Reverse engineering CRC?
Steven D'Aprano
steve at REMOVE-THIS-cybersource.com.au
Mon Mar 8 04:41:35 CET 2010
On Mon, 08 Mar 2010 16:09:12 +1300, Gregory Ewing wrote:
> Given some known data/crc pairs, how feasible is it to figure out the
> polynomial being used to generate the crc?
Can you just ask the application developer what CRC is being used? Or
look at the source code? Disassemble the binary?
> 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.
"At least"? Can't you tell by looking at them?
A good place to start is here:
http://en.wikipedia.org/wiki/Cyclic_redundancy_check
http://en.wikipedia.org/wiki/List_of_checksum_algorithms
You can at least eliminate known CRCs. There doesn't appear to be any 24-
bit CRCs in common use, and very few other 24-bit checksums either, so
you're probably looking at a custom CRC.
> 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 you're reduced to trying to determine the polynomial from a sample of
checksums, that's a curve fitting problem. There are various ways to fit
polynomials through a set of known points, but as far as I know none of
them are designed for reverse-engineering CRCs.
You may be able to find something about curve-fitting using discrete
maths (i.e. where all values are limited to integers) or with constraints.
You should probably take this to somewhere like sci.crypt. Here's a
thread from a few years back which might give you some hints:
http://www.derkeiler.com/Newsgroups/sci.crypt/2008-07/msg00035.html
Or here:
http://stackoverflow.com/questions/401231/determining-crc-algorithm-from-
data-crc-embedded-application
--
Steven
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