IBM BSC CRC?

[Jon Elson]

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