linux-kernel - Re: [PATCH, RFC 10/12] random: cap the rate which the /dev/urandom pool gets reseeded

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Date:	Sun, 22 Sep 2013 17:26:14 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Theodore Ts'o" <tytso@....edu>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	joern@...fs.org, macro@...ux-mips.org, ralf@...ux-mips.org,
	dave.taht@...il.com, blogic@...nwrt.org, andrewmcgr@...il.com,
	smueller@...onox.de, geert@...ux-m68k.org, tg@...bsd.de
Subject: Re: [PATCH, RFC 10/12] random: cap the rate which the /dev/urandom
 pool gets reseeded

On 09/22/2013 04:23 PM, Theodore Ts'o wrote:
> On Sun, Sep 22, 2013 at 03:45:11PM -0700, H. Peter Anvin wrote:
>> I understand the motivation, but I question basing it in a fixed amount of time.
> 
> We already have a threshold based on the amount of the entropy in the
> input pool.  I could increase that threshold, but then instead of
> having the entropy hover between say, 192 and 128, it would hover
> between say, 576 and 512.  What that means in practice is that there
> will be a higher baseline, but we will still end up reseeding every 10
> seconds or so (that being approximately how much entropy I've seen
> flowing into the input pool on my laptop system --- 64 bits every 10
> seconds or so).
> 
> By basing it on a time-based threshold, it means that the input pool
> can build up faster such that over time, such that entropy pool ends
> up hovering around 3400 bits.
> 

So make it a threshold around 2048-3072 bits.  A.k.a. "if the input pool
is filling up fast enough, take advantage of it."

> What is your concern is about having it being time-based --- or more
> accurately, being partially time-based, since we also keep the entorpy
> count based threshold?  I'll note that the Fortuna Random Number
> Generator, devised by Bruce Schneier and Niels Ferguson also uses as
> part of its design a time-based reseed limit.

Just that it is unnecessary in a scenario when entropy is plentiful.

	-hpa


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