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From: sja@tekla.fi (Sakari Jalovaara)
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Date: Thu, 23 Jan 1997 19:26:49 +0200
Message-Id: <9701231726.AA15141@poveri.tekla.fi>
To: hackers@freebsd.org
Cc: msmith@atrad.adelaide.edu.au
Subject: Re: CRC-16 algorithms?
Sender: owner-hackers@freebsd.org
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Precedence: bulk

> 00000000  00 39 3c 3e 01 02 03 00  80 00 00 00 00 00 00 00  |.9<>............|
> 00000010  08 00 0e 20 3d 4b 00 00  00 00 00 00 00 02 01 00  |... =K..........|
> 00000020  01 03 00 de ad 01 00 00  00 00 00 00 00 00 00 01  |....-...........|
> 00000030  00 00 00 00 00 00 00 00  00 00 00 00 01 26 3c aa  |.............&<.|
>
> The bytes at 0x3d and 0x3e are the CRC-16 of the bytes from 0-0x3c

Just checking: without the _three_ last bytes of that data, the CRC-16
checksum is 0x3d3e?

I don't seem to find such a CRC-16 algorithm (unless we assume
an arbitrary "XorOut" parameter.)

For trying CRC-16 with different parameters, get "crcmodel.c" from
http://kbs.cs.tu-berlin.de/isis/crc and use the following little main
program.

I didn't try all different "cm_init" values, just -1 and 0.
Write your own for loop for that.  You'll need another set of
test data if you try that.  Or if you try different final xor
parameters.


int main (void)
{
    cm_t cm;
    int x, crc;
    static unsigned char data[] = {
	0x00, 0x39, 0x3c, 0x3e, 0x01, 0x02, 0x03, 0x00,
	0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
	0x08, 0x00, 0x0e, 0x20, 0x3d, 0x4b, 0x00, 0x00,
	0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0x01, 0x00,
	0x01, 0x03, 0x00, 0xde, 0xad, 0x01, 0x00, 0x00,
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
	0x00, 0x00, 0x00, 0x00, 0x01,
    };
    int n;

    for (x = 1; x <= 0xffff; x += 2)
    {
	cm.cm_width = 16;
	cm.cm_poly  = x;
	cm.cm_init  = 0L;
	cm.cm_refin = FALSE;
	cm.cm_refot = FALSE;
	cm.cm_xorot = 0L;

	cm_ini(&cm);

	for (n = 0; n < sizeof (data); n++)
	    cm_nxt(&cm, data[n]);

	crc = cm_crc(&cm);
	if (crc == 0x3d3e)
	    printf("%x %x\n", crc, x);
    }

    return 0;
}
									++sja