[Cryptech Tech] RAM as source of entropy

[Joachim Strömbergson]

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Aloha!

I just had a (not very unique) idea of using RAM memories as source of
entropy. The idea is to power cycle an external memory and read the
contents. Due to how memories work the contents will be in more or less
random state.

The SRAM memory, which basically consists of a D-flipflop built with 6
transistors will be in either 0 or 1 state. The state based on previous
state, random charge fluctuations in the transistors, temperature of
device and whatnot.

For a DRAM the contents of a power cycle is some sort of decay state
based on the state before power (or refresh) was dropped. This means
that using a DRAM would require some sore of pattern written into the
memory, drop power for x amount of time, read out the contents and repeat.

The good things with memories as entropy source are:
* Capacity - should be able to generate quite a lot of entropy/s

* Digital - No A/D converter required, simply treat the source as a
normal memory, just add an extra I/O to control the power cycle.

* Cheap and easy to integrate. We should be able to use small serial
connected memories for this. This baby here costs less than 1 USD:

http://se.farnell.com/microchip/23a640-i-p/ic-sram-serial-64k-1-7v-pdip8/dp/1695544

These entropy sources would be fairly robust. Temperature seems to
affect the DRAM decay more than SRAM. Basically dropping the DRAM memory
into a freezer slows decay down very much. But the Crypech HSM should be
able to check its ambient temperature anyway.

I've found a few papers that looks at DRAM and SRAM memories as entropy
sources [1][2][3]. I'm quite infatuated with this idea and would really
appreciate some clear headed thoughts. I'm willing to test this in hw.

FredrikT from your work on the YubiHSM can you say that you considered
this memory based type of entropy source. And if so, why not using it?
And if you can't speak about this, just say "NDA". ;-)


[1] http://goo.gl/25TFov
[2] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.164.6432
[3] https://eprint.iacr.org/2013/304.pdf

- -- 
Med vänlig hälsning, Yours

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