linux-kernel - Re: [PATCH 5/7] random: replace non-blocking pool with a Chacha20-based CRNG

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Message-ID: <20160620234819.GF9848@thunk.org>
Date:	Mon, 20 Jun 2016 19:48:19 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org, andi@...stfloor.org,
	sandyinchina@...il.com, jsd@...n.com, hpa@...or.com
Subject: Re: [PATCH 5/7] random: replace non-blocking pool with a
 Chacha20-based CRNG

On Mon, Jun 20, 2016 at 05:49:17PM +0200, Stephan Mueller wrote:
> 
> Is speed everything we should care about? What about:
> 
> - offloading of crypto operation from the CPU

In practice CPU offland is not helpful, and in fact, in most cases is
harmful, when one is only encrypting a tiny amount of data.  That's
because the cost of setup and teardown, not to mention key scheduling,
dominate.  This is less of the case in the case of the SIMD / AVX
optimizations --- but that's because these are CPU instructions, and
there really isn't any CPU offloading going on.

> - potentially additional security features a hardware cipher may provide like 
> cache coloring attack resistance?

Um.... have you even taken a *look* at how ChaCha20 is implemented?
*What* cache coloring attack is possible at all, period?

Hint: where are the lookup tables?  Where are the data-dependent
memory accesses in the ChaCha20 core?

						- Ted