linux-kernel - Re: Why does reading from /dev/urandom deplete entropy so much?

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Message-ID: <20071205212646.GP17536@waste.org>
Date:	Wed, 5 Dec 2007 15:26:47 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Adrian Bunk <bunk@...nel.org>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: Why does reading from /dev/urandom deplete entropy so much?

On Tue, Dec 04, 2007 at 07:17:58PM +0100, Eric Dumazet wrote:
> Alan Cox a ?crit :
> >>No matter what you consider as being better, changing a 12 years old and 
> >>widely used userspace interface like /dev/urandom is simply not an 
> >>option.
> >>    
> >
> >Fixing it to be more efficient in its use of entropy and also fixing the
> >fact its not actually a good random number source would be worth looking
> >at however.
> >  
> Yes, since current behavior on network irq is very pessimistic.

No, it's very optimistic. The network should not be trusted.

The distinction between /dev/random and /dev/urandom boils down to one
word: paranoia. If you are not paranoid enough to mistrust your
network, then /dev/random IS NOT FOR YOU. Use /dev/urandom. Do not
send patches to make /dev/random less paranoid, kthxbye.

> If you have some trafic, (ie more than HZ/2  interrupts per second), 
> then add_timer_randomness() feeds
> some entropy but gives no credit (calling credit_entropy_store() with 
> nbits=0)
> 
> This is because we take into account only the jiffies difference, and 
> not the get_cycles() that should give
> us more entropy on most plaforms.

If we cannot measure a difference, we should nonetheless assume there
is one?
 
> In this patch, I suggest that we feed only one u32 word of entropy, 
> combination of the previous distinct
> words (with some of them being constant or so), so that the nbits 
> estimation is less pessimistic, but also to
> avoid injecting false entropy.

Umm.. no, that's not how it works at all.

Also, for future reference, patches for /dev/random go through me, not
through Dave.

-- 
Mathematics is the supreme nostalgia of our time.
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