From: "George Spelvin" <linux@horizon.com>
To: herbert@gondor.apana.org.au, linux@horizon.com
Cc: dborkman@redhat.com, hannes@stressinduktion.org,
linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
tgraf@suug.ch
Subject: Re: Where exactly will arch_fast_hash be used
Date: 7 Dec 2014 05:02:52 -0500 [thread overview]
Message-ID: <20141207100252.6707.qmail@ns.horizon.com> (raw)
In-Reply-To: <20141207092828.GA8623@gondor.apana.org.au>
> 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?
next prev parent reply other threads:[~2014-12-07 10:02 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-12-07 5:20 Where exactly will arch_fast_hash be used George Spelvin
2014-12-07 9:28 ` Herbert Xu
2014-12-07 10:02 ` George Spelvin [this message]
2014-12-07 12:51 ` Herbert Xu
2014-12-07 13:23 ` George Spelvin
2014-12-07 14:06 ` Hannes Frederic Sowa
2014-12-07 21:33 ` George Spelvin
2014-12-08 11:25 ` Hannes Frederic Sowa
2014-12-08 16:19 ` George Spelvin
2014-12-08 16:32 ` Hannes Frederic Sowa
2014-12-09 14:24 ` Herbert Xu
2014-12-07 13:14 ` Hannes Frederic Sowa
2014-12-07 13:30 ` George Spelvin
2014-12-07 13:41 ` Hannes Frederic Sowa
2014-12-07 13:52 ` Hannes Frederic Sowa
-- strict thread matches above, loose matches on Subject: below --
2014-12-04 8:11 Herbert Xu
2014-12-04 12:34 ` Hannes Frederic Sowa
2014-12-04 13:14 ` Daniel Borkmann
2014-12-04 15:26 ` Thomas Graf
2014-12-04 15:29 ` Herbert Xu
2014-12-04 15:39 ` Thomas Graf
2014-12-04 15:43 ` Daniel Borkmann
2014-12-04 15:47 ` Herbert Xu
2014-12-04 15:51 ` Daniel Borkmann
2014-12-04 15:56 ` David Laight
2014-12-04 16:10 ` Herbert Xu
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