Reverse engineering CRC?

[Albert van der Horst]

In article <7vj7fdFnnjU1 at mid.individual.net>,
Gregory Ewing  <greg.ewing 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.

If it is really a CRC, it is doable.

You can have an indication, if the intention is to detect
machine errors (transmission or disk errors) or they want
you to prevent tampering with the file.
In the latter case it may be a one-way hash. Then it is near
impossible, as this is the design criterion for a one-way hash.

>--
>Greg

Groetjes Albert

--
-- 
Albert van der Horst, UTRECHT,THE NETHERLANDS
Economic growth -- being exponential -- ultimately falters.
albert at spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van.der.horst