[Cryptech Tech] RAM as source of entropy

[Joachim Strömbergson]

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Aloha!

Warren Kumari wrote:
> It seems that the initial state of DRAM and SRAM is fairly far from 
> random -- there was some research (that I can try drudge up) after
> the "Lest We Remember: Cold Boot Attacks on Encryption Keys" paper
> that showed that the initial state of DRAM and SRAM (weeeellll CPU
> cache) is different for each unit, but fairly predictable within the
> unit...

Judging by the papers some memories actually are pretty random at
initial state.

If you look at the papers I referenced they talk precisely about this.
For DRAM memories there are cells that decay much slower than others,
and they alway do this. And some SRAM cells seem to move into the same
initial state for every cycle. This is used to fingerprint devices. But
the fingerprinting is done by reading massive amounts of times and
averaging to remove noise and extract the unique pattern. There seems to
be enough entropy generated to (via whitening, conditioning) extract
good entropy. At least from some memories. The paper that tested
internal RAMs in PIC MCU vs STM32 showed that the former had very bad
entropy, while the latter was a seemingly good source.

I.e. using this requires testing of the SRAM chips (at least the type
and revision). But then you have to test all your entropy sources.

Also it is not clear how much time one needs to give the memory in
between extraction. But the SRAM seems to be a better choice.

- -- 
Med vänlig hälsning, Yours

Joachim Strömbergson - Alltid i harmonisk svängning.
========================================================================
 Joachim Strömbergson          Secworks AB          joachim at secworks.se
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