Re: Calculating CRC5 of 11-bit data

[Ross Jacobs <rossbjacobs () gmail com>]

Hi Tomasz,

My vote is for table lookup along a one-byte boundary as all of the
existing CRC checkers in /wsutil currently use one. Notice how in our CRC16
and CRC32 implementations, we bitshift by 8 and 24 respectively. It should
be possible to compute any CRC along a one-byte boundary. This article
<http://www.sunshine2k.de/articles/coding/crc/understanding_crc.html> goes
over how to implement non-CRC8 algorithms to do just that with a table
lookup.

PyCRC <https://pycrc.org/> was used to generate Wireshark's CRC6, CRC7, and
CRC11 algorithms. You may want to take a look at it and see if it can help
here. In this case, width=5 and poly=0x05 (assuming CRC5/USB found here
<http://reveng.sourceforge.net/crc-catalogue/all.htm>).

Cheers,
Ross

On Fri, Jul 19, 2019 at 9:28 AM Tomasz Moń <desowin () gmail com> wrote:

> Hello,
>
> I am currently working on USB Link Layer dissector. It is supposed to
> be used with OpenVizsla FPGA-based USB hardware sniffer. USB Packets
> feature two CRC types: CRC-5/USB and CRC-16/USB.
>
> The CRC-16/USB is pretty straightforward. I have implemented it in [1]
> and it works fine with my captures.
>
> The CRC-5/USB is not so straightforward as it does not operate on the
> byte boundary. The CRC-5/USB is calculated on 11 bits of data. I think
> there are two possible approaches to check the CRC-5/USB in Wireshark:
> 1. Compute the CRC5 bitwise on the 11 bits of data, or
> 2. Compute the CRC5 with table-based algorithm on 2 bytes (11 bits
> data + 5 bits CRC) and relying on the CRC properties expect fixed
> output.
>
> Which approach would be preferred?
>
> Best Regards,
> Tomasz Moń
>
> [1] https://code.wireshark.org/review/#/c/34016/
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