the ability to generate test cases
> with known message contents to some extent, although
> I don't have complete control over the contents. Also
> it's a manual process, so generating large numbers of
> them automatically isn't an option.
>


Hi Greg.  I would at least flip one bit at a time on the first byte of
your data to see if the transformation is bitwise.