MRAM, persistence of memory

Thomas Shaddack shaddack at
Tue Jul 15 11:58:05 PDT 2003

On Tue, 15 Jul 2003, Sampo Syreeni wrote:

> There are plenty of examples like printer buffers,

They shall be on encrypted partition, if possible by a session key (as
they are temporary files which we won't need after next powerup), which
gets lost after powerdown.

> random pools,

Why not initialize them after each powerup with fresh data from a hardware

> already keyed smartcards,

...whose status gets lost with powerdown,

> anything you're currently showing on-screen (I wouldn't be surprised if
> that could be read after sustained powerdown),

An issue (though I don't know how serious) with CRTs, where there could be
detectable residual charge, but with a bit of luck that could get
undetectable in just couple seconds in normal room temperatures.

> any battery backed memory,

IMPORTANT issue. But as NVRAM is a kind of permanent storage, nothing
unencrypted shall touch it anyway.

> off-the-shelf peripherals which do not admit crypto,

If we consider them plaintext, it doesn't matter here if they contain
crypto or not.

> and so on. Probably
> we'd also want to make the machine as dead as possible, shutting down any
> remote power-on features,

Why? The adversary can power up the machine once he gets physical access,
anyway. By then, though, he shouldn't get anything more than a machine
with data with either missing session key (and hence unrecoverable) or
encrypted data missing the key that has to be supplied by the user to
complete the power-up.

> network connections (otherwise retries might go
> on for a fair while),

...which is no harm, as the data should be encrypted and without the key,
lost to the powerdown, they won't reveal anything than that there was
some connection in progress. Which is something the adversary most likely
knows anyway; before physical intrusions, passive surveillance usually
takes place. Keyword: Carnivore.

> open phone lines,

...hung up by power loss to the modems...

> whathaveyou.

Everything must lose critical inner state with the powerdown. Everything
that carries plaintext data has to be power-dependent.

> That's why we might need software, and not just the power switch.

That's why we need the software designed to cope with the possibility of
unpredictable power-down. There should be no dependability on proper
function of software at the moment of the power-down itself, as we need to
keep the system as reliable (and as simple) as possible. Remember that
years in jail, and possibly even lives, are in stake.

The software could be able to communicate with the hardware though,
issuing "locks" to prevent power-down in the middle of a critical
operation (eg, writing to disk) and delay it a little. The locks should be
issued for maximum time of some few 100s milliseconds, expiring in
hardware if not cancelled from the software. There should be a period of
allowed shutdown every period (100s msec?) even in the worst possible
combination of issued locks. This should prevent damage to data during an
emergency shutdown event while not compromising the security due to
software bug (eg, forgotten unlock).

The delay time has to be determined to the worst possible combination of
events, to not allow the best-trained best-equipped adversary to prevent
the machine from shutting down in that time.

> I can also imagine situations where we'd want to be able to perform
> manual shutdowns remotely.

That's the simple part. Just take the output of the computer (or an
external device, eg. a cellphone, a radio receiver, anything suitable) and
use it as one of the event inputs of the power-down system. You raise the
event (tell the computer over SSH, send a SMS, broadcast a tone...), the
computer shuts down.

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