Reverse CRC ...

[chants]

Hi CZC, is there a name for the polynomial finding tool? Also can you tell us for CRC-N bits how many CRC of N bits are needed to recover the polynomial and what if there are more or less bits input into CRC function does it change this requirement? For N to N bit mapping I suppose only 1 CRC value determines it, or less than N bits due to 1 to 1 mapping. But if CRC of large data, due to collisions or seems each extra bit adds a power of 2 more values needed to determine it. Anyway, this is interesting enough that it sounds you have a tool to make it easily done.

Also for questioners asking for mapping functions, remember to provide huge amounts of values or white box info if not absolutely sure. Because even CRC with an XOR which flips some bits of the result would likely throw off any ordinary detection. E.g. CRC16 ^ 0x1f1f. But as part of protocol reversing it would be nice to have powerful numeric identification tools where computation is all done server side. Of course no guarantee server is not using random numbers and tracking them with a database but it's more expensive to do such overkill solutions.

[CZC]

Originally Posted by chants

Hi CZC, is there a name for the polynomial finding tool? Also can you tell us for CRC-N bits how many CRC of N bits are needed to recover the polynomial and what if there are more or less bits input into CRC function does it change this requirement?

When bruteforcing, the program finds all possible polynomials. You then narrow them down if you have several examples.

At some point in the past I was using CRCs a lot in microcontrollers and I decided to write my own tool for that.

The source code is attached, it compiles in linux and windows as a command line program.

You can use shortcuts like using hex or strings directly on the command line.

CRC generator V1.4 $

./getcrc [options] file(s)
        -z #    polynom size
        -p #    CRC polynom
        -r      reverse polynom
        -m      mirror (reflect) data
        -n      non-direct init value
        -i #    initial value
        -e #    exit value
        -c      reverse CRC
        -a      all known CRCs
        -b #    bruteforce to target CRC
        -v      verbose bruteforce
        -s      interpret filename as a string
        -f      interpret filename as a hex string
        -C      check CRC values for the "123456789" input
        -t      trace CRC generation
        -T      dump CRC table and exit