linux-kernel - Re: random(4) driver questions

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Date:	Tue, 28 Jun 2011 14:02:58 +0800
From:	Sandy Harris <sandyinchina@...il.com>
To:	"Ted Ts'o" <tytso@....edu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: random(4) driver questions

On Mon, Jun 27, 2011 at 10:54 PM, Ted Ts'o <tytso@....edu> wrote:

> Suffice it to say the random generator has changed significantly since
> 2006.  There is now a secondary pool, which uses catastrophic
> reseeding, etc.

Why does the secondary pool use another hash, rather than a
block cipher? I can see using a hash for the primary pool; at
the original design time, export laws were a concern, and in
any case a hash is the obvious mixing primitive there.

However, for the secondary pools, a block cipher seems to
me to be the obvious thing to use because there is plenty
of analysis, in the Yarrow paper and follow-ups, of that
technique. Also, I think it might be faster.

An AES-128 context, 11 128-bit round keys, is roughly the
same size as one of the current secondary pools, 32 32-bit
chunks. What would maintainers think of a patch along
those lines?

Another question is whether and when we might replace
SHA-1 with a more modern hash. Jeff Garzik has a patch
to add Skein to the crypto API. That would be faster
than SHA-1 and perhaps more easily analyzed since
the compression function is a block cipher. Of course
the SHA-3 Advanced Hash Standard process is not
scheduled to finish for another year and there's a
good argument that we should wait for that.

Also, though there are some attacks on SHA-1, none
of them appear to matter for this application, so
perhaps :If it ain't broke, don't fix it".
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