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Re: [BugTraq] Peter Gutmann data deletion theaory?
From: Robin Whittle <rw () firstpr com au>
Date: Fri, 22 Jul 2005 13:03:18 +1000

Peter Gutmann's 1996 paper is at:

  Secure Deletion of Data from Magnetic and Solid-State Memory
  http://www.treachery.net/~jdyson/infosec/secure_del.html

I will discuss four types of memory: Magnetic media (tape, hard-drive,
floppy etc.), Static RAM, Dynamic RAM and FLASH (or EPROM or EEPROM).

Magnetic media typically involves a flat recording surface, with a read
head, write head and sometimes an erase head before the write head.  The
width of the read head may be less than that of the write head, so that
slight misalignments in reading or writing still cause the read signal
to be picked up from within the wider written band.

Magnetic media probably has some kind of depth effect too - it is a
three-dimensional object which is magnetised in one step and read in
another.  Unless all trace of previous writes can be removed, some
remnant of the original signal will remain.  To what extent this can be
read - either by the standard read system or by fancy forensic
techniques - would vary enormously.

Any recovered original signal will be only a small fraction of the
actual read signal.  The rest will be noise, the last recorded
signal, and remnants of other recordings in the past.  There needs to be
a certain signal-to-noise ratio before data can be recovered in any
meaningful manner.

I can't imagine how these forensic techniques work with modern hard
drives, where data is packed so densely and recorded with highly complex
encoding systems.

However, to ensure the destruction of previously written data, I think
its easier to physically destroy the media than do enough research to
prove beyond doubt that there is no possible recovery technique.

Probably what I have written above also applies in principle to optical
media as well.

Semiconductor memory (and core memory) is completely different.  There
is no three-dimensional recording medium - so there is no misalignment
between conventional read and write operations.  There is a single
quantity which is used to encode a bit of information - the voltage of a
capacitor (Dynamic and Flash RAM) or of one side of a flip-flop (Static
RAM).

A Static RAM cell consists of a four transistor flip-flop with two
additional components, either transistors or resistors, as loads for
each side.  Each side is an inverting amplifier, usually made with an N-
and a P-channel Field Effect Transistor.  Reading involves switching one
or both of the sides of the flip-flop (using additional transistors) to
bit lines, and then reading these lines.  Writing involves a similar
connection to the bit lines, but then forcing the state of the flip-flop
to one state or the other.

At the end of a write operation, the flip-flop is constantly powered and
is free from external intervention.  Each inverting amplifier constantly
inverts its input and forms the input to the other.  This constant
amplification would quickly (fractions of nanoseconds) move the voltage
of one side towards ground and the other towards the power supply
voltage.  I can't imagine any detectable difference in the physical
state of the flip-flop due to its previous state surviving more than
tiny fractions of a nanosecond.  Since all these flip-flops are
operational as soon as power is applied, any charge states after the
chip was turned off would soon be over-ridden by the amplifying nature
of the flip-flops.  The question is whether the chip could be powered up
in a way which would amplify slight charge differences remaining in the
chip since it was last operating.  Such differences would need to be
significant compared to the natural bias in each cell towards powering
up one way or another, due to atom-scale differences in dimensions of
channels, gates and doping.  This seems unlikely to me, but I am not a
semiconductor engineer.

I recall reading, via some third-hand account, of some very old memory
chips from the 1970s which could have their internal structure changed
by the sub-micron voltage gradients over long periods of time spent with
a particular data pattern, and this could be perceived in the power-up
state of each bit, as the usual randomness was skewed by these
particular changes.

Peter's paper discusses changes to the device due to repeated writes of
1 and 0.  Only a semiconductor engineer would be able to advise how
valid this theory is, and whether any such changes could possibly be
detected by powering-up the chip in a special way, or by some other
external test mode or invasive microscopical technique.

Dynamic RAM involves a single transistor connecting a single capacitor
to a single bit line, to which many other capacitors could also be
connected - but only one at a time.  Reading involves measuring the
voltage of the bit line to discern whether it is high or low.  In
practice, this involves a non-inverting amplifier connected to the bit
line, with its output connected to its input, and therefore to the bit
line.  There is a pre-charge system to bias the amplifier in the middle
of the range of voltages before connecting the bit line and turning it
on, so that, for instance, in a 5 volt DRAM cell (now they are all 3.3
volts or less) any bit-line voltage above 2.5 volts will be amplified up
to 5 volts (in a nanosecond or so) and any voltage below this will
likewise be amplified to 0 volts.

Thus a read involves a "refresh" - forcing memory cells which are deemed
to be "low" to (or strongly towards and usually very close to) 0 volts
and those which are deemed to be "high" to 5 volts.

Since capacitors leak, DRAM systems repeatedly read and refresh all the
capacitors, such as every 16 milliseconds. (Dipping at random into a
1990 Hitachi memory data book.  Nonetheless, I recall interrupting
refresh in a Z80 CPM system and having the data, or at least enough of
it to run the system, survive for 10 seconds or so.)  Each such refresh
or read effectively erases any detail of the prior voltage of the
capacitor.  There might be some extremely small difference in the final
voltage of the capacitor in the following situations:

1 - Initially 5 volts, then written to "low", so almost 0 volts, with
    just a little of the residual charge, due to limited resistance of
    the bit line and the limited time of the write cycle.

2 - Initially 0 volts then written or refreshed to "low".  This would be
    effectively 0 volts - there is no other charge for there to be a
    residual.

3 - Initially 2.4 volts, so deemed to be low, but refreshed or written
    to low.  (There's no reason in ordinary operation for a capacitor to
    every be at this voltage, unless it was left to leak due to there
    being no refreshes for a long time).

So I can't see how any trace of previous states could possibly be
measured, including by directly probing the capacitor by some means -
rather than by using the sense amplifiers, which are only making a
decision about whether the voltage is above or below 2.5 volts - after
one or more read, refresh or write cycles.

Perhaps keeping a DRAM chip for weeks, months or years with a particular
state of data, as may well happen, could result in long-term changes in
the exact physical nature of the semiconductor material, insulators etc.
 of the capacitor and its immediate surrounds.  However, unlike Static
RAM, I can't see how this could be measured (except by some direct
probing of the chip, which seems highly impractical, since such probes
would be noisy compared to the slight changes) because the base state of
the capacitors at power up is 0 volts or close to it, and the sense
amplifiers can only discern fine differences in voltage around the 2.5
volt range.

So I think that short term storage of data in Static or Dynamic RAM
leaves absolutely no detectable trace after it has been over-written.  I
can't imagine how long-term storage patterns could be detected in DRAM
and I think it is probably impossible, by even the most heroic means, to
discern any such long-term patterns in SRAM.

There are two classes of data recovery here:

1 - Turning the chip on in a special way to discern its state when it
    was turned off.

2 - Trying to sense long-term changes in the device to find out what
    data it has stored for long periods of time.

The first approach might involve removing the chip from its system by
desoldering it (or maybe unplugging a SIMM) or by cutting the PCB and
making low-temperature bond connections to the chip to it in a forensic
test rig.  Alternatively, the device may be left in the system and
powered up with different signals being forced onto the memory chip to
stop the usual system operations, which usually include erasing DRAM or
SRAM (except for battery backed up SRAM, in which case the previous data
is easily readable).

So maybe someone could build a special system to plug a DRAM SIMM into
in order to detect tiny charges remaining in the capacitors from the
state the chip was in when turned off.  There could be many reasons why
this is impossible, not least the highly complex nature of modern DRAM
chips, with their fancy protocols, compared to the much more direct
nature of chips from the 70s and 80s.

Flash, EEPROM and EPROM memories also use a single capacitor, but this
is not connected or sensed directly by using a transistor to connect it
to a bit line.  The aim of data recovery in these cases is to find what
data was stored in the chip prior to the data which was most recently
written.  Flash memory has block erase modes, so it is possible that an
area has just been erased, without new data being written.

The capacitor of each memory cell is a small island, typically of
silicon, entirely surrounded by quartz or some other insulator.  Its
voltage (Flash and EEPROM) is raised and lowered by high voltage
temporary breakdown (quantum mechanical tunnelling of electrons) through
the insulator.  The capacitor's voltage is sensed by it part of a gate
of a MOSFET - so it can be read without any current entering or leaving.
 EPROM capacitors are charged and read as described above, but they can
only be discharged by illumination with short wavelength UV light, which
gives individual electrons enough energy to tunnel through the
capacitors insulator.

I expect that in all these cases programming and erasure is not an
exactly complete process - that some trace would remain of the previous
state.  To what extent this could be used it hard to say, but perhaps
there is a way in some cases, for instance with a FLASH or EEPROM
memory.   Lets say the chip was programmed with some data we wish to
recover.  Then lets say it was all erased to "1".  Now we power up the
chip and try to discern individual voltages of each cell.  Maybe there's
a way of increasing the power supply voltage so that the sense
amplifiers are discerning fine voltage difference in the range of the
normal "1" voltage.  (Flash, EEPROM and EPROM typically use internal or
external power supplies to create voltages much higher than the standard
power supply rails used for reading, so the exact voltage of the
capacitor, may be rather high.  What counts is the much higher power
supply voltage of the cell or read amplifier which is needed to read a
"0" when the cell is actually programmed as a "1".  Then by changing the
power supply voltage slightly, multiple reads can be used to finely
measure the supply voltage at which this cells reads as a 0 instead of a
1.  Maybe we could discern some variations which would have something to
do with the original data.  I can't see how this could work if the
capacitors were written to 0 volts.

Unlike DRAM and SRAM, Flash etc. has no continual or repetitive
amplification function.  Traces of previous charges may remain, but I
doubt they would be recoverable, except perhaps by the most drastic
forensic techniques - and even then, each write or erase operation would
reduce the remnants further still.


  - Robin        http://www.firstpr.com.au




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