netdev - Re: Where exactly will arch_fast

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Date:	7 Dec 2014 05:02:52 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	herbert@...dor.apana.org.au, linux@...izon.com
Cc:	dborkman@...hat.com, hannes@...essinduktion.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, tgraf@...g.ch
Subject: Re: Where exactly will arch_fast_hash be used

> For a start why don't you print out the hashes of 1-255 and then
> find out how easy it is to deduce the last bit of the hash result.

They're available in lib/crc32table.h, as crc32ctable_le[0].

As a CRC is a linear function, every bit is the XOR of some
selected bits of the input, i.e. the parity of the input and
some bit-specific mask sequence.

Furthermore, CRCs are cyclic, so the mask sequences for adjacent bits are
shifts of each other.

The lsbit of the CRC32c of x is the parity of x & 0x1f.

This is because the LFSR sequence generated by the polynomial
starts 0001111110010001110010101111011000111000011011110010110000100101...

The first bit corresponds to the msbit of the last byte.

How does this implicate the low bits specifically?
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