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List:       cryptography
Subject:    Re: [Cryptography] Photon beam splitters for "true" random number generation ?
From:       Jerry Leichter <leichter () lrw ! com>
Date:       2015-12-31 1:54:27
Message-ID: AE637F3C-89BC-4EA8-B27B-E29873DF6BED () lrw ! com
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> That's known to be not good enough.  The device could quite
> plausibly have twice as much storage as you think it does,
> and who knows what is lurking in the hidden half.
It could be much more than half.  Looking just at USB sticks, you can buy them with \
capacities from 16MB (pretty rare these days) up to at least 256GB, in multiples of \
two.  No one runs lines at all those points.  They run a couple of lines at some \
maximum size, then bin the chips based on how much memory is actually good, rounding \
down to the nearest power of two since that's the way the industry has chosen to \
market them.  (There's absolutely nothing other than marketing preventing someone \
from selling 765MB USB sticks.  In fact, some SSD's are sold at non-power-of-two \
sizes - e.g., 480 and 960GB.)

The "half" you refer to may well be the almost-half that's left over in rounding down \
to a power of two - at least some part of which will be dedicated to spares.  \
(Actually, a chip might have to be rounded down by a further factor of two to provide \
some minimum number of spares.)

However, just because the rest of the stick tested bad, doesn't mean it's actually \
fully unusable.  It may work but not completely reliably, or some cells may work \
while others don't.  This stuff will be useless for the purposes for which the stick \
is sold  - but given suitably written software, it could be used for all kinds of \
special, hidden purposes.

Story from years back:  The CDC 6600 - the supercomputer of the early 1970's - could \
be purchased with either (I think) 128KW (Kilo-Words - a word was 60 bits) or 256KW \
of main memory.  NYU purchased one.  They asked for the 128KW version.  CDC tried to \
convince them that they should really get the full 256KW.  But the institution had a \
grant and couldn't swing the extra money - probably a couple of hundred thousand in \
those days.  So CDC finally delivered a 128KW version.  But ... the developers who \
played around with the machine found that they could actually get at memory beyond \
the 128KW limit.  Writes and reads to it ... worked just fine.  In fact, what they \
eventually determined was that the machine they had came equipped with the full 256KW \
- CDC had just made a special patch to their copy of the OS to have it limit itself \
to the lower half.  A source within CDC eventually explained to them that CDC listed \
a 128KW option, but that they didn't really expect anyone to buy one: If you were \
spending the tens of millions one of these things cost, you were not going to go \
short on main memory.  NYU was, in fact, the first customer to buy one of the "small \
memory" configurations - and CDC hadn't actually worked out the necessary \
manufacturing changes to build one.  So they just shipped the configuration they had \
and patched the OS.

(During the anti-Vietnam-war protests of the late 1960's, a group took over the \
machine room in which the CDC 6600 was stored.  They held it hostage - and when they \
left, the left behind some incendiary devices on long fuses.  The machine was barely \
saved from destruction by some faculty members - the story was recently retold.  What \
I heard from people there was that they would not have minded so much if the machine \
had been torched:  Insurance would have replaced it, but the replacement would not \
have been one of the very early runs - I think serial number 4 - which was too early \
to support Extended Core Storage, which used slower but cheaper memory - we're \
talking magnetic core in those days - as a kind of I/O device.  The fact the machine \
was such an early model makes the story about main memory more believable.)

                                                       -- Jerry

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