Reverse engineering CRC?

[Gregory Ewing]

Steven D'Aprano wrote:

> Can you just ask the application developer what CRC is being used? Or 
> look at the source code? Disassemble the binary?

There's no source, and the binary is enormous. I could ask,
but I wouldn't hold out much hope of them being willing to
tell me.

>>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?

It's not entirely clear exactly which bytes are part of the
CRC. There are 3 adjacent bytes in the header of the file
that change when I modify the contents, which led me to
think it was a 24-bit CRC. But I now believe that one of
them is not part of the CRC, and it's actually 16 bits.

Using pycrc, I've now tried all possible 16-bit polynomials,
with various combinations of bit and byte reversal, but I
haven't found one that works consistently, so I'm wondering
whether it's using some non-standard algorithm.

-- 
Greg