Re: CCITT-CRC16 in kernel

[linux]

>> Using this bit-ordering, and omitting the x^16 term as is
>> conventional (it's implicit in the implementation), the polynomials
>> come out as:
>> CRC-16: 0xa001
>> CRC-CCITT: 0x8408
> 
> Huh? That's the problem.
> 
> X^16 + X^12 + X^5 + X^0 = 0x1021, not 0xa001
> 
> Also,
> 
> X^16 + X^15 + X^2 + X^0 = 0x8005, not 0x8408

You're wrong in two ways:
1) You've got CRC-16 and CRC-CCITT mixed up, and
2) You've got the bit ordering backwards.  Remember, I said very clearly,
   the lsbit is the first bit, and the first bit is the highest power
   of x.  You can reverse the convention and still have a CRC, but that's
   not the way it's usually done and it's more awkward in software.

CRC-CCITT = X^16 + X^12 + X^5 + X^0 = 0x8408, and NOT 0x1021
CRC-16 =  X^16 + X^15 + X^2 + X^0 = 0xa001, and NOT 0x8005

> Attached is a program that will generate a table of polynomials
> for the conventional CRC lookup-table code. If you look at
> the table in the kernel code, offset 1, you will see that
> the polynomial is 0x1189. This corresponds to the CRC of
> the value 1. It does not correspond to either your polynomials
> or the ones documented on numerous web pages.

No, it doesn't.  The table entry at offset *128* is the CRC polynomial,
which is 0x8408, exactly as the comment just above the table says.


> I think somebody just guessed and came up with "magic" because the
> table being used isn't correct.

The table being used is 100% correct.  There is no mistake.
If you think you've found a mistake, there's something you're not
understanding.

Sorry to be so blunt, but it's true.

>> The *other* think you have to do is append the checksum to the message
>> correctly.  As mentioned earlier, the lsbit of a byte is considered
>> first, so the lsbyte of the 16-bit accumulator is appended first.

> Right, but the hardware did that. I have no control over that. I
> have to figure out if:
> 
> (1) It started with 0xffff or something else.
> (2) It was inverted after.
> (3) The result was byte-swapped.
> 
> With the "usual" CRC-16 that I used before, using the lookup-
> table that is for the 0x1021 polynomial, hardware was found
> to have inverted and byte-swapped, but started with 0xefde
> (0x1021 inverted). Trying to use the in-kernel CRC, I was
> unable to find anything that made sense.

You can get rid of the starting value and inversion by XORing together
two messages (with valid CRCs) of equal length.  The result has a valid
CRC with preset to 0 and no inversion.  You can figure that out later.

Then, the only questions are the polynomial and bit ordering.
(You can also have a screwed-up CRC byte ordering, but that's rare
except in software written by people who don't know better.  Hardware
invariably gets it right.)

As I said, the commonest case is to consider the lsbit first.
However, some implementations take the msbit of each byte first.

Here's code to do it both ways.  This is the bit-at-a-time version,
not using a table.  You can verify that the first implementation,
fed an initial crc=0, poly=0x8408, and all possible 1-byte messages,
produces the table in crc-ccitt.c.

/*
 * Expects poly encoded so 0x8000 is x^0 and 0x0001 is x^15.
 * CRC should be appended lsbyte first.
 */
uint16_t
crc_lsb_first(uint16_t crc, uint16_t poly, unsigned char const *p, size_t len)
{
	while (len--) {
		unsigned i;
		crc ^= (unsigned char)*p++;
		for (i = 0; i < 8; i++)
			crc = (crc >> 1) ^ ((crc & 1) ? poly : 0);
	}
	return crc;
}

/*
 * Expects poly encoded so 0x0001 is x^0 and 0x8000 is x^15.
 * CRC should be appended msbyte first.
 */
uint16_t
crc_msb_first(uint16_t crc, uint16_t poly, unsigned char const *p, size_t len)
{
	while (len--) {
		unsigned i;
		crc ^= (uint16_t)(unsigned char)*p++ << 8;
		for (i = 0; i < 8; i++)
			crc = (crc << 1) ^ ((crc & 0x8000) ? poly : 0);
	}
	return crc;
}

If you're trying to reverse-engineer an unknown CRC, get two valid
messages of the same length, form their XOR, and try a few different
polynomials.  (There's a way to do it more efficiently using a GCD, but
on a modern machine, it's faster to try all 32768 possible polynomials
than to write and debug the GCD code.)

After that, you can figure out the preset and final inversion, if any.
For fixed-length messages, you can merge them into a single 16-bit
constant that you can include at the beginning or the end, but if
you have variable-length messages, it matters.
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