linux-kernel - Re: Entropy sources (was: /dev/random

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Date:   Thu, 25 Aug 2016 14:30:02 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     noloader@...il.com
Cc:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Entropy sources (was: /dev/random - a new approach)

On 08/20/16 22:37, Jeffrey Walton wrote:
>>
>> The biggest problem there is that the timer interrupt adds *no* entropy
>> unless there is a source of asynchronicity in the system.  On PCs,
>> traditionally the timer has been run from a completely different crystal
>> (14.31818 MHz) than the CPU, which is the ideal situation, but if they
>> are run off the same crystal and run in lockstep, there is very little
>> if anything there.  On some systems, the timer may even *be* the only
>> source of time, and the entropy truly is zero.
> 
> It seems like a networked computer should have an abundance on entropy
> available from the network stack. Every common case I can come up with
> includes a networked computer. If a handheld is outside of coverage,
> then it probably does not have the randomness demands because it can't
> communicate (i.e., TCP sequence numbers, key agreement, etc).
> 
> In fact, there are at least two papers that use bits from the network stack:
> 

The network stack is a good source of entropy, *once it is online*.
However, the most serious case is while the machine is still booting,
when the network will not have enabled yet.

	-hpa