Bruteforce CRC

Given some strings and their expected CRC finds the possible generating polynomials. We use GAP (A Computer Algebra System) and some C programs.

How does it work?

Calculating the CRC code of a given "message" is really simple: we think of the message as if it were a polynomial $m(x)$ with coefficients in the field with two elements GF(2).

A type of CRC code is defined by a polynomial $g(x)$. Let's call its degree $d = Deg g$. The degree it's usually a power of two for reasons that will be soon clear.

The CRC of the message $m(x)$ is the remainder in the Euclidean Division of the polynomial $m(x) * x^d$ by $g(x)$. That is, we can write $$ m(x) * x^d = g(x) q(x) + r(x) $$, where $r(x)$ is the remainder, that is to be interpreted as the final CRC.

We simply noticed that $g(x) \mid m(x) * x^d - r(x)$, that is the defining polynomial divides the message minus the CRC number. So we can compute, given enough messages and CRC, the MCD of all the polynomials $m(x) * x^d - r(x)$, knowing that $g(x)$ must be one of the divisors of it of degree d.

How to use it

Basic step are:

• Using file2binary convert your files to the bit representation

• Using variant generate all possible modes of operation of the machine generating the CRCs.

• Using calculateCRCpolynomial finally solve all CRCs challenges and hope there will be some useful output.

COMING SOON: You can also let the system generate the infos regarding the CRC found and generate CRC for you from your files