linux-kernel - Re: [PATCH] /dev/random: Insufficient of entropy on many architectures

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Date:	Sun, 15 Sep 2013 13:12:15 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Jörn Engel <joern@...fs.org>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>, dave.taht@...ferbloat.net,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] /dev/random: Insufficient of entropy on many architectures

Am Freitag, 13. September 2013, 14:59:31 schrieb Theodore Ts'o:

Hi Theodore,

>On Fri, Sep 13, 2013 at 07:36:20AM +0200, Stephan Mueller wrote:
>
>However, if you are worried about a malicious entropy source, things
>are a little bit different.  Suppose RDRAND == AES(i++, NSA_KEY),
>where the NSA doesn't know the starting value of i.  But if it get can
>get a raw RDRAND value (say, someone uses it without doing any
>whitening as a session key or as a D-H parameter), it can decrypt the
>output using the NSA_KEY, and then now that it knows i, it can brute
>force break the RDRAND output, even if it's not entirely sure how many
>times RDRAND has been called between that cleanb RDRAND value and the
>RDRAND output it is trying to break.
>
>In *this* case, smearing out the value of RDRAND across the entropy
>pool does help, becuase it makes it significantly harder to get a
>clean RDRAND value to decrypt.

Agreed. I was only talking about "well-behaved" entropy sources.
>
>
>That being said, the much bigger problem that I'm worried about is not
>necessarily a trojan'ed RDRAND, but rather on embedded ARM and MIPS
>devices where we have unsufficient entropy, and on the first boot out
>of the box, there is no random seed file that can be fixed in at boot

Yes, my local MIPS-based router which is a very ubiquitous one in 
Germany (Fritz Box) does not seed /dev/random but yet starts using 
/dev/urandom during boot cycle.

>time.  Mixing in personalization information (serial numbers, MAC
>addresses) which *hopefully* the NSA wouldn't know in the case of
>pervasive, bulk surveillance, is a bit of a help.  But it's certainly
>no help against a direct, targetted attack.

That is why I am hoping that the CPU jitter as harvested by my RNG will 
help as it delivers entropy on demand in a bandwidth where the initial 
seeding may not needed any more and we have sufficient entropy during 
boot sequence.
>
>Regards,
>
>						- Ted


Ciao
Stephan
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