----- Forwarded message from Theodore Ts'o <tytso@mit.edu> ----- Date: Thu, 17 Oct 2013 09:08:00 -0400 From: Theodore Ts'o <tytso@mit.edu> To: Adam Back <adam@cypherspace.org> Cc: Jerry Leichter <leichter@lrw.com>, Sandy Harris <sandyinchina@gmail.com>, Cryptography <cryptography@metzdowd.com> Subject: Re: [Cryptography] /dev/random is not robust Message-ID: <20131017130800.GE11932@thunk.org> User-Agent: Mutt/1.5.21 (2010-09-15) On Thu, Oct 17, 2013 at 02:32:57PM +0200, Adam Back wrote:

Yarrow, and the replacement Fortuna try to address this problem by accumulating entropy and adding it in bigger lumps..

... and Linux's /dev/random driver does this. Post July 2012, most of the entropy is gathered via a per-CPU (to a avoid cache line bouncing effects and so it can be lockless) entropy pool, where we sample the high resolution cycle counter (or whatever the highest granularity clock / memory refresh control register / etc. we have access to on the archtecture) and the interrupted IP, and mix that into the per-CPU fast mix pool on every interrupt. We do *not* use an entropy estimator for this interrupt fast mix pool. Instead, we sample Every 64 interrupts, we transfer entropy from the fast mix pool to the input pool, and we credit the input pool with a single bit of entropy. (There is very likely much more than a single bit of entropy that has gotten accumulated during those 64 interrupts, but out of an abundance of caution, we're using a very conservative estimate for administrative concerns.) In both the pre and post July 2012 designs, using a Yarrow-like approach, we only transfer entropy from the input pool to the output pool when there is sufficient entropy estimated to be in the input pool so that we can do a "catastrophic ressed". The "/dev/random is not robust paper" assumed that the attacker could control the interrupt timings such that estimate of entropy in the input pool was incorrect, and thus the catastrophic reseed aspect of the design could be bypassed. I've already discussed why I don't believe that the assumption that the attacker could control the interrupt timings to such an extent is not realistic, and analysis of the entropy estimator (as used in the pre-July 2012 design) showed that in fact, it was quite good. But in the post July 2012 design, we no longer use an interrupt estimator for the interrupt fast mix pool. We abandoned it for efficiency concerns, since we wanted to make the cpu count on the global interrupt fast path as low overhead as possible; instead, we traded this off by a brute force quantity argument --- if we can collect the timing for every single interrupt we're much better off than collecting it only for some interrupts, especially when in the old design (which involved CPU cache line bouncing and potential lock contention) device driver authors were disabling the entropy collection more often than not. So in the new design, we aren't using an dynamic entropy estimator --- instead, we're assuming that after collecting the timings for 64 interrupts, we've collecting a single bit of entropy, which is really a static entropy measure. Could this be spoofed if the attacker has control of the interrupt timings of the system? Sure, but if the attacker has that level of control on the system, then then pretty much all generators would be seriously compromised as well. The only way the paper could show that their proposed generator was "robust" was based on the assumption that it would be possible for the attacker to control the entropy inputs in such a way that entropy estimator would be spoofed, but the attacker might still not know some of the bits of the entropy inputs. After all, if the attacker knows all of the bits, then by definition all generators would be screwed. However, what has not been demonstrated in the paper is a real life scenario where the attacker would have that level of control over the entropy inputs --- enough that entrpoy estimators would be fooled, but not enough control that their constuction could be considered robust. Regards, - Ted _______________________________________________ The cryptography mailing list cryptography@metzdowd.com http://www.metzdowd.com/mailman/listinfo/cryptography ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org ______________________________________________________________ ICBM: 48.07100, 11.36820 http://ativel.com http://postbiota.org AC894EC5: 38A5 5F46 A4FF 59B8 336B 47EE F46E 3489 AC89 4EC5