27 Jan
2020
27 Jan
'20
8:42 a.m.
On 01/27/2020 02:20 AM, Mattis Lind via cctalk wrote:
Yes, there are the crc reveng for example. But I
cannot get it to generate
the CRC digits that matches. Neither does it give me anything when I ask it
to search for algorithm used.
Of course it is probably just me that don't understand it well enough.
Soem years ago I needed to crack the CRC on a Fanuc serial
encoder. Luckily, the CRC value
was just 5 bits. I built a device to read out and store the
data from the encoder on a PC.
Then, I wrote a little c program that had a "universal" CRC
implementation, where the polynomial
was supplied as a command line parameter. It would read the
stored file, apply the polynomial
as given and report the residuals. With just a 32 value
search space, I just entered them one at a time,
and a few minutes later, I had found the polynomial that
gave all zero residuals.
With a larger search space, you might construct such a
program and let it run all possible polynomial
values and report how many all zero residuals you got for
each. Given just a few short message blocks,
it shouldn't take more than a few minutes to find the magic
number.
Jon