Date: Mon, 19 Aug 2013 01:24:19 -0600 From: Warner Losh <imp@bsdimp.com> To: Mark R V Murray <mark@grondar.org> Cc: Tim Kientzle <tim@kientzle.com>, FreeBSD-arch Arch <freebsd-arch@freebsd.org>, secteam@freebsd.org, =?iso-8859-1?Q?Dag-Erling_Sm=F8rgrav?= <des@des.no> Subject: Re: random(4) plugin infrastructure for mulitple RNG in a modular fashion Message-ID: <EE0B6A6A-38CD-4DB9-A811-F4C0BF83109E@bsdimp.com> In-Reply-To: <3513A465-AD8D-4DDC-9408-2F89F9B86404@grondar.org> References: <20130807183112.GA79319@dragon.NUXI.org> <86pptfnu33.fsf@nine.des.no> <20130815231713.GD76666@x96.org> <20130816002625.GE76666@x96.org> <9B274F48-0C88-4117-BEAC-1A555772A3C5@grondar.org> <86a9kf733d.fsf@nine.des.no> <0C97B866-A169-4141-8368-AA7F5B5382F4@grondar.org> <861u5r71zi.fsf@nine.des.no> <892B11BD-396D-4F82-B97C-753F72CA494D@grondar.org> <86r4dr5j3p.fsf@nine.des.no> <4C1BD77C-8C6B-4044-9285-5978A3BC4B70@kientzle.com> <12B58C72-CFE3-4AD4-AD03-462A10E431D9@bsdimp.com> <3513A465-AD8D-4DDC-9408-2F89F9B86404@grondar.org>
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On Aug 19, 2013, at 1:13 AM, Mark R V Murray wrote: >=20 > On 19 Aug 2013, at 08:09, Warner Losh <imp@bsdimp.com> wrote: >>> Besides Yarrow and Fortuna mixers, we could then >>> offer a "null mixer" option that selected the single >>> "best" entropy source and passed it directly through. >>=20 >> I'm still wondering why timecounters aren't the right model to follow = here, where you can have several compiled into the kernel and the one = with the best score wins. >=20 > How would they get a score, and how would it be decided which is = better? How is the score "calibrated"? For timecounters, we make judgements based on how good or bad we think = the timekeeping ability of the underlying device. I'd imagine that we'd = rate the hardware RNGs high, and the fallback means of harvesting = entropy from interrupts medium, and anything that's really really bad as = low. This would allow for the hardware RNGs to override the other = sources of entropy, while still allowing fallback to reasonable entropy = on devices that are known suspect (While still allowing the pig-headed = and/or externally constrained folks to use the bad sources). For the mixers, the scoring mechanism makes less sense. You'd want more = of an ordered list specified by the user to dictate policy to choose = between nothing, fortuna and yarrow. You'd also want a parameter to deal with failure here: panic or block. >>> Users could compile the null mixer into the kernel >>> and load a single HW RNG driver to have precise >>> control over /dev/random. Interrupt harvesting would >>> be the lowest-quality source as a fall back. >>>=20 >>> In particular, this has a reasonable failure mode if >>> someone built a kernel with only a single HW entropy >>> source and the null mixer: >>> * On hardware with that source, they would get >>> full-speed HW entropy. >>> * On hardware without that source, they would get >>> the old blocking /dev/random that we had before >>> Yarrow, the one that used only interrupt harvesting. >>=20 >> Assuming there was enough interrupt entropy to generate bits=85 >=20 > See Ferguson & Schneier on this (qv my follow-up). Saw that. I was worried only about starvation, but there's much more to = worry about than that it seems. Warner
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